Product SiteDocumentation Site

Fedora 15

Deployment Guide

Deployment, Configuration and Administration of Fedora 15

Edition 1

Jaromír Hradílek

Red Hat, Inc. Engineering Content Services

Douglas Silas

Red Hat, Inc. Engineering Content Services

Martin Prpič

Red Hat, Inc. Engineering Content Services

Eva Kopalová

Red Hat, Inc. Engineering Content Services

Eliška Slobodová

Red Hat, Inc. Engineering Content Services

John Ha

Red Hat, Inc. Engineering Content Services

David O'Brien

Red Hat, Inc. Engineering Content Services

Michael Hideo

Red Hat, Inc. Engineering Content Services

Don Domingo

Red Hat, Inc. Engineering Content Services

Legal Notice

Copyright © 2011 Red Hat, Inc. and others.
The text of and illustrations in this document are licensed by Red Hat under a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. The original authors of this document, and Red Hat, designate the Fedora Project as the "Attribution Party" for purposes of CC-BY-SA. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version.
Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law.
Red Hat, Red Hat Enterprise Linux, the Shadowman logo, JBoss, MetaMatrix, Fedora, the Infinity Logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries.
For guidelines on the permitted uses of the Fedora trademarks, refer to https://fedoraproject.org/wiki/Legal:Trademark_guidelines.
Linux® is the registered trademark of Linus Torvalds in the United States and other countries.
Java® is a registered trademark of Oracle and/or its affiliates.
XFS® is a trademark of Silicon Graphics International Corp. or its subsidiaries in the United States and/or other countries.
MySQL® is a registered trademark of MySQL AB in the United States, the European Union and other countries.
All other trademarks are the property of their respective owners.
Abstract
The Deployment Guide documents relevant information regarding the deployment, configuration, and administration of Fedora 15. It is oriented towards system administrators with a basic understanding of the system.

Preface
1. Target Audience
2. How to Read this Book
3. Document Conventions
3.1. Typographic Conventions
3.2. Pull-quote Conventions
3.3. Notes and Warnings
4. Feedback
5. Acknowledgments
I. Basic System Configuration
1. Language and Keyboard Configuration
1.1. Changing the Language
1.2. Changing the Keyboard Layout
2. Date and Time Configuration
2.1. Date and Time Configuration Tool
2.2. Command Line Configuration
2.2.1. Configuring the Date and Time
2.2.2. Configuring the Network Time Protocol
2.3. Additional Resources
2.3.1. Additional Resources
3. Managing Users and Groups
3.1. User Accounts Tool
3.1.1. Configuring an Account
3.1.2. Adding a New User
3.1.3. Removing a User
3.2. User Manager Tool
3.2.1. Adding a New User
3.2.2. Adding a New Group
3.2.3. Modifying Group Properties
3.3. User and Group Management Tools
3.3.1. Command Line Configuration
3.3.2. Explaining the Process
3.4. Standard Users
3.5. Standard Groups
3.6. User Private Groups
3.6.1. Group Directories
3.7. Shadow Passwords
3.8. Additional Resources
3.8.1. Installed Documentation
II. Package Management
4. Yum
4.1. Checking For and Updating Packages
4.1.1. Checking For Updates
4.1.2. Updating Packages
4.1.3. Preserving Configuration File Changes
4.2. Packages and Package Groups
4.2.1. Searching Packages
4.2.2. Listing Packages
4.2.3. Displaying Package Information
4.2.4. Installing Packages
4.2.5. Removing Packages
4.3. Configuring Yum and Yum Repositories
4.3.1. Setting [main] Options
4.3.2. Setting [repository] Options
4.3.3. Using Yum Variables
4.3.4. Creating a Yum Repository
4.4. Yum Plugins
4.4.1. Enabling, Configuring and Disabling Yum Plugins
4.4.2. Installing Additional Yum Plugins
4.4.3. Plugin Descriptions
4.5. Additional Resources
5. PackageKit
5.1. Updating Packages with Software Update
5.1.1. Setting the Update-Checking Interval
5.1.2. Setting the Software Sources
5.2. Using Add/Remove Software
5.2.1. Refreshing Software Sources (Yum Repositories)
5.2.2. Finding Packages with Filters
5.2.3. Installing and Removing Packages (and Dependencies)
5.2.4. Installing and Removing Package Groups
5.2.5. Viewing the Transaction Log
5.3. PackageKit Architecture
5.4. Additional Resources
III. Networking
6. Network Interfaces
6.1. Network Configuration Files
6.2. Interface Configuration Files
6.2.1. Ethernet Interfaces
6.2.2. Channel Bonding Interfaces
6.2.3. Alias and Clone Files
6.2.4. Dialup Interfaces
6.2.5. Other Interfaces
6.3. Interface Control Scripts
6.4. Configuring Static Routes
6.5. Network Function Files
6.6. Additional Resources
6.6.1. Installed Documentation
IV. Infrastructure Services
7. Services and Daemons
7.1. Configuring Services
7.1.1. Enabling the Service
7.1.2. Disabling the Service
7.2. Running Services
7.2.1. Checking the Service Status
7.2.2. Running the Service
7.2.3. Stopping the Service
7.2.4. Restarting the Service
7.3. Additional Resources
7.3.1. Installed Documentation
7.3.2. Related Books
8. Configuring Authentication
8.1. The Authentication Configuration Tool
8.1.1. Identity & Authentication
8.1.2. Advanced Options
8.1.3. Command Line Version
8.2. The System Security Services Daemon (SSSD)
8.2.1. What is SSSD?
8.2.2. SSSD Features
8.2.3. Setting Up SSSD
8.2.4. Configuring Services
8.2.5. Configuring Domains
8.2.6. Setting Up Kerberos Authentication
8.2.7. Configuring a Proxy Domain
8.2.8. Troubleshooting
8.2.9. SSSD Configuration File Format
9. OpenSSH
9.1. The SSH Protocol
9.1.1. Why Use SSH?
9.1.2. Main Features
9.1.3. Protocol Versions
9.1.4. Event Sequence of an SSH Connection
9.2. An OpenSSH Configuration
9.2.1. Configuration Files
9.2.2. Starting an OpenSSH Server
9.2.3. Requiring SSH for Remote Connections
9.2.4. Using a Key-Based Authentication
9.3. OpenSSH Clients
9.3.1. Using the ssh Utility
9.3.2. Using the scp Utility
9.3.3. Using the sftp Utility
9.4. More Than a Secure Shell
9.4.1. X11 Forwarding
9.4.2. Port Forwarding
9.5. Additional Resources
9.5.1. Installed Documentation
9.5.2. Useful Websites
V. Servers
10. DHCP Servers
10.1. Why Use DHCP?
10.2. Configuring a DHCP Server
10.2.1. Configuration File
10.2.2. Lease Database
10.2.3. Starting and Stopping the Server
10.2.4. DHCP Relay Agent
10.3. Configuring a DHCP Client
10.4. Configuring a Multihomed DHCP Server
10.4.1. Host Configuration
10.5. DHCP for IPv6 (DHCPv6)
10.6. Additional Resources
10.6.1. Installed Documentation
11. DNS Servers
11.1. Introduction to DNS
11.1.1. Nameserver Zones
11.1.2. Nameserver Types
11.1.3. BIND as a Nameserver
11.2. BIND
11.2.1. Configuring the named Service
11.2.2. Editing Zone Files
11.2.3. Using the rndc Utility
11.2.4. Using the dig Utility
11.2.5. Advanced Features of BIND
11.2.6. Common Mistakes to Avoid
11.2.7. Additional Resources
12. Web Servers
12.1. The Apache HTTP Server
12.1.1. New Features
12.1.2. Notable Changes
12.1.3. Updating the Configuration
12.1.4. Running the httpd Service
12.1.5. Editing the Configuration Files
12.1.6. Working with Modules
12.1.7. Setting Up Virtual Hosts
12.1.8. Setting Up an SSL Server
12.1.9. Additional Resources
13. Mail Servers
13.1. Email Protocols
13.1.1. Mail Transport Protocols
13.1.2. Mail Access Protocols
13.2. Email Program Classifications
13.2.1. Mail Transport Agent
13.2.2. Mail Delivery Agent
13.2.3. Mail User Agent
13.3. Mail Transport Agents
13.3.1. Postfix
13.3.2. Sendmail
13.3.3. Fetchmail
13.3.4. Mail Transport Agent (MTA) Configuration
13.4. Mail Delivery Agents
13.4.1. Procmail Configuration
13.4.2. Procmail Recipes
13.5. Mail User Agents
13.5.1. Securing Communication
13.6. Additional Resources
13.6.1. Installed Documentation
13.6.2. Useful Websites
13.6.3. Related Books
14. Directory Servers
14.1. OpenLDAP
14.1.1. Introduction to LDAP
14.1.2. Installing the OpenLDAP Suite
14.1.3. Configuring an OpenLDAP Server
14.1.4. Running an OpenLDAP Server
14.1.5. Configuring a System to Authenticate Using OpenLDAP
14.1.6. Additional Resources
15. File and Print Servers
15.1. Samba
15.1.1. Introduction to Samba
15.1.2. Samba Daemons and Related Services
15.1.3. Connecting to a Samba Share
15.1.4. Configuring a Samba Server
15.1.5. Starting and Stopping Samba
15.1.6. Samba Server Types and the smb.conf File
15.1.7. Samba Security Modes
15.1.8. Samba Account Information Databases
15.1.9. Samba Network Browsing
15.1.10. Samba with CUPS Printing Support
15.1.11. Samba Distribution Programs
15.1.12. Additional Resources
15.2. FTP
15.2.1. The File Transfer Protocol
15.2.2. FTP Servers
15.2.3. Files Installed with vsftpd
15.2.4. Starting and Stopping vsftpd
15.2.5. vsftpd Configuration Options
15.2.6. Additional Resources
15.3. Printer Configuration
15.3.1. Starting the Printer Configuration Tool
15.3.2. Starting Printer Setup
15.3.3. Adding a Local Printer
15.3.4. Adding an AppSocket/HP JetDirect printer
15.3.5. Adding an IPP Printer
15.3.6. Adding an LPD/LPR Host or Printer
15.3.7. Adding a Samba (SMB) printer
15.3.8. Selecting the Printer Model and Finishing
15.3.9. Printing a test page
15.3.10. Modifying Existing Printers
15.3.11. Additional Resources
VI. Monitoring and Automation
16. System Monitoring Tools
16.1. System Processes
16.2. Memory Usage
16.3. File Systems
16.4. Hardware
16.5. Additional Resources
16.5.1. Installed Documentation
17. Viewing and Managing Log Files
17.1. Configuring rsyslog
17.1.1. Global Directives
17.1.2. Modules
17.1.3. Rules
17.1.4. rsyslog Command Line Configuration
17.2. Locating Log Files
17.2.1. Configuring logrotate
17.3. Viewing Log Files
17.4. Adding a Log File
17.5. Monitoring Log Files
17.6. Additional Resources
17.6.1. Installed Documentation
17.6.2. Useful Websites
18. Automating System Tasks
18.1. Cron and Anacron
18.1.1. Starting and Stopping the Service
18.1.2. Configuring Anacron Jobs
18.1.3. Configuring Cron Jobs
18.1.4. Controlling Access to Cron
18.1.5. Black/White Listing of Cron Jobs
18.2. At and Batch
18.2.1. Configuring At Jobs
18.2.2. Configuring Batch Jobs
18.2.3. Viewing Pending Jobs
18.2.4. Additional Command Line Options
18.2.5. Controlling Access to At and Batch
18.2.6. Starting and Stopping the Service
18.3. Additional Resources
18.3.1. Installed Documentation
19. Automatic Bug-Reporting Tool (ABRT)
19.1. Overview
19.2. Installing and Running ABRT
19.3. ABRT Plugins
19.3.1. Analyzer Plugins
19.3.2. Reporter Plugins
19.3.3. Plugin Configuration in the GUI
19.4. Generating Backtraces
19.4.1. Troubleshooting Backtrace Generation
19.5. Using the Command Line Interface
19.5.1. Viewing Crashes
19.5.2. Reporting Crashes
19.5.3. Deleting Crashes
19.6. Configuring ABRT
19.7. Configuring Centralized Crash Collection
19.7.1. Testing ABRT's Crash Detection
19.7.2. Testing the Upload Method
19.8. Configuring Automatic Reporting
VII. Kernel, Module and Driver Configuration
20. Manually Upgrading the Kernel
20.1. Overview of Kernel Packages
20.2. Preparing to Upgrade
20.3. Downloading the Upgraded Kernel
20.4. Performing the Upgrade
20.5. Verifying the Initial RAM Disk Image
20.6. Verifying the Boot Loader
20.6.1. Configuring the GRUB Boot Loader
20.6.2. Configuring the OS/400 Boot Loader
20.6.3. Configuring the YABOOT Boot Loader
21. Working with Kernel Modules
21.1. Listing Currently-Loaded Modules
21.2. Displaying Information About a Module
21.3. Loading a Module
21.4. Unloading a Module
21.5. Setting Module Parameters
21.6. Persistent Module Loading
21.7. Specific Kernel Module Capabilities
21.7.1. Using Multiple Ethernet Cards
21.7.2. Using Channel Bonding
21.8. Additional Resources
21.8.1. Installed Documentation
21.8.2. Useful Websites
22. The kdump Crash Recovery Service
22.1. Configuring the kdump Service
22.1.1. Using the Kernel Dump Configuration Utility
22.1.2. Configuring kdump on the Command Line
22.1.3. Testing the Configuration
22.2. Analyzing the Core Dump
22.2.1. Running the crash Utility
22.2.2. Displaying the Message Buffer
22.2.3. Displaying a Backtrace
22.2.4. Displaying a Process Status
22.2.5. Displaying Virtual Memory Information
22.2.6. Displaying Open Files
22.2.7. Exiting the Utility
22.3. Additional Resources
22.3.1. Installed Documentation
22.3.2. Useful Websites
A. RPM
A.1. RPM Design Goals
A.2. Using RPM
A.2.1. Finding RPM Packages
A.2.2. Installing and Upgrading
A.2.3. Configuration File Changes
A.2.4. Uninstalling
A.2.5. Freshening
A.2.6. Querying
A.2.7. Verifying
A.3. Checking a Package's Signature
A.3.1. Importing Keys
A.3.2. Verifying Signature of Packages
A.4. Practical and Common Examples of RPM Usage
A.5. Additional Resources
A.5.1. Installed Documentation
A.5.2. Useful Websites
A.5.3. Related Books
B. The sysconfig Directory
B.1. Files in the /etc/sysconfig/ Directory
B.1.1. /etc/sysconfig/arpwatch
B.1.2. /etc/sysconfig/authconfig
B.1.3. /etc/sysconfig/autofs
B.1.4. /etc/sysconfig/clock
B.1.5. /etc/sysconfig/dhcpd
B.1.6. /etc/sysconfig/firstboot
B.1.7. /etc/sysconfig/i18n
B.1.8. /etc/sysconfig/init
B.1.9. /etc/sysconfig/ip6tables-config
B.1.10. /etc/sysconfig/keyboard
B.1.11. /etc/sysconfig/ldap
B.1.12. /etc/sysconfig/named
B.1.13. /etc/sysconfig/network
B.1.14. /etc/sysconfig/ntpd
B.1.15. /etc/sysconfig/quagga
B.1.16. /etc/sysconfig/radvd
B.1.17. /etc/sysconfig/samba
B.1.18. /etc/sysconfig/selinux
B.1.19. /etc/sysconfig/sendmail
B.1.20. /etc/sysconfig/spamassassin
B.1.21. /etc/sysconfig/squid
B.1.22. /etc/sysconfig/system-config-users
B.1.23. /etc/sysconfig/vncservers
B.1.24. /etc/sysconfig/xinetd
B.2. Directories in the /etc/sysconfig/ Directory
B.3. Additional Resources
B.3.1. Installed Documentation
C. The proc File System
C.1. A Virtual File System
C.1.1. Viewing Virtual Files
C.1.2. Changing Virtual Files
C.2. Top-level Files within the proc File System
C.2.1. /proc/buddyinfo
C.2.2. /proc/cmdline
C.2.3. /proc/cpuinfo
C.2.4. /proc/crypto
C.2.5. /proc/devices
C.2.6. /proc/dma
C.2.7. /proc/execdomains
C.2.8. /proc/fb
C.2.9. /proc/filesystems
C.2.10. /proc/interrupts
C.2.11. /proc/iomem
C.2.12. /proc/ioports
C.2.13. /proc/kcore
C.2.14. /proc/kmsg
C.2.15. /proc/loadavg
C.2.16. /proc/locks
C.2.17. /proc/mdstat
C.2.18. /proc/meminfo
C.2.19. /proc/misc
C.2.20. /proc/modules
C.2.21. /proc/mounts
C.2.22. /proc/mtrr
C.2.23. /proc/partitions
C.2.24. /proc/slabinfo
C.2.25. /proc/stat
C.2.26. /proc/swaps
C.2.27. /proc/sysrq-trigger
C.2.28. /proc/uptime
C.2.29. /proc/version
C.3. Directories within /proc/
C.3.1. Process Directories
C.3.2. /proc/bus/
C.3.3. /proc/bus/pci
C.3.4. /proc/driver/
C.3.5. /proc/fs
C.3.6. /proc/irq/
C.3.7. /proc/net/
C.3.8. /proc/scsi/
C.3.9. /proc/sys/
C.3.10. /proc/sysvipc/
C.3.11. /proc/tty/
C.3.12. /proc/PID/
C.4. Using the sysctl Command
C.5. References
D. Revision History
Index

Preface

The Deployment Guide contains information on how to customize the Fedora 15 system to fit your needs. If you are looking for a comprehensive, task-oriented guide for configuring and customizing your system, this is the manual for you.
This manual discusses many intermediate topics such as the following:
  • Installing and managing packages using the graphical PackageKit and command line Yum package managers
  • Setting up a network—from establishing an Ethernet connection using NetworkManager to configuring channel bonding interfaces to increase server bandwidth
  • Configuring DHCP, BIND, Apache HTTP Server, Postfix, Sendmail and other enterprise-class servers and software
  • Gathering information about your system, including obtaining user-space crash data with the Automatic Bug Reporting Tool, and kernel-space crash data with kdump
  • Easily working with kernel modules and upgrading the kernel

1. Target Audience

The Deployment Guide assumes you have a basic understanding of the Fedora operating system. If you need help with the installation of this system, refer to the Fedora 15 Installation Guide.

2. How to Read this Book

This manual is divided into the following main categories:
Part I, “Basic System Configuration”
This part covers basic system administration tasks such as keyboard configuration, date and time configuration, and managing users and groups.
Chapter 1, Language and Keyboard Configuration covers basic language and keyboard setup. Read this chapter if you need to configure the language of your desktop, change the keyboard layout, or add the keyboard layout indicator to the panel.
Chapter 2, Date and Time Configuration covers the configuration of the system date and time. Read this chapter if you need to change the date and time setup, or configure the system to synchronize the clock with a remote Network Time Protocol (NTP) server.
Chapter 3, Managing Users and Groups covers the management of users and groups in a graphical user interface and on the command line. Read this chapter if you need to manage users and groups on your system, or enable password aging.
Part II, “Package Management”
This part describes how to manage software packages on Fedora using both Yum and the PackageKit suite of graphical package management tools.
Chapter 4, Yum describes the Yum package manager. Read this chapter for information how to search, install, update, and uninstall packages on the command line.
Chapter 5, PackageKit describes the PackageKit suite of graphical package management tools. Read this chapter for information how to search, install, update, and uninstall packages using a graphical user interface.
Part III, “Networking”
This part describes how to configure the network on Fedora.
Chapter 6, Network Interfaces explores various interface configuration files, interface control scripts, and network function files located in the /etc/sysconfig/network-scripts/ directory. Read this chapter for information how to use these files to configure network interfaces.
Part IV, “Infrastructure Services”
This part provides information how to configure services and daemons, configure authentication, and enable remote logins.
Chapter 7, Services and Daemons covers the configuration of the services to be run when a system is started, and provides information on how to start, stop, and restart the services on the command line using the systemctl utility.
Chapter 8, Configuring Authentication describes how to configure user information retrieval from Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind user account databases, and provides an introduction to the System Security Services Daemon (SSSD). Read this chapter if you need to configure authentication on your system.
Chapter 9, OpenSSH describes how to enable a remote login via the SSH protocol. It covers the configuration of the sshd service, as well as a basic usage of the ssh, scp, sftp client utilities. Read this chapter if you need a remote access to a machine.
Part V, “Servers”
This part discusses various topics related to servers such as how to set up a Web server or share files and directories over the network.
Chapter 10, DHCP Servers guides you through the installation of a Dynamic Host Configuration Protocol (DHCP) server and client. Read this chapter if you need to configure DHCP on your system.
Chapter 11, DNS Servers introduces you to Domain Name System (DNS), explains how to install, configure, run, and administer the BIND DNS server. Read this chapter if you need to configure a DNS server on your system.
Chapter 12, Web Servers focuses on the Apache HTTP Server 2.2, a robust, full-featured open source web server developed by the Apache Software Foundation. Read this chapter if you need to configure a web server on your system.
Chapter 13, Mail Servers reviews modern email protocols in use today, and some of the programs designed to send and receive email, including Postfix, Sendmail, Fetchmail, and Procmail. Read this chapter if you need to configure a mail server on your system.
Chapter 14, Directory Servers covers the installation and configuration of OpenLDAP 2.4, an open source implementation of the LDAPv2 and LDAPv3 protocols. Read this chapter if you need to configure a directory server on your system.
Chapter 15, File and Print Servers guides you through the installation and configuration of Samba, an open source implementation of the Server Message Block (SMB) protocol, and vsftpd, the primary FTP server shipped with Fedora. Additionally, it explains how to use the Printer Configuration tool to configure printers. Read this chapter if you need to configure a file or print server on your system.
Part VI, “Monitoring and Automation”
This part describes various tools that allow system administrators to monitor system performance, automate system tasks, and report bugs.
Chapter 16, System Monitoring Tools discusses applications and commands that can be used to retrieve important information about the system. Read this chapter to learn how to gather essential system information.
Chapter 17, Viewing and Managing Log Files describes the configuration of the rsyslog daemon, and explains how to locate, view, and monitor log files. Read this chapter to learn how to work with log files.
Chapter 18, Automating System Tasks provides an overview of the cron, at, and batch utilities. Read this chapter to learn how to use these utilities to perform automated tasks.
Chapter 19, Automatic Bug-Reporting Tool (ABRT) concentrates on ABRT, a system service and a set of tools to collect crash data and send a report to the relevant issue tracker. Read this chapter to learn how to use ABRT on your system.
Part VII, “Kernel, Module and Driver Configuration”
This part covers various tools that assist administrators with kernel customization.
Chapter 20, Manually Upgrading the Kernel provides important information how to manually update a kernel package using the rpm command instead of yum. Read this chapter if you cannot update a kernel package with the Yum package manager.
Chapter 21, Working with Kernel Modules explains how to display, query, load, and unload kernel modules and their dependencies, and how to set module parameters. Additionally, it covers specific kernel module capabilities such as using multiple Ethernet cards and using channel bonding. Read this chapter if you need to work with kernel modules.
Chapter 22, The kdump Crash Recovery Service explains how to configure, test, and use the kdump service in Fedora, and provides a brief overview of how to analyze the resulting core dump using the crash debugging utility. Read this chapter to learn how to enable kdump on your system.
Appendix A, RPM
This appendix concentrates on the RPM Package Manager (RPM), an open packaging system used by Fedora, and the use of the rpm utility. Read this appendix if you need to use rpm instead of yum.
Appendix B, The sysconfig Directory
This appendix outlines some of the files and directories located in the /etc/sysconfig/ directory. Read this appendix if you want to learn more about these files and directories, their function, and their contents.
Appendix C, The proc File System
This appendix explains the concept of a virtual file system, and describes some of the top-level files and directories within the proc file system (that is, the /proc/ directory). Read this appendix if you want to learn more about this file system.

3. Document Conventions

This manual uses several conventions to highlight certain words and phrases and draw attention to specific pieces of information.
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. The Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not, alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes the Liberation Fonts set by default.

3.1. Typographic Conventions

Four typographic conventions are used to call attention to specific words and phrases. These conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight keycaps and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current working directory, enter the cat my_next_bestselling_novel command at the shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a keycap, all presented in mono-spaced bold and all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key combination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to the first virtual terminal. Press Ctrl+Alt+F1 to return to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two key combinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned values mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog box text; labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example:
Choose SystemPreferencesMouse from the main menu bar to launch Mouse Preferences. In the Buttons tab, click the Left-handed mouse check box and click Close to switch the primary mouse button from the left to the right (making the mouse suitable for use in the left hand).
To insert a special character into a gedit file, choose ApplicationsAccessoriesCharacter Map from the main menu bar. Next, choose SearchFind… from the Character Map menu bar, type the name of the character in the Search field and click Next. The character you sought will be highlighted in the Character Table. Double-click this highlighted character to place it in the Text to copy field and then click the Copy button. Now switch back to your document and choose EditPaste from the gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific menu names; and buttons and text found within a GUI interface, all presented in proportional bold and all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or variable text. Italics denotes text you do not input literally or displayed text that changes depending on circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at a shell prompt. If the remote machine is example.com and your username on that machine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file system. For example, to remount the /home file system, the command is mount -o remount /home.
To see the version of a currently installed package, use the rpm -q package command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and release. Each word is a placeholder, either for text you enter when issuing a command or for text displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and important term. For example:
Publican is a DocBook publishing system.

3.2. Pull-quote Conventions

Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus:
books        Desktop   documentation  drafts  mss    photos   stuff  svn
books_tests  Desktop1  downloads      images  notes  scripts  svgs
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows:
package org.jboss.book.jca.ex1;

import javax.naming.InitialContext;

public class ExClient
{
   public static void main(String args[]) 
       throws Exception
   {
      InitialContext iniCtx = new InitialContext();
      Object         ref    = iniCtx.lookup("EchoBean");
      EchoHome       home   = (EchoHome) ref;
      Echo           echo   = home.create();

      System.out.println("Created Echo");

      System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
   }
}

3.3. Notes and Warnings

Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.

Note

Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should have no negative consequences, but you might miss out on a trick that makes your life easier.

Important

Important boxes detail things that are easily missed: configuration changes that only apply to the current session, or services that need restarting before an update will apply. Ignoring a box labeled 'Important' will not cause data loss but may cause irritation and frustration.

Warning

Warnings should not be ignored. Ignoring warnings will most likely cause data loss.

4. Feedback

If you find a typographical error in this manual, or if you have thought of a way to make this manual better, we would love to hear from you! Please submit a report in Bugzilla against the product Fedora Documentation.
When submitting a bug report, be sure to provide the following information:
  • Manual's identifier: deployment-guide
  • Version number: 15
If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding text so we can find it easily.

5. Acknowledgments

Certain portions of this text first appeared in the Deployment Guide, copyright © 2007 Red Hat, Inc., available at http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Deployment_Guide/index.html.
The authors of this book would like to thank the following people for their valuable contributions: Adam Tkáč, Andrew Fitzsimon, Andrius Benokraitis, Brian Cleary Edward Bailey, Garrett LeSage, Jeffrey Fearn, Joe Orton, Joshua Wulf, KarstenWade, Lucy Ringland, Marcela Mašláňová, Mark Johnson, Michael Behm, Michael Behm, Miroslav Lichvár, Radek Vokál, Rahul Kavalapara, Rahul Sundaram, Sandra Moore, Zbyšek Mráz and Jan Včelák, among many others.

Part I. Basic System Configuration

Chapter 1. Language and Keyboard Configuration

This chapter describes how to change the language of your desktop, configure the keyboard layout, and allow a user to switch between different keyboard layouts using the Region and Language configuration tool.

1.1. Changing the Language

To configure the language of your desktop, open the System Settings dialog window by selecting ApplicationsSystem ToolsSystem Settings from the Activities menu, click Region and Language, and select the Language tab. You will be presented with a list of available languages.
Changing the language
Changing the language
Figure 1.1. Changing the language

To choose a particular language, select it from the list and either close the window, or click the All Settings button in the upper-left corner of the window to return to the System Settings window. The changes will take effect the next time you log in to your system.

1.2. Changing the Keyboard Layout

Although the installation program allows a system administrator to configure a keyboard layout during the system installation, the default settings may not always suit your current needs. To add a new keyboard layout, open the System Settings window by selecting ApplicationsSystem ToolsSystem Settings from the Activities menu, click the Region and Language icon, and select the Layouts tab.
Changing the keyboard layout
Changing the keyboard layout
Figure 1.2. Changing the keyboard layout

The left part of the window provides a list of currently enabled layouts, while the right part allows you to choose whether to use the same keyboard layout for all windows, or if you prefer to use a different layout for each window.
To add a keyboard layout, click the + (that is, the plus sign) button below the list. A dialog box will appear, allowing you to select the desired layout. The input field in the bottom part of the dialog box allows you to reduce the number of displayed items by typing part of the layout name (such as slov for Slovak layouts) in it. Once you are done, click the Add button to confirm your choice.
Adding a keyboard layout
Adding a keyboard layout
Figure 1.3. Adding a keyboard layout

To remove a keyboard layout, click the (that is, the minus sign) button below the list.
When more than one layout is enabled, a keyboard indicator appears on the panel in order to allow you to switch between the layouts, and the first layout in the list is considered the default. To move a particular layout up or down in the list, select it from the list and click the (the upwards arrow) or (the downwards arrow) buttons respectively.
The keyboard layout indicator
The keyboard layout indicator
Figure 1.4. The keyboard layout indicator

The changes take effect immediately.

Chapter 2. Date and Time Configuration

This chapter covers setting the system date and time in Fedora, both manually and using the Network Time Protocol (NTP), as well as setting the adequate time zone. Two methods are covered: setting the date and time using the Date and Time configuration tool, and doing so on the command line.

2.1. Date and Time Configuration Tool

The Date and Time configuration tool allows a user to change the system date and time, to configure the time zone used by the system, and to set up the Network Time Protocol daemon to synchronize the system clock with a time server.
To start the tool, select ApplicationsSystem ToolsSystem Settings from the Activities menu and click the Date and Time icon, or click the time in the panel and select Date and Time Settings from the drop-down menu.
Running the Date and Time configuration tool
Running the Date and Time configuration tool
Figure 2.1. Running the Date and Time configuration tool

By default, the tool only allows you to review the current settings. This is because only root is allowed to set the system date and time. To unlock the configuration tool for changes, click the Unlock button in the bottom-left corner of the window, and provide the root password when prompted.
The Date and Time configuration tool
The Date and Time configuration tool
Figure 2.2. The Date and Time configuration tool

As you can see in Figure 2.2, “The Date and Time configuration tool”, the main window of the Date and Time configuration tool is divided into two parts: while the left side of the window is dedicated to the time zone settings, the right part allows you to configure the date and time itself.
To change the time zone, click on the map, or select the region and city from the Region and City drop-down lists.
To change the current time of your system, either configure the system to synchronize it over the network by clicking the Network Time switch, or set it manually by clicking the up and down arrows above and below the numbers. Use the 24-Hour Time switch to enable or disable the 24-hour time format.
To change the current date of your system, select a month from the drop-down list below the time, and use the up and down arrows to choose the day and year.
The changes take effect immediately.

2.2. Command Line Configuration

In case your system does not have the Date/Time Properties tool installed, or the X Window Server is not running, you can change the system date and time on the command line. Note that in order to perform actions described in this section, you have to be logged in as root. To do so, type the following at a shell prompt:
su -

2.2.1. Configuring the Date and Time

The date command allows the superuser to set the system date and time manually:
  1. To change the current date, type the command in the following form at a shell prompt, replacing the YYYY with a four-digit year, MM with a two-digit month, and DD with a two-digit day of the month:
    date +%D -s YYYY-MM-DD
    For example, to set the date to 2 June 2010, type:
    ~]# date +%D -s 2010-06-02
  2. To change the current date, use the following command, where HH stands for an hour, MM is a minute, and SS is a second, all typed in a two-digit form:
    date +%T -s HH:MM:SS
    If your system clock is set to use UTC (Coordinated Universal Time), add the following option:
    date +%T -s HH:MM:SS -u
    For instance, to set the system clock to 11:26 PM using the UTC, type:
    ~]# date +%T -s 23:26:00 -u
You can check your current settings by typing date without any additional argument, for example:
~]$ date
Wed Jun  2 11:58:48 CEST 2010

2.2.2. Configuring the Network Time Protocol

As opposed to the manual setup described above, you can also synchronize the system clock with a remote server over the Network Time Protocol (NTP). For the one-time synchronization only, use the ntpdate command:
  1. Check whether the selected NTP server is accessible by using the ntpdate command in the following form:
    ntpdate -q server_address
    For example, to connect to 0.fedora.pool.ntp.org, type:
    ~]# ntpdate -q 0.fedora.pool.ntp.org
    server 204.15.208.61, stratum 2, offset -39.275438, delay 0.16083
    server 69.65.40.29, stratum 2, offset -39.269122, delay 0.17191
    server 148.167.132.201, stratum 2, offset -39.270239, delay 0.20482
    23 May 01:05:54 ntpdate[10619]: step time server 204.15.208.61 offset -39.275438 sec
  2. When you find a satisfactory server, run the ntpdate command followed with one or more server addresses:
    ntpdate server_address...
    For instance:
    ~]# ntpdate 0.fedora.pool.ntp.org 1.fedora.pool.ntp.org
    23 May 01:07:13 ntpdate[10669]: step time server 204.15.208.61 offset -39.275436 sec
    Unless an error message is displayed, the system time should now be set. You can check the current by setting typing date without any additional arguments as shown in Section 2.2.1, “Configuring the Date and Time”.
  3. In most cases, these steps are sufficient. Only if you really need one or more system services to always use the correct time, enable running the ntpdate at boot time:
    systemctl enable ntpdate.service
    For more information about system services and their setup, refer to Chapter 7, Services and Daemons.

    Note

    If the synchronization with the time server at boot time keeps failing, that is, you find a relevant error message in the /var/log/boot.log system log, try to add the following line to /etc/sysconfig/network:
    NETWORKWAIT=1
However, the more convenient way is to set the ntpd daemon to synchronize the time at boot time automatically:
  1. Open the NTP configuration file /etc/ntp.conf in a text editor, creating a new one if it does not already exist.
  2. Add or edit the list of public NTP servers. If you are using Fedora 15, the file should already contain the following lines, but feel free to change or expand these according to your needs:
    server 0.fedora.pool.ntp.org iburst
    server 1.fedora.pool.ntp.org iburst
    server 2.fedora.pool.ntp.org iburst

    Speeding up initial synchronization

    To speed the initial synchronization up, it is recommended that the iburst directive is added at the end of each server line.
  3. In the same file, set the proper permissions, giving the unrestricted access to localhost only:
    restrict default kod nomodify notrap nopeer noquery
    restrict -6 default kod nomodify notrap nopeer noquery
    restrict 127.0.0.1
    restrict -6 ::1
  4. Save the changes, exit the editor, and restart the NTP daemon:
    systemctl restart ntpd.service
  5. Additionally, make sure that ntpd daemon is started at boot time:
    systemctl enable ntpd.service

2.3. Additional Resources

For more information about the date and time configuration, refer to the following resources.

2.3.1. Additional Resources

  • date(1) — The manual page for the date utility.
  • ntpdate(8) — The manual page for the ntpdate utility.
  • ntpd(8) — The manual page for the ntpd service.

Chapter 3. Managing Users and Groups

The control of users and groups is a core element of Fedora system administration. Users can be either people (meaning accounts tied to physical users) or accounts which exist for specific applications to use. Groups are logical expressions of organization, tying users together for a common purpose. Users within a group can read, write, or execute files owned by that group.
Each user is associated with a unique numerical identification number called a userid (UID); likewise, each group is associated with a groupid (GID).
A user who creates a file is also the owner and group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file owner can be changed only by the root user, and access permissions can be changed by both the root user and file owner.
Fedora also supports access control lists (ACLs) for files and directories which allow permissions for specific users outside of the owner to be set. For more information about ACLs, refer to the Access Control Lists chapter of the Fedora Storage Administration Guide.

3.1. User Accounts Tool

The User Accounts configuration tool allows you to view, modify, add, and delete local users. To run the utility, either select ApplicationsSystem ToolsSystem Settings from the Activities menu and click the User Accounts icon, or click your name on the panel and choose My Account from the drop-down menu.
The User Accounts configuration tool
The User Accounts configuration tool
Figure 3.1. The User Accounts configuration tool

The main window of the User Accounts configuration tool is divided into two parts: The left side of the window contains a list of available user accounts. The right side provides details on a particular account.
By default, the tool only allows you to change certain settings regarding your account. This is because only root is allowed to configure users and groups. To unlock the configuration tool for all kinds of changes, click the Unlock button in the bottom-left corner of the window, and provide the root password when prompted.

3.1.1. Configuring an Account

To change the image associated with an account, click the icon next to the account name and either select a picture from the drop-down list, or click Browse for more pictures... to use an image from your local drive.
To change the name associated with an account, click the name next to the icon to edit it.
To change the account type, use the Account type drop-down list. However, this change requires the configuration tool to be unlocked even if it is your account.
To change the default language for an account, click the button next to the Language label, and select the desired language from the list.
To change the password, click the button next to the Password label. A dialog box will appear, allowing you to set the new password. Note that the current password must be provided in order to confirm the change. Once done, click the Change button to save the change.
Changing the password
Changing the password
Figure 3.2. Changing the password

Password security advice

It is advisable to use a much longer password, as this makes it more difficult for an intruder to guess it and access the account without permission. It is also recommended that the password not be based on a dictionary term: use a combination of letters, numbers and special characters.
Finally, to set up automatic login for a particular account, enable the Automatic Login switch. The configuration tool must be unlocked to make this change.

3.1.2. Adding a New User

To add a new user, make sure the configuration tool is unlocked, and click the + button (that is, the plus sign) below the account list. A dialog box as shown in Figure 3.3, “Creating a new account” will appear.
Creating a new account
Creating a new account
Figure 3.3. Creating a new account

Take the followign steps to create an account:
  1. Select an account type from the Account type drop-down list. Available account types are Administrator and Standard (the default option).
  2. Fill in the Full name input field to set the name associated with the account. This name will be used by the login manager, and will be displayed on the panel.
  3. Either select a suggested username from the Username drop-down list, or fill in the corresponding input field.
  4. Click the Create button to confirm the settings.
Fedora uses a user private group (UPG) scheme. The UPG scheme does not add or change anything in the standard UNIX way of handling groups; it offers a new convention. Whenever you create a new user, a unique group with the same name as the user is created.
When a new account is created, default configuration files are copied from the /etc/skel/ directory into the new home directory.

3.1.3. Removing a User

To remove a user, make sure the configuration tool is unlocked, select the desired account from the account list, and click the button (that is, the minus sign) below the account list. A dialog box as shown in Figure 3.4, “Removing an account” will appear.
Removing an account
Removing an account
Figure 3.4. Removing an account

To delete files and directories that belong to the user (that is, the home directory, mail spool, and temporary files), click the Delete Files button. To keep these files intact and only delete the user account, click Keep Files. To abort the deletion, click Cancel.

3.2. User Manager Tool

The User Manager allows you to view, modify, add, and delete local users and groups.
The GNOME User Manager
The GNOME User Manager allows you to manage users
Figure 3.5. The GNOME User Manager

You can start the User Manager by clicking ApplicationsOtherUsers and Groups from the Activities menu. Alternatively, you can enter system-config-users at the shell prompt to open the User Manager. Viewing and modifying user and group information requires superuser privileges. If you are not the superuser when you open the User Manager, it will prompt you for the superuser password.
To view a list of local users on the system, click the Users tab. To view a list of local groups on the system, click the Groups tab.
To find a specific user or group, type the first few letters of the name in the Search filter field. Press Enter or click the Apply filter button. The filtered list is displayed.
To sort the users, click on the column User Name and for groups click on Group Name. The users or groups are sorted according to the value of that column.
Fedora reserves user IDs below 500 for system users. By default, the User Manager does not display system users. To view all users, including the system users, go to EditPreferences and uncheck Hide system users and groups from the dialog box.

3.2.1. Adding a New User

To add a new user, click the Add User button. A window as shown in Figure 3.6, “Creating a new user” appears. Type the username and full name for the new user in the appropriate fields. Type the user's password in the Password and Confirm Password fields. The password must be at least six characters.

Password security advice

It is advisable to use a much longer password, as this makes it more difficult for an intruder to guess it and access the account without permission. It is also recommended that the password not be based on a dictionary term: use a combination of letters, numbers and special characters.
Select a login shell from the pulldown list. If you are not sure which shell to select, accept the default value of /bin/bash. The default home directory is /home/username/. You can change the home directory that is created for the user, or you can choose not to create the home directory by unselecting Create home directory.
If you select to create the home directory, default configuration files are copied from the /etc/skel/ directory into the new home directory.
Fedora uses a user private group (UPG) scheme. The UPG scheme does not add or change anything in the standard UNIX way of handling groups; it offers a new convention. Whenever you create a new user, by default, a unique group with the same name as the user is created. If you do not want to create this group, unselect Create a private group for the user.
To specify a user ID for the user, select Specify user ID manually. If the option is not selected, the next available user ID above 500 is assigned to the new user. Because Fedora reserves user IDs below 500 for system users, it is not advisable to manually assign user IDs 1-499.
Click OK to create the user.
Creating a new user
Creating a new user with the Create New User dialog
Figure 3.6. Creating a new user

To configure more advanced user properties, such as password expiration, modify the user's properties after adding the user.

Modifying User Properties

To view the properties of an existing user, click on the Users tab, select the user from the user list, and click Properties from the menu (or choose FileProperties from the pulldown menu). A window similar to Figure 3.7, “User Properties” appears.
User Properties
Modifying user properties
Figure 3.7. User Properties

The User Properties window is divided into multiple tabbed pages:
  • User Data — Shows the basic user information configured when you added the user. Use this tab to change the user's full name, password, home directory, or login shell.
  • Account Info — Select Enable account expiration if you want the account to expire on a certain date. Enter the date in the provided fields. Select Local password is locked to lock the user account and prevent the user from logging into the system.
  • Password Info — Displays the date that the user's password last changed. To force the user to change passwords after a certain number of days, select Enable password expiration and enter a desired value in the Days before change required: field. The number of days before the user's password expires, the number of days before the user is warned to change passwords, and days before the account becomes inactive can also be changed.
  • Groups — Allows you to view and configure the Primary Group of the user, as well as other groups that you want the user to be a member of.

3.2.2. Adding a New Group

To add a new user group, select Add Group from the toolbar. A window similar to Figure 3.8, “New Group” appears. Type the name of the new group. To specify a group ID for the new group, select Specify group ID manually and select the GID. Note that Fedora also reserves group IDs lower than 500 for system groups.
New Group
Creating a new group
Figure 3.8. New Group

Click OK to create the group. The new group appears in the group list.

3.2.3. Modifying Group Properties

To view the properties of an existing group, select the group from the group list and click Properties from the menu (or choose FileProperties from the pulldown menu). A window similar to Figure 3.9, “Group Properties” appears.
Group Properties
Modifying group properties
Figure 3.9. Group Properties

The Group Users tab displays which users are members of the group. Use this tab to add or remove users from the group. Click OK to save your changes.

3.3. User and Group Management Tools

Managing users and groups can be tiresome; this is why Fedora provides tools and conventions to make this task easier to manage.
The easiest way to manage users and groups is through the graphical application, User Manager (system-config-users). For more information on User Manager, refer to Section 3.2, “User Manager Tool”.
The following command line tools can also be used to manage users and groups:
  • useradd, usermod, and userdel — Industry-standard methods of adding, deleting and modifying user accounts
  • groupadd, groupmod, and groupdel — Industry-standard methods of adding, deleting, and modifying user groups
  • gpasswd — Industry-standard method of administering the /etc/group file
  • pwck, grpck — Tools used for the verification of the password, group, and associated shadow files
  • pwconv, pwunconv — Tools used for the conversion of passwords to shadow passwords and back to standard passwords

3.3.1. Command Line Configuration

If you prefer command line tools or do not have the X Window System installed, use following to configure users and groups.

Adding a User

To add a user to the system:
  1. Issue the useradd command to create a locked user account:
    useradd username
  2. Unlock the account by issuing the passwd command to assign a password and set password aging guidelines:
    passwd username
Command line options for useradd are detailed in Table 3.1, “useradd Command Line Options”.
Table 3.1. useradd Command Line Options
Option Description
-c 'comment' comment can be replaced with any string. This option is generally used to specify the full name of a user.
-d home_directory Home directory to be used instead of default /home/username/.
-e date Date for the account to be disabled in the format YYYY-MM-DD.
-f days Number of days after the password expires until the account is disabled. If 0 is specified, the account is disabled immediately after the password expires. If -1 is specified, the account is not be disabled after the password expires.
-g group_name Group name or group number for the user's default group. The group must exist prior to being specified here.
-G group_list List of additional (other than default) group names or group numbers, separated by commas, of which the user is a member. The groups must exist prior to being specified here.
-m Create the home directory if it does not exist.
-M Do not create the home directory.
-N Do not create a user private group for the user.
-p password The password encrypted with crypt.
-r Create a system account with a UID less than 500 and without a home directory.
-s User's login shell, which defaults to /bin/bash.
-u uid User ID for the user, which must be unique and greater than 499.

Adding a Group

To add a group to the system, use the command groupadd:
groupadd group_name
Command line options for groupadd are detailed in Table 3.2, “groupadd Command Line Options”.
Table 3.2. groupadd Command Line Options
Option Description
-f, --force When used with -g gid and gid already exists, groupadd will choose another unique gid for the group.
-g gid Group ID for the group, which must be unique and greater than 499.
-K, --key key=value Override /etc/login.defs defaults.
-o, --non-unique Allow to create groups with duplicate.
-p, --password password Use this encrypted password for the new group.
-r Create a system group with a GID less than 500.

Password Aging

For security reasons, it is advisable to require users to change their passwords periodically. This can be done when adding or editing a user on the Password Info tab of the User Manager.
To configure password expiration for a user from a shell prompt, use the chage command with an option from Table 3.3, “chage Command Line Options”, followed by the username.

Shadow passwords must be enabled to use chage

Shadow passwords must be enabled to use the chage command. For more information, see Section 3.7, “Shadow Passwords”.
Table 3.3. chage Command Line Options
Option Description
-d days Specifies the number of days since January 1, 1970 the password was changed.
-E date Specifies the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used.
-I days Specifies the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not locked after the password expires.
-l Lists current account aging settings.
-m days Specify the minimum number of days after which the user must change passwords. If the value is 0, the password does not expire.
-M days Specify the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account.
-W days Specifies the number of days before the password expiration date to warn the user.

chage interactivity

If the chage command is followed directly by a username (with no options), it displays the current password aging values and allows them to be changed interactively.
You can configure a password to expire the first time a user logs in. This forces users to change passwords immediately.
  1. Set up an initial password — There are two common approaches to this step: the administrator can assign a default password, or he can use a null password.
    To assign a default password, type the following at a shell prompt:
    passwd username
    To assign a null password instead, use the following command:
    passwd -d username

    Avoid using null passwords whenever possible

    Using a null password, while convenient, is a highly insecure practice, as any third party can log in first and access the system using the insecure username. Always make sure that the user is ready to log in before unlocking an account with a null password.
  2. Force immediate password expiration — Type the following command:
    chage -d 0 username
    This command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
Upon the initial log in, the user is now prompted for a new password.

3.3.2. Explaining the Process

The following steps illustrate what happens if the command useradd juan is issued on a system that has shadow passwords enabled:
  1. A new line for juan is created in /etc/passwd:
    juan:x:501:501::/home/juan:/bin/bash
    The line has the following characteristics:
    • It begins with the username juan.
    • There is an x for the password field indicating that the system is using shadow passwords.
    • A UID greater than 499 is created. Under Fedora, UIDs and GIDs below 500 are reserved for system use. These should not be assigned to users.
    • A GID greater than 499 is created.
    • The optional GECOS information is left blank.
    • The home directory for juan is set to /home/juan/.
    • The default shell is set to /bin/bash.
  2. A new line for juan is created in /etc/shadow:
    juan:!!:14798:0:99999:7:::
    The line has the following characteristics:
    • It begins with the username juan.
    • Two exclamation marks (!!) appear in the password field of the /etc/shadow file, which locks the account.

      Note

      If an encrypted password is passed using the -p flag, it is placed in the /etc/shadow file on the new line for the user.
    • The password is set to never expire.
  3. A new line for a group named juan is created in /etc/group:
    juan:x:501:
    A group with the same name as a user is called a user private group. For more information on user private groups, refer to Section 3.2.1, “Adding a New User”.
    The line created in /etc/group has the following characteristics:
    • It begins with the group name juan.
    • An x appears in the password field indicating that the system is using shadow group passwords.
    • The GID matches the one listed for user juan in /etc/passwd.
  4. A new line for a group named juan is created in /etc/gshadow:
    juan:!::
    The line has the following characteristics:
    • It begins with the group name juan.
    • An exclamation mark (!) appears in the password field of the /etc/gshadow file, which locks the group.
    • All other fields are blank.
  5. A directory for user juan is created in the /home/ directory:
    ~]# ls -l /home
    total 4
    drwx------. 4 juan juan 4096 Mar  3 18:23 juan
    This directory is owned by user juan and group juan. It has read, write, and execute privileges only for the user juan. All other permissions are denied.
  6. The files within the /etc/skel/ directory (which contain default user settings) are copied into the new /home/juan/ directory:
    ~]# ls -la /home/juan
    total 28
    drwx------. 4 juan juan 4096 Mar  3 18:23 .
    drwxr-xr-x. 5 root root 4096 Mar  3 18:23 ..
    -rw-r--r--. 1 juan juan   18 Jun 22  2010 .bash_logout
    -rw-r--r--. 1 juan juan  176 Jun 22  2010 .bash_profile
    -rw-r--r--. 1 juan juan  124 Jun 22  2010 .bashrc
    drwxr-xr-x. 2 juan juan 4096 Jul 14  2010 .gnome2
    drwxr-xr-x. 4 juan juan 4096 Nov 23 15:09 .mozilla
At this point, a locked account called juan exists on the system. To activate it, the administrator must next assign a password to the account using the passwd command and, optionally, set password aging guidelines.

3.4. Standard Users

Table 3.4, “Standard Users” lists the standard users configured in the /etc/passwd file by an Everything installation. The group ID (GID) in this table is the primary group for the user. See Section 3.5, “Standard Groups” for a listing of standard groups.
Table 3.4. Standard Users
User UID GID Home Directory Shell Packages
root 0 0 /root /bin/bash setup
bin 1 1 /bin /sbin/nologin setup
daemon 2 2 /sbin /sbin/nologin setup
sys 3 setup
adm 3 4 /var/adm /bin/bash setup
tty 5 setup
disk 6 setup
lp 4 7 /var/spool/lpd /sbin/nologin setup
mem 8 setup
kmem 9 setup
wheel 10 setup
cdrom 11 udev, MAKEDEV
sync 5 (0) /sbin /bin/sync setup
shutdown 6 (0) /sbin /sbin/shutdown setup
halt 7 (0) /sbin /sbin/halt setup
mail 8 12 /var/spool/mail /sbin/nologin setup
news 9 13 /var/spool/news /sbin/nologin setup
uucp 10 14 /var/spool/uucp /sbin/nologin setup
operator 11 (0) /root /sbin/nologin setup
games 12 (100) /usr/games /sbin/nologin setup
gopher 13 30 /usr/lib/gopher-data /sbin/nologin setup
ftp 14 50 /var/ftp /sbin/nologin setup
man 15 setup
oprofile 16 16 /home/oprofile /sbin/nologin oprofile
pkiuser 17 17 /usr/share/pki /sbin/nologin pki-ca, rhpki-ca
dialout 18 udev, MAKEDEV
floppy 19 dev, MAKEDEV
games 20 setup
slocate 21 slocate
utmp 22 initscripts, libutempter
squid 23 23 /var/spool/squid /dev/null squid
pvm 24 24 /usr/share/pvm3 /bin/bash pvm
named 25 25 /var/named /bin/false bind
postgres 26 26 /var/lib/pgsql /bin/bash postgresql-server
mysql 27 27 /var/lib/mysql /bin/bash mysql
nscd 28 28 / /bin/false nscd
rpcuser 29 29 /var/lib/nfs /bin/false nfs-utils
console 31 dev
rpc 32 32 / /bin/false portmap
amanda 33 (6) /var/lib/amanda /bin/false amanda
tape 33 udev, MAKEDEV
netdump 34 34 /var/crash /bin/bash netdump-client, netdump-server
utempter 35 libutempter
vdsm 36 / /bin/bash kvm, vdsm
kvm 36 kvm, vdsm, libvirt
rpm 37 37 /var/lib/rpm /bin/bash rpm
ntp 38 38 /etc/ntp /sbin/nologin ntp
video 39 setup
dip 40 setup
mailman 41 41 /var/mailman /bin/false mailman
gdm 42 42 /var/gdm /bin/bash gdm
xfs 43 43 /etc/X11/fs /bin/false XFree86-xfs
pppusers 44 linuxconf
popusers 45 linuxconf
slipusers 46 linuxconf
mailnull 47 47 /var/spool/mqueue /dev/null sendmail
apache 48 48 /var/www /bin/false apache
wnn 49 49 /home/wnn /bin/bash FreeWnn
smmsp 51 51 /var/spool/mqueue /dev/null sendmail
puppet 52 52 /var/lib/puppet /sbin/nologin puppet
tomcat 53 53 /var/lib/tomcat /sbin/nologin tomcat
lock 54 lockdev
ldap 55 55 /var/lib/ldap /bin/false openldap-servers
frontpage 56 56 /var/www /bin/false mod_frontpage
nut 57 57 /var/lib/ups /bin/false nut
beagleindex 58 58 /var/cache/beagle /bin/false beagle
tss 59 59 /sbin/nologin trousers
piranha 60 60 /etc/sysconfig/ha /dev/null piranha
prelude-manager 61 61 /sbin/nologin prelude-manager
snortd 62 62 /sbin/nologin snortd
audio 63 setup
condor 64 64 /var/lib/condor /sbin/nologin condord
nslcd 65 (55) / /sbin/nologin nslcd
wine 66 wine
pegasus 66 65 /var/lib/Pegasus /sbin/nologin tog-pegasus
webalizer 67 67 /var/www/html/usage /sbin/nologin webalizer
haldaemon 68 68 / /sbin/nologin hal
vcsa 69 69 /sbin/nologin dev, MAKEDEV
avahi 70 70 /var/run/avahi-daemon /sbin/nologin avahi
realtime 71
tcpdump 72 72 / /sbin/nologin tcpdump
privoxy 73 73 /etc/privoxy /bin/bash privoxy
sshd 74 74 /var/empty/sshd /sbin/nologin openssh-server
radvd 75 75 / /bin/false radvd
cyrus 76 (12) /var/imap /bin/bash cyrus-imapd
saslauth 76 cyrus-imapd
arpwatch 77 77 /var/lib/arpwatch /sbin/nologin arpwatch
fax 78 78 /var/spool/fax /sbin/nologin mgetty
nocpulse 79 79 /etc/sysconfig/nocpulse /bin/bash nocpulse
desktop 80 80 /sbin/nologin desktop-file-utils
dbus 81 81 / /sbin/nologin dbus
jonas 82 82 /var/lib/jonas /sbin/nologin jonas
clamav 83 83 /tmp /sbin/nologin clamav
screen 84 screen
quaggavt 85 quagga
sabayon 86 86 /sbin/nologin sabayon
polkituser 87 87 / /sbin/nologin PolicyKit
wbpriv 88 samba-common
postfix 89 89 /var/spool/postfix /bin/true postfix
postdrop 90 postfix
majordomo 91 91 /usr/lib/majordomo /bin/bash majordomo
quagga 92 92 / /sbin/nologin quagga
exim 93 93 /var/spool/exim /sbin/nologin exim
distcache 94 94 / /sbin/nologin distcache
radiusd 95 95 / /bin/false freeradius
hsqldb 96 96 /var/lib/hsqldb /sbin/nologin hsqldb
dovecot 97 97 /usr/libexec/dovecot /sbin/nologin dovecot
ident 98 98 / /sbin/nologin ident
nobody 99 99 / /sbin/nologin setup
users 100 setup
qemu 107 107 / /sbin/nologin libvirt
ovirt 108 108 / /sbin/nologin libvirt
saned 111 111 / /sbin/nologin sane-backends
vhostmd 112 112 /usr/share/vhostmd /sbin/nologin vhostmd
usbmuxd 113 113 / /sbin/nologin usbmuxd
bacula 133 133 /var/spool/bacula /sbin/nologin bacula
ricci 140 140 /var/lib/ricci /sbin/nologin ricci
luci 141 141 /var/lib/luci /sbin/nologin luci
stap-server 155 155 /var/lib/stap-server /sbin/nologin systemtap
avahi-autoipd 170 170 /var/lib/avahi-autoipd /sbin/nologin avahi
pulse 171 171 /var/run/pulse /sbin/nologin pulseaudio
rtkit 172 172 /proc /sbin/nologin rtkit
nfsnobody 65534[a] 65534 /var/lib/nfs /sbin/nologin nfs-utils
[a] nfsnobdy is 4294967294 on 64-bit platforms

3.5. Standard Groups

Table 3.5, “Standard Groups” lists the standard groups configured by an Everything installation. Groups are stored in the /etc/group file.
Table 3.5. Standard Groups
Group GID Members
root 0 root
bin 1 root, bin, daemon
daemon 2 root, bin, daemon
sys 3 root, bin, adm
adm 4 root, adm, daemon
tty 5
disk 6 root
lp 7 daemon, lp
mem 8
kmem 9
wheel 10 root
mail 12 mail, postfix
uucp 14 uucp
man 15
games 20
gopher 30
video 39
dip 40
ftp 50
lock 54
audio 63
nobody 99
users 100
dbus 81
usbmuxd 113
utmp 22
utempter 35
avahi-autoipd 170
floppy 19
vcsa 69
rpc 32
rtkit 499
abrt 498
nscd 28
desktop_admin_r 497
desktop_user_r 496
cdrom 11
tape 33
dialout 18
haldaemon 68 haldaemon
apache 48
ldap 55
saslauth 495
postdrop 90
postfix 89
avahi 70
ntp 38
rpcuser 29
nfsnobody 65534
pulse 494
pulse-access 493
fuse 492
gdm 42
stapdev 491
stapusr 490
sshd 74
tcpdump 72
slocate 21
dovecot 97
dovenull 489
mailnull 47
smmsp 51

3.6. User Private Groups

Fedora uses a user private group (UPG) scheme, which makes UNIX groups easier to manage. A UPG is created whenever a new user is added to the system. It has the same name as the user for which it was created and that user is the only member of the UPG.
UPGs make it safe to set default permissions for a newly created file or directory, allowing both the user and the group of that user to make modifications to the file or directory.
The setting which determines what permissions are applied to a newly created file or directory is called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX systems, the umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator's group, are not allowed to make any modifications. However, under the UPG scheme, this "group protection" is not necessary since every user has their own private group.

3.6.1. Group Directories

System administrators usually like to create a group for each major project and assign people to the group when they need to access that project's files. With this traditional scheme, file managing is difficult; when someone creates a file, it is associated with the primary group to which they belong. When a single person works on multiple projects, it becomes difficult to associate the right files with the right group. However, with the UPG scheme, groups are automatically assigned to files created within a directory with the setgid bit set. The setgid bit makes managing group projects that share a common directory very simple because any files a user creates within the directory are owned by the group which owns the directory.
For example, a group of people need to work on files in the /opt/myproject/ directory. Some people are trusted to modify the contents of this directory, but not everyone.
  1. As root, create the /opt/myproject/ directory by typing the following at a shell prompt:
    mkdir /opt/myproject
  2. Add the myproject group to the system:
    groupadd myproject
  3. Associate the contents of the /opt/myproject/ directory with the myproject group:
    chown root:myproject /opt/myproject
  4. Allow users to create files within the directory, and set the setgid bit:
    chmod 2775 /opt/myproject
At this point, all members of the myproject group can create and edit files in the /opt/myproject/ directory without the administrator having to change file permissions every time users write new files. To verify that the permissions have been set correctly, run the following command:
~]# ls -l /opt
total 4
drwxrwsr-x. 3 root myproject 4096 Mar  3 18:31 myproject

3.7. Shadow Passwords

In multiuser environments it is very important to use shadow passwords (provided by the shadow-utils package). Doing so enhances the security of system authentication files. For this reason, the installation program enables shadow passwords by default.
The following list shows the advantages shadow passwords have over the traditional way of storing passwords on UNIX-based systems:
  • Improves system security by moving encrypted password hashes from the world-readable /etc/passwd file to /etc/shadow, which is readable only by the root user.
  • Stores information about password aging.
  • Allows the /etc/login.defs file to enforce security policies.
Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /etc/shadow file, any commands which create or modify password aging information do not work.
The following is a list of commands which do not work without first enabling shadow passwords:
  • chage
  • gpasswd
  • usermod -e or -f options
  • useradd -e or -f options

3.8. Additional Resources

For more information about users and groups, and tools to manage them, refer to the following resources.

3.8.1. Installed Documentation

  • Related man pages — There are a number of man pages for the various applications and configuration files involved with managing users and groups. Some of the more important man pages have been listed here:
    User and Group Administrative Applications
    • man chage — A command to modify password aging policies and account expiration.
    • man gpasswd — A command to administer the /etc/group file.
    • man groupadd — A command to add groups.
    • man grpck — A command to verify the /etc/group file.
    • man groupdel — A command to remove groups.
    • man groupmod — A command to modify group membership.
    • man pwck — A command to verify the /etc/passwd and /etc/shadow files.
    • man pwconv — A tool to convert standard passwords to shadow passwords.
    • man pwunconv — A tool to convert shadow passwords to standard passwords.
    • man useradd — A command to add users.
    • man userdel — A command to remove users.
    • man usermod — A command to modify users.
    Configuration Files
    • man 5 group — The file containing group information for the system.
    • man 5 passwd — The file containing user information for the system.
    • man 5 shadow — The file containing passwords and account expiration information for the system.

Part II. Package Management

Chapter 4. Yum

Yum is the The Fedora Project package manager that is able to query for information about packages, fetch packages from repositories, install and uninstall packages using automatic dependency resolution, and update an entire system to the latest available packages. Yum performs automatic dependency resolution on packages you are updating, installing or removing, and thus is able to automatically determine, fetch and install all available dependent packages. Yum can be configured with new, additional repositories, or package sources, and also provides many plugins which enhance and extend its capabilities. Yum is able to perform many of the same tasks that RPM can; additionally, many of the command line options are similar. Yum enables easy and simple package management on a single machine or on groups of them.

Secure package management with GPG-signed packages

Yum provides secure package management by enabling GPG (Gnu Privacy Guard; also known as GnuPG) signature verification on GPG-signed packages to be turned on for all package repositories (i.e. package sources), or for individual repositories. When signature verification is enabled, Yum will refuse to install any packages not GPG-signed with the correct key for that repository. This means that you can trust that the RPM packages you download and install on your system are from a trusted source, such as The Fedora Project, and were not modified during transfer. Refer to Section 4.3, “Configuring Yum and Yum Repositories” for details on enabling signature-checking with Yum, or Section A.3, “Checking a Package's Signature” for information on working with and verifying GPG-signed RPM packages in general.
Yum also enables you to easily set up your own repositories of RPM packages for download and installation on other machines.
Learning Yum is a worthwhile investment because it is often the fastest way to perform system administration tasks, and it provides capabilities beyond those provided by the PackageKit graphical package management tools. Refer to Chapter 5, PackageKit for details on using PackageKit.

Yum and superuser privileges

You must have superuser privileges in order to use yum to install, update or remove packages on your system. All examples in this chapter assume that you have already obtained superuser privileges by using either the su or sudo command.

4.1. Checking For and Updating Packages

4.1.1. Checking For Updates

You can use the yum check-update command to see which installed packages on your system have updates available:
~]# yum check-update
Loaded plugins: langpacks, presto, refresh-packagekit

PackageKit.x86_64                    0.6.14-2.fc15                 fedora
PackageKit-command-not-found.x86_64  0.6.14-2.fc15                 fedora
PackageKit-device-rebind.x86_64      0.6.14-2.fc15                 fedora
PackageKit-glib.x86_64               0.6.14-2.fc15                 fedora
PackageKit-gstreamer-plugin.x86_64   0.6.14-2.fc15                 fedora
PackageKit-gtk-module.x86_64         0.6.14-2.fc15                 fedora
PackageKit-gtk3-module.x86_64        0.6.14-2.fc15                 fedora
PackageKit-yum.x86_64                0.6.14-2.fc15                 fedora
PackageKit-yum-plugin.x86_64         0.6.14-2.fc15                 fedora
gdb.x86_64                           7.2.90.20110429-36.fc15       fedora
kernel.x86_64                        2.6.38.6-26.fc15              fedora
rpm.x86_64                           4.9.0-6.fc15                  fedora
rpm-libs.x86_64                      4.9.0-6.fc15                  fedora
rpm-python.x86_64                    4.9.0-6.fc15                  fedora
yum.noarch                           3.2.29-5.fc15                 fedora
The packages in the above output are listed as having updates available. The first package in the list is PackageKit, the graphical package manager. The line in the example output tells us:
  • PackageKit — the name of the package
  • x86_64 — the CPU architecture the package was built for
  • 0.6.14 — the version of the updated package to be installed
  • fedora — the repository in which the updated package is located
The output also shows us that we can update the kernel (the kernel package), Yum and RPM themselves (the yum and rpm packages), as well as their dependencies (such as the rpm-libs and rpm-python packages), all using yum.

4.1.2. Updating Packages

You can choose to update a single package, multiple packages, or all packages at once. If any dependencies of the package (or packages) you update have updates available themselves, then they are updated too.

Updating a Single Package

To update a single package, enter yum update package_name, for example:
~]# yum update gdb
Loaded plugins: langpacks, presto, refresh-packagekit
Setting up Update Process
Resolving Dependencies
--> Running transaction check
---> Package gdb.x86_64 0:7.2.90.20110411-34.fc15 will be updated
---> Package gdb.x86_64 0:7.2.90.20110429-36.fc15 will be an update
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package     Arch         Version                          Repository      Size
================================================================================
Updating:
 gdb         x86_64       7.2.90.20110429-36.fc15          fedora         1.9 M

Transaction Summary
================================================================================
Upgrade       1 Package(s)

Total download size: 1.9 M
Is this ok [y/N]:
This output contains several items of interest:
  1. Loaded plugins:yum always informs you which Yum plugins are installed and enabled. Here, yum is using the langpacks, presto, and refresh-packagekit plugins. Refer to Section 4.4, “Yum Plugins” for general information on Yum plugins, or to Section 4.4.3, “Plugin Descriptions” for descriptions of specific plugins.
  2. gdb.x86_64 — you can download and install new gdb package.
  3. yum presents the update information and then prompts you as to whether you want it to perform the update; yum runs interactively by default. If you already know which transactions yum plans to perform, you can use the -y option to automatically answer yes to any questions yum may ask (in which case it runs non-interactively). However, you should always examine which changes yum plans to make to the system so that you can easily troubleshoot any problems that might arise.
If a transaction does go awry, you can view Yum's log of transactions by typing the cat /var/log/yum.log command at a shell prompt. The most recent transactions are listed at the end of the log file.

Updating and installing kernels with Yum

yum always installs a new kernel in the same sense that RPM installs a new kernel when you use the command rpm -i kernel. Therefore, you do not need to worry about the distinction between installing and upgrading a kernel package when you use yum: it will do the right thing, regardless of whether you are using the yum update or yum install command.
When using RPM, on the other hand, it is important to use the rpm -i kernel command (which installs a new kernel) instead of rpm -u kernel (which replaces the current kernel). Refer to Section A.2.2, “Installing and Upgrading” for more information on installing/updating kernels with RPM.

Updating All Packages and Their Dependencies

To update all packages and their dependencies, simply enter the yum update command (without any arguments):
~]# yum update

Updating Security-Related Packages

Discovering which packages have security updates available and then updating those packages quickly and easily is important. Yum provides the plugin for this purpose. The security plugin extends the yum command with a set of highly-useful security-centric commands, subcommands and options. Refer to Section 4.4.3, “Plugin Descriptions” for specific information.

4.1.3. Preserving Configuration File Changes

You will inevitably make changes to the configuration files installed by packages as you use your Fedora system. RPM, which Yum uses to perform changes to the system, provides a mechanism for ensuring their integrity. Refer to Section A.2.2, “Installing and Upgrading” for details on how to manage changes to configuration files across package upgrades.

4.2. Packages and Package Groups

4.2.1. Searching Packages

You can search all RPM package names, descriptions and summaries by using the yum search term [more_terms] command. yum displays the list of matches for each term, for example:
~]# yum search meld kompare
Loaded plugins: langpacks, presto, refresh-packagekit
============================== N/S Matched: meld ===============================
meld.noarch : Visual diff and merge tool
python-meld3.x86_64 : HTML/XML templating system for Python

============================= N/S Matched: kompare =============================
komparator.x86_64 : Kompare and merge two folders

  Name and summary matches only, use "search all" for everything.
The yum search command is useful for searching for packages you do not know the name of, but for which you know a related term.

4.2.2. Listing Packages

yum list and related commands provide information about packages, package groups, and repositories.
All of Yum's list commands allow you to filter the results by appending one or more glob expressions as arguments. Glob expressions are normal strings of characters which contain one or more of the wildcard characters * (which expands to match any character multiple times) and ? (which expands to match any one character).

Filtering results with glob expressions

Be careful to escape the glob expressions when passing them as arguments to a yum command, otherwise the Bash shell will interpret these expressions as pathname expansions, and potentially pass all files in the current directory that match the globs to yum. To make sure the glob expressions are passed to yum as intended, either:
  • escape the wildcard characters by preceding them with a backslash character
  • double-quote or single-quote the entire glob expression.
yum list glob_expr [more_glob_exprs]
Lists information on installed and available packages matching all glob expressions.
Example 4.1. Listing all ABRT addons and plugins using glob expressions
~]# yum list abrt-addon\* abrt-plugin\*
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
abrt-addon-ccpp.x86_64               2.0.2-5.fc15     @fedora
abrt-addon-kerneloops.x86_64         2.0.2-5.fc15     @fedora
abrt-addon-python.x86_64             2.0.2-5.fc15     @fedora
abrt-plugin-bugzilla.x86_64          2.0.2-5.fc15     @fedora
abrt-plugin-logger.x86_64            2.0.2-5.fc15     @fedora
Available Packages
abrt-plugin-mailx.x86_64             2.0.2-5.fc15     updates
abrt-plugin-reportuploader.x86_64    2.0.2-5.fc15     updates
abrt-plugin-rhtsupport.x86_64        2.0.2-5.fc15     updates

yum list all
Lists all installed and available packages.
Example 4.2. Listing all installed and available packages
~]# yum list all
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
ConsoleKit.x86_64                       0.4.4-1.fc15                  @fedora
ConsoleKit-libs.x86_64                  0.4.4-1.fc15                  @fedora
ConsoleKit-x11.x86_64                   0.4.4-1.fc15                  @fedora
GConf2.x86_64                           2.32.3-1.fc15                 @fedora
GConf2-gtk.x86_64                       2.32.3-1.fc15                 @fedora
ModemManager.x86_64                     0.4-7.git20110201.fc15        @fedora
NetworkManager.x86_64                   1:0.8.998-4.git20110427.fc15  @fedora
NetworkManager-glib.x86_64              1:0.8.998-4.git20110427.fc15  @fedora
NetworkManager-gnome.x86_64             1:0.8.998-4.git20110427.fc15  @fedora
NetworkManager-openconnect.x86_64       0.8.1-9.git20110419.fc15      @fedora
[output truncated]

yum list installed
Lists all packages installed on your system. The rightmost column in the output lists the repository from which the package was retrieved.
Example 4.3. Listing installed packages using a double-quoted glob expression
~]# yum list installed "krb?-*"
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
krb5-libs.x86_64              1.9-7.fc15              @fedora

yum list available
Lists all available packages in all enabled repositories.
Example 4.4. Listing available packages using a single glob expression with escaped wildcard characters
~]# yum list available gstreamer\*plugin\*
Loaded plugins: langpacks, presto, refresh-packagekit
Available Packages
gstreamer-plugin-crystalhd.x86_64               3.5.1-1.fc14       fedora
gstreamer-plugins-bad-free.x86_64               0.10.22-1.fc15     updates
gstreamer-plugins-bad-free-devel.x86_64         0.10.22-1.fc15     updates
gstreamer-plugins-bad-free-devel-docs.x86_64    0.10.22-1.fc15     updates
gstreamer-plugins-bad-free-extras.x86_64        0.10.22-1.fc15     updates
gstreamer-plugins-base.x86_64                   0.10.33-1.fc15     updates
gstreamer-plugins-base-devel.x86_64             0.10.33-1.fc15     updates
gstreamer-plugins-base-devel-docs.noarch        0.10.33-1.fc15     updates
gstreamer-plugins-base-tools.x86_64             0.10.33-1.fc15     updates
gstreamer-plugins-espeak.x86_64                 0.3.3-3.fc15       fedora
gstreamer-plugins-fc.x86_64                     0.2-2.fc15         fedora
gstreamer-plugins-good.x86_64                   0.10.29-1.fc15     updates
gstreamer-plugins-good-devel-docs.noarch        0.10.29-1.fc15     updates

yum grouplist
Lists all package groups.
Example 4.5. Listing all package groups
~]# yum grouplist
Loaded plugins: langpacks, presto, refresh-packagekit
Setting up Group Process
Installed Groups:
   Administration Tools
   Design Suite
   Dial-up Networking Support
   Fonts
   GNOME Desktop Environment
[output truncated]

yum repolist
Lists the repository ID, name, and number of packages it provides for each enabled repository.
Example 4.6. Listing enabled repositories
~]# yum repolist
Loaded plugins: langpacks, presto, refresh-packagekit
repo id                      repo name                                    status
fedora                       Fedora 15 - i386                             19,365
updates                      Fedora 15 - i386 - Updates                   3,848
repolist: 23,213

4.2.3. Displaying Package Information

The yum info package_name [more_names] command displays information about one or more packages (glob expressions are valid here as well). For example, to display information about the abrt package, type:
~]# yum info abrt
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
Name        : abrt
Arch        : x86_64
Version     : 2.0.1
Release     : 2.fc15
Size        : 806 k
Repo        : installed
From repo   : fedora
Summary     : Automatic bug detection and reporting tool
URL         : https://fedorahosted.org/abrt/
License     : GPLv2+
Description : abrt is a tool to help users to detect defects in applications and
            : to create a bug report with all informations needed by maintainer
            : to fix it. It uses plugin system to extend its functionality.
yum info package_name is similar to the rpm -q --info package_name command, but provides as additional information the ID of the Yum repository the RPM package is found in (look for the From repo line in the output).
The yumdb info package_name [more_names] command can be used to query the Yum database for alternative and useful information about a package, including the checksum of the package (and algorithm used to produce it, such as SHA-256), the command given on the command line that was invoked to install the package (if any), and the reason that the package is installed on the system (where user indicates it was installed by the user, and dep means it was brought in as a dependency). For example, to display additional information about the yum package, type:
~]# yumdb info yum
Loaded plugins: langpacks, presto, refresh-packagekit
yum-3.2.29-4.fc15.noarch
     checksum_data = 249f21fb43c41381c8c9b0cd98d2ea5fa0aa165e81ed2009cfda74c05af67246
     checksum_type = sha256
     from_repo = fedora
     from_repo_revision = 1304429533
     from_repo_timestamp = 1304442346
     installed_by = 0
     reason = user
     releasever = $releasever
For more information on the yumdb command, see man yumdb.
Finally, the yum history command can be used to show a timeline of Yum transactions, the dates and times on when they occurred, the number of packages affected, whether transactions succeeded or were aborted, and if the RPM database was changed between transactions. Refer to the history section of man yum for details.

4.2.4. Installing Packages

You can install a package and all of its non-installed dependencies by entering:
yum install package_name
You can install multiple packages simultaneously by appending their names as arguments:
yum install package_name [more_names]
If you are installing packages on a multilib system, such as an AMD64 or Intel 64 machine, you can specify the architecture of the package (as long as it is available in an enabled repository) by appending .arch to the package name. For example, to install the sqlite2 package for i686, type:
~]# yum install sqlite2.i686
You can use glob expressions to quickly install multiple similarly-named packages:
~]# yum install audacious-plugins-\*
In addition to package names and glob expressions, you can also provide file names to yum install. If you know the name of the binary you want to install, but not its package name, you can give yum install the path name:
~]# yum install /usr/sbin/named
yum then searches through its package lists, finds the package which provides /usr/sbin/named, if any, and prompts you as to whether you want to install it.

Finding which package owns a file

If you know you want to install the package that contains the named binary, but you do not know in which bin or sbin directory is the file installed, use the yum provides command with a glob expression:
~]# yum provides "*bin/named"
Loaded plugins: langpacks, presto, refresh-packagekit
32:bind-9.8.0-3.P1.fc15.i686 : The Berkeley Internet Name Domain (BIND) DNS
                             : (Domain Name System) server
Repo        : fedora
Matched from:
Filename    : /usr/sbin/named
yum provides "*/file_name" is a common and useful trick to find the packages that contain file_name.

Installing a Package Group

A package group is similar to a package: it is not useful by itself, but installing one pulls a group of dependent packages that serve a common purpose. A package group has a name and a groupid. The yum grouplist -v command lists the names of all package groups, and, next to each of them, their groupid in parentheses. The groupid is always the term in the last pair of parentheses, such as kde-desktop in the following example:
~]# yum -v grouplist kde\*
Not loading "blacklist" plugin, as it is disabled
Loading "langpacks" plugin
Loading "presto" plugin
Loading "refresh-packagekit" plugin
Not loading "whiteout" plugin, as it is disabled
Adding en_US to language list
Config time: 0.900
Yum Version: 3.2.29
Setting up Group Process
rpmdb time: 0.002
group time: 0.995
Available Groups:
   KDE Software Compilation (kde-desktop)
   KDE Software Development (kde-software-development)
Done
You can install a package group by passing its full group name (without the groupid part) to groupinstall, for example:
~]# yum groupinstall "KDE Software Compilation"
You can also install by groupid:
~]# yum groupinstall kde-desktop
You can even pass the groupid (or quoted name) to the install command if you prepend it with an @-symbol (which tells yum that you want to perform a groupinstall):
~]# yum install @kde-desktop

4.2.5. Removing Packages

The yum remove package_name uninstalls (removes in RPM and Yum terminology) the package, as well as any packages that depend on it. As when you install multiple packages, you can remove several at once by adding more package names to the command. For example, to remove totem, rhythmbox, and sound-juicer, type the following at a shell prompt:
~]# yum remove totem rhythmbox sound-juicer
Similar to install, remove can take these arguments:
  • package names
  • glob expressions
  • file lists
  • package provides

Removing a package when other packages depend on it

Yum is not able to remove a package without also removing packages which depend on it. This type of operation can only be performed by RPM, is not advised, and can potentially leave your system in a non-functioning state or cause applications to misbehave and/or crash. For further information, refer to Section A.2.4, “Uninstalling” in the RPM chapter.

Removing a Package Group

You can remove a package group using syntax congruent with the install syntax. The following are alternative but equivalent ways of removing the KDE Software Compilation group:
~]# yum groupremove "KDE Software Compilation"
~]# yum groupremove kde-desktop
~]# yum remove @kde-desktop

Intelligent package group removal

When you tell yum to remove a package group, it will remove every package in that group, even if those packages are members of other package groups or dependencies of other installed packages. However, you can instruct yum to remove only those packages which are not required by any other packages or groups by adding the groupremove_leaf_only=1 directive to the [main] section of the /etc/yum.conf configuration file. For more information on this directive, refer to Section 4.3.1, “Setting [main] Options”.

4.3. Configuring Yum and Yum Repositories

This section shows you how to:
  • set global Yum options by editing the [main] section of the /etc/yum.conf configuration file;
  • set options for individual repositories by editing the [repository] sections in /etc/yum.conf and .repo files in the /etc/yum.repos.d/ directory;
  • use Yum variables in /etc/yum.conf and files in /etc/yum.repos.d/ so that dynamic version and architecture values are handled correctly; and,
  • set up your own custom Yum repository.
The /etc/yum.conf configuration file contains one mandatory [main] section under which you can set Yum options. The values that you define in the [main] section of yum.conf have global effect, and may override values set in individual [repository] sections. You can also add [repository] sections to /etc/yum.conf; however, best practice is to define individual repositories in new or existing .repo files in the /etc/yum.repos.d/directory. Refer to Section 4.3.2, “Setting [repository] Options” if you need to add or edit repository-specific information.

4.3.1. Setting [main] Options

The /etc/yum.conf configuration file contains exactly one [main] section. You can add many additional options under the [main] section heading in /etc/yum.conf. Some of the key-value pairs in the [main] section affect how yum operates; others affect how Yum treats repositories. The best source of information for all Yum options is in the [main] OPTIONS and [repository] OPTIONS sections of man yum.conf.
A sample /etc/yum.conf configuration file can look like this:
[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=3
[comments abridged]
# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d
The following is a list of the most commonly-used options in the [main] section, and descriptions for each:
assumeyes=value
...where value is one of:
0yum should prompt for confirmation of critical actions it performs. This is the default.
1 — Do not prompt for confirmation of critical yum actions. If assumeyes=1 is set, yum behaves in the same way that the command line option -y does.
cachedir=/var/cache/yum/$basearch/$releasever
This option specifies the directory where Yum should store its cache and database files. By default, Yum's cache directory is /var/cache/yum/$basearch/$releasever. See Section 4.3.3, “Using Yum Variables” for descriptions of the $basearch and $releasever Yum variables.
debuglevel=value
...where value is an integer between 1 and 10. Setting a higher debuglevel value causes yum to display more detailed debugging output. debuglevel=0 disables debugging output, while debuglevel=2 is the default.
exactarch=value
...where value is one of:
0 — Do not take into account the exact architecture when updating packages.
1 — Consider the exact architecture when updating packages. With this setting, yum will not install an i686 package to update an i386 package already installed on the system. This is the default.
exclude=package_name [more_package_names]
This option allows you to exclude packages by keyword during installation/updates. Listing multiple packages for exclusion can be accomplished by quoting a space-delimited list of packages. Shell globs using wildcards (for example, * and ?) are allowed.
gpgcheck=value
...where value is one of:
0 — Disable GPG signature-checking on packages in all repositories, including local package installation.
1 — Enable GPG signature-checking on all packages in all repositories, including local package installation. gpgcheck=1 is the default, and thus all packages' signatures are checked.
If this option is set in the [main] section of the /etc/yum.conf file, it sets the GPG-checking rule for all repositories. However, you can also set gpgcheck=value for individual repositories instead; i.e., you can enable GPG-checking on one repository while disabling it on another. Setting gpgcheck=value for an individual repository in its corresponding .repo file overrides the default if it is present in /etc/yum.conf. Refer to Section A.3, “Checking a Package's Signature” for further information on GPG signature-checking.
groupremove_leaf_only=value
...where value is one of:
0yum should not check the dependencies of each package when removing a package group. With this setting, yum removes all packages in a package group, regardless of whether those packages are required by other packages or groups. groupremove_leaf_only=0 is the default.
1yum should check the dependencies of each package when removing a package group, and remove only those packages which are not not required by any other package or group.
For more information on removing packages, refer to Intelligent package group removal.
installonlypkgs=space separated list of packages
Here you can provide a space-separated list of packages which yum can install, but will never update. Refer to man yum.conf for the list of packages which are install-only by default. If you add the installonlypkgs directive to /etc/yum.conf, you should ensure that you list all of the packages that should be install-only, including any of those listed under the installonlypkgs section of man yum.conf. In particular, kernel packages should always be listed in installonlypkgs (as they are by default), and installonly_limit should always be set to a value greater than 2 so that a backup kernel is always available in case the default one fails to boot. Refer to installonly_limit=value for details on the installonly_limit directive.
installonly_limit=value
...where value is an integer representing the maximum number of versions that can be installed simultaneously for any single package listed in the installonlypkgs directive. The defaults for the installonlypkgs directive include several different kernel packages, so be aware that changing the value of installonly_limit will also affect the maximum number of installed versions of any single kernel package. The default value listed in /etc/yum.conf is installonly_limit=3, and it is not recommended to decrease this value, particularly below 2.
keepcache=value
...where value is one of:
0 — Do not retain the cache of headers and packages after a successful installation. This is the default.
1 — Retain the cache after a successful installation.
logfile=/var/log/yum.log
This option specifies where yum should send its logging output. By default, yum logs to /var/log/yum.log.
multilib_policy=value
...where value is one of:
best — install the best-choice architecture for this system. For example, setting multilib_policy=best on an AMD64 system causes yum to install 64-bit versions of all packages.
all — always install every possible architecture for every package. For example, with multilib_policy set to all on an AMD64 system, yum would install both the i686 and AMD64 versions of a package, if both were available.
obsoletes=value
...where value is one of:
0 — Disable yum's obsoletes processing logic when performing updates.
1 — Enable yum's obsoletes processing logic when performing updates. When one package declares in its spec file that it obsoletes another package, the latter package will be replaced by the former package when the former package is installed. Obsoletes are declared, for example, when a package is renamed. obsoletes=1 the default.
plugins=value
...where value is one of:
0 — Disable all Yum plugins globally.

Disabling all plugins is not advised

Disabling all plugins is not advised, because certain plugins provide important Yum services. Disabling plugins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem with Yum.
1 — Enable all Yum plugins globally. With plugins=1, you can still disable a specific Yum plugin by setting enabled=0 in that plugin's configuration file. Refer to Section 4.4, “Yum Plugins” for more information about various Yum plugins, or to Section 4.4.1, “Enabling, Configuring and Disabling Yum Plugins” for further information on controlling plugins.
reposdir=/absolute/path/to/directory/containing/repo/files
This option allows you to specify a directory where .repo files are located. All .repo files contain repository information (similar to the [repository] sections of /etc/yum.conf). yum collects all repository information from .repo files and the [repository] section of the /etc/yum.conf file to create a master list of repositories to use for transactions. Refer to Section 4.3.2, “Setting [repository] Options” for more information about options you can use for both the [repository] section and .repo files. If reposdir is not set, yum uses the default directory /etc/yum.repos.d/.
retries=value
...where value is an integer 0 or greater. This value sets the number of times yum should attempt to retrieve a file before returning an error. Setting this to 0 makes yum retry forever. The default value is 10.

4.3.2. Setting [repository] Options

The [repository] sections (where repository is a unique repository ID, such as my_personal_repo) allow you to define individual Yum repositories. To define a new repository, either add this section to the /etc/yum.conf file, or to a .repo file in the /etc/yum.repos.d/ directory.
All .repo files in /etc/yum.repos.d/are read by yum, which allows you to create new, custom .repo files in this directory. Best practice is to define your repositories here instead of in /etc/yum.conf.
The following is a (bare-minimum) example of the form a .repo file takes:
[repository_ID]
name=A Repository Name
baseurl=http://path/to/repo or ftp://path/to/repo or file:///path/to/local/repo
Every [repository] section must contain the following minimum directives:
[repository_ID]
The repository ID is a unique, one-word (no spaces; underscores are allowed) string of characters (enclosed by brackets) that serves as a repository identifier.
name=A Repository Name
This is a human-readable string describing the repository.
baseurl=http://path/to/repo, ftp://path/to/repo, file:///path/to/local/repo
This is a URL to the directory where the repodata directory of a repository is located. Usually this URL is an HTTP link, such as:
baseurl=http://path/to/repo/releases/$releasever/server/$basearch/os/
Note that Yum always expands the $releasever, $arch and $basearch variables in URLs. See Section 4.3.3, “Using Yum Variables” for explanations of all Yum variables.
  • If the repository is available over FTP, use: ftp://path/to/repo
  • If the repository is local to the machine, use file:///path/to/local/repo
  • If a specific online repository requires basic HTTP authentication, you can specify your username and password in the http://path/to/repo by prepending it as username:password@link. For example, if a repository on http://www.example.com/repo/ requires a username of user and a password of password, then the baseurl link could be specified as:
    baseurl=http://user:password@www.example.com/repo/
The following is another useful [repository] directive:
enabled=value
...where value is one of:
0 — do not include this repository as a package source when performing updates and installs. This is an easy way of quickly turning repositories on and off, which is useful when you desire a single package from a repository that you do not want to enable for updates or installs.
1 — include this repository as a package source.
Turning repositories on and off can also be performed by passing either the --enablerepo=repo_name or --disablerepo=repo_name option to yum, or through the Add/Remove Software window of the PackageKit utility.
Many more [repository] options exist. Refer to the [repository] OPTIONS section of man yum.conf for the exhaustive list and descriptions for each.

4.3.3. Using Yum Variables

You can use and reference the following built-in variables in yum commands and in all Yum configuration files (that is, /etc/yum.conf and all .repo files in the /etc/yum.repos.d/ directory):
$releasever
You can use this variable to reference the release version of Fedora. Yum obtains the value of $releasever from the distroverpkg=value line in the /etc/yum.conf configuration file. If there is no such line in /etc/yum.conf, then yum infers the correct value by deriving the version number from the redhat-release package.
$arch
You can use this variable to refer to the system's CPU architecture as returned when calling Python's os.uname() function. Valid values for $arch include: i586, i686, and x86_64.
$basearch
You can use $basearch to reference the base architecture of the system. For example, i686 and i586 machines both have a base architecture of i386, and AMD64 and Intel 64 machines have a base architecture of x86_64.
$uuid
You can use this variable to refer to a universally unique identifier (UUID) for the machine.
$YUM0-9
These ten variables are each replaced with the value of any shell environment variables with the same name. If one of these variables is referenced (in /etc/yum.conf for example) and a shell environment variable with the same name does not exist, then the configuration file variable is not replaced.
To define a custom variable or to override the value of an existing one, create a file with the same name as the variable (without the $ sign) in the /etc/yum/vars/ directory, and add the desired value on its first line.
Example 4.7. Using a custom Yum variable
Repository descriptions often include the operating system name. To define a new variable called $osname, create a new file with Fedora on the first line and save it as /etc/yum/vars/osname. For example:
~]# echo "Fedora" > /etc/yum/vars/osname
Instead of Fedora 15, you can now use the following in the .repo files:
name=$osname $releasever

4.3.4. Creating a Yum Repository

To set up a Yum repository, follow these steps:
  1. Install the createrepo package:
    ~]# yum install createrepo
  2. Copy all of the packages into one directory, such as /mnt/local_repo/.
  3. Run the createrepo --database command on that directory:
    ~]# createrepo --database /mnt/local_repo/
This will create the necessary metadata for your Yum repository, as well as the sqlite database for speeding up yum operations.

4.4. Yum Plugins

Yum provides plugins that extend and enhance its operations. Certain plugins are installed by default. Yum always informs you which plugins, if any, are loaded and active whenever you call any yum command:
~]# yum info yum
Loaded plugins: langpacks, presto, refresh-packagekit
[output truncated]
Note that the plugin names which follow Loaded plugins are the names you can provide to the --disableplugins=plugin_name option.

4.4.1. Enabling, Configuring and Disabling Yum Plugins

To enable Yum plugins, ensure that a line beginning with plugins= is present in the [main] section of /etc/yum.conf, and that its value is set to 1:
plugins=1
You can disable all plugins by changing this line to plugins=0.

Disabling all plugins is not advised

Disabling all plugins is not advised because certain plugins provide important Yum services. Disabling plugins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem with Yum.
Every installed plugin has its own configuration file in the /etc/yum/pluginconf.d/ directory. You can set plugin-specific options in these files. For example, the following is the content of refresh-packagekit.conf, the configuration file for the refresh-packagekit plugin:
[main]
enabled=1
Plugin configuration files always contain a [main] section (similar to Yum's /etc/yum.conf file) in which there is (or you can place if it is missing) an enabled= option that controls whether the plugin is enabled when you run yum commands.
If you disable all plugins by setting enabled=0 in /etc/yum.conf, then all plugins are disabled regardless of whether they are enabled in their individual configuration files.
If you merely want to disable all Yum plugins for a single yum command, use the --noplugins option.
If you want to disable one or more Yum plugins for a single yum command, add the --disableplugin=plugin_name option to the command. For example, to disable the presto plugin while updating a system, type:
~]# yum update --disableplugin=presto
The plugin names you provide to the --disableplugin= option are the same names listed after the Loaded plugins line in the output of any yum command. You can disable multiple plugins by separating their names with commas. In addition, you can match multiple plugin names or shorten long ones by using glob expressions:
~]# yum update --disableplugin=presto,refresh-pack*

4.4.2. Installing Additional Yum Plugins

Yum plugins usually adhere to the yum-plugin-plugin_name package-naming convention, but not always: the package which provides the presto plugin is named yum-presto, for example. You can install a Yum plugin in the same way you install other packages. For instance, to install the security plugin, type the following at a shell prompt:
~]# yum install yum-plugin-security

4.4.3. Plugin Descriptions

The following are descriptions of a few commonly installed Yum plugins:
presto (yum-presto)
The presto plugin adds support to Yum for downloading delta RPM packages, during updates, from repositories which have presto metadata enabled. Delta RPMs contain only the differences between the version of the package installed on the client requesting the RPM package and the updated version in the repository.
Downloading a delta RPM is much quicker than downloading the entire updated package, and can speed up updates considerably. Once the delta RPMs are downloaded, they must be rebuilt to apply the difference to the currently-installed package and thus create the full, updated package. This process takes CPU time on the installing machine. Using delta RPMs is therefore a tradeoff between time-to-download, which depends on the network connection, and time-to-rebuild, which is CPU-bound. Using the presto plugin is recommended for fast machines and systems with slower network connections, while slower machines on very fast connections may benefit more from downloading normal RPM packages, that is, by disabling presto.
refresh-packagekit (PackageKit-yum-plugin)
The refresh-packagekit plugin updates metadata for PackageKit whenever yum is run. The refresh-packagekit plugin is installed by default.
rhnplugin (yum-rhn-plugin)
The rhnplugin provides support for connecting to RHN Classic. This allows systems registered with RHN Classic to update and install packages from this system.
Refer to man rhnplugin for more information about the plugin.
security (yum-plugin-security)
Discovering information about and applying security updates easily and often is important to all system administrators. For this reason Yum provides the security plugin, which extends yum with a set of highly-useful security-related commands, subcommands and options.
You can check for security-related updates as follows:
~]# yum check-update --security
Loaded plugins: langpacks, presto, refresh-packagekit, security
Limiting package lists to security relevant ones
updates-testing/updateinfo                               | 329 kB     00:00
9 package(s) needed for security, out of 270 available

ConsoleKit.x86_64                    0.4.5-1.fc15                  updates
ConsoleKit-libs.x86_64               0.4.5-1.fc15                  updates
ConsoleKit-x11.x86_64                0.4.5-1.fc15                  updates
NetworkManager.x86_64                1:0.8.999-2.git20110509.fc15  updates
NetworkManager-glib.x86_64           1:0.8.999-2.git20110509.fc15  updates
[output truncated]
You can then use either yum update --security or yum update-minimal --security to update those packages which are affected by security advisories. Both of these commands update all packages on the system for which a security advisory has been issued. yum update-minimal --security updates them to the latest packages which were released as part of a security advisory, while yum update --security will update all packages affected by a security advisory to the latest version of that package available.
In other words, if:
  • the kernel-2.6.38.4-20 package is installed on your system;
  • the kernel-2.6.38.6-22 package was released as a security update;
  • then kernel-2.6.38.6-26 was released as a bug fix update,
...then yum update-minimal --security will update you to kernel-2.6.38.6-22, and yum update --security will update you to kernel-2.6.38.6-26. Conservative system administrators may want to use update-minimal to reduce the risk incurred by updating packages as much as possible.
Refer to man yum-security for usage details and further explanation of the enhancements the security plugin adds to yum.

4.5. Additional Resources

http://yum.baseurl.org/wiki/Guides
The Yum Guides section of the Yum wiki contains more documentation.

Chapter 5. PackageKit

Fedora provides PackageKit for viewing, managing, updating, installing and uninstalling packages compatible with your system. PackageKit consists of several graphical interfaces that can be opened from the GNOME panel menu, or from the Notification Area when PackageKit alerts you that updates are available. For more information on PackageKit's architecture and available front ends, refer to Section 5.3, “PackageKit Architecture”.

5.1. Updating Packages with Software Update

You can open Software Updates by clicking ApplicationsSystem ToolsSoftware Update from the Activities menu, or running the gpk-update-viewer command at the shell prompt. In the Software Updates window, all available updates are listed along with the names of the packages being updated (minus the .rpm suffix, but including the CPU architecture), a short summary of the package, and, usually, short descriptions of the changes the update provides. Any updates you do not wish to install can be de-selected here by unchecking the checkbox corresponding to the update.
Installing updates with Software Update
installing 56 updates with PackageKit's software update window
Figure 5.1. Installing updates with Software Update

The updates presented in the Software Updates window only represent the currently-installed packages on your system for which updates are available; dependencies of those packages, whether they are existing packages on your system or new ones, are not shown until you click Install Updates.
PackageKit utilizes the fine-grained user authentication capabilities provided by the PolicyKit toolkit whenever you request it to make changes to the system. Whenever you instruct PackageKit to update, install or remove packages, you will be prompted to enter the superuser password before changes are made to the system.
If you instruct PackageKit to update the kernel package, then it will prompt you after installation, asking you whether you want to reboot the system and thereby boot into the newly-installed kernel.

5.1.1. Setting the Update-Checking Interval

Selecting ApplicationsOtherSoftware Updates from the Activities menu opens the Software Update Preferences window. The Update Settings tab allows you to define the interval at which PackageKit checks for package updates, as well as whether or not to automatically install all updates or only security updates. Leaving the Check for updates when using mobile broadband box unchecked is handy for avoiding extraneous bandwidth usage when using a wireless connection on which you are charged for the amount of data you download.
Setting PackageKit's update-checking interval
Setting the update-checking interval for PackageKit
Figure 5.2. Setting PackageKit's update-checking interval

5.1.2. Setting the Software Sources

To select which package repositories to use to install software updates, select ApplicationsOtherSoftware Updates from the Activities menu, and click the Software Sources tab of the Software Update Preferences window.
Setting PackageKit's software sources
Setting the software sources for PackageKit
Figure 5.3. Setting PackageKit's software sources

PackageKit refers to Yum repositories as software sources. It obtains all packages from enabled software sources.The Software Sources tab shows the repository name, as written on the name=My Repository Name field of all [repository] sections in the /etc/yum.conf configuration file, and in all repository.repo files in the /etc/yum.repos.d/ directory.
Entries which are checked in the Enabled column indicate that the corresponding repository will be used to locate packages to satisfy all update and installation requests (including dependency resolution). The Enabled column corresponds to the enabled=<1 or 0> field in [repository] sections. Checking an unchecked box enables the Yum repository, and unchecking it disables it. Performing either function causes PolicyKit to prompt for superuser authentication to enable or disable the repository. PackageKit actually inserts the enabled=<1 or 0> line into the correct [repository] section if it does not exist, or changes the value if it does. This means that enabling or disabling a repository through the Software Sources window causes that change to persist after closing the window or rebooting the system. The ability to quickly enable and disable repositories based on our needs is a highly-convenient feature of PackageKit.
Note that it is not possible to add or remove Yum repositories through PackageKit.

Showing source RPM, test, and debuginfo repositories

Checking the box at the bottom of the Software Sources tab causes PackageKit to display source RPM, testing and debuginfo repositories as well. This box is unchecked by default.

5.2. Using Add/Remove Software

PackageKit's Software Update GUI window is a separate application from its Add/Remove Software application, although the two have intuitively similar interfaces. To find and install a new package, select ApplicationsSystem ToolsAdd/Remove Software from the Activities menu, or run the gpk-application command at the shell prompt.
PackageKit's Add/Remove Software window
Viewing PackageKit's Add/Remove Software window
Figure 5.4. PackageKit's Add/Remove Software window

5.2.1. Refreshing Software Sources (Yum Repositories)

To enable or disable a Yum repository, open a dialog box by sclicking SystemSoftware Sources, and select the Software Sources tab. Refer to Section 5.1.2, “Setting the Software Sources” for more information on available configuration options.
After enabling and/or disabling the correct Yum repositories, make sure that you have the latest list of available packages. Click on SystemRefresh Package Lists and PackageKit will obtain the latest lists of packages from all enabled software sources, that is, Yum repositories.

5.2.2. Finding Packages with Filters

You can view the list of all configured and unfiltered (see below) Yum repositories by opening Add/Remove Software and clicking SystemSoftware Sources. Once the software sources have been updated, it is often beneficial to apply some filters so that PackageKit retrieves the results of our Find queries faster. This is especially helpful when performing many package searches. Four of the filters in the Filters drop-down menu are used to split results by matching or not matching a single criterion. By default when PackageKit starts, these filters are all unapplied (No Filter), but once you do filter by one of them, that filter remains set until you either change it or close PackageKit.
Because you are usually searching for available packages that are not installed on the system, click FiltersInstalled and select the Only Available radio button.
Filtering out already-installed packages
filtering out packages which are already installed
Figure 5.5. Filtering out already-installed packages

Also, unless we require development files such as C header files, we can filter for Only End User Files and, in doing so, filter out all of the package_name-devel packages we are not interested in.
Filtering out development packages from the list of Find results
filtering out development packages from our results
Figure 5.6. Filtering out development packages from the list of Find results

The two remaining filters with submenus are:
Graphical
Narrows the search to either applications which provide a GUI interface (Only Graphical) or those that do not. This filter is useful when browsing for GUI applications that perform a specific function.
Free
Search for packages which are considered to be free software Refer to the Fedora Licensing List for details on approved licenses.
The remaining checkbox filters are always either checked or unchecked. They are:
Hide Subpackages
Checking the Hide Subpackages checkbox filters out generally-uninteresting packages that are typically only dependencies of other packages that we want. For example, checking Hide Subpackages and searching for package would cause the following related packages to be filtered out of the Find results (if it exists):
  • package-devel
  • package-libs
  • package-libs-devel
  • package-debuginfo
Only Newest Packages
Checking Only Newest Packages filters out all older versions of the same package from the list of results, which is generally what we want.

Using the Only Newest Packages filter

Checking Only Newest Packages filters out all but the most recent version of any package from the results list. This filter is often combined with the Only Available filter to search for the latest available versions of new (not installed) packages.
Only native packages
Checking the Only Native Packages box on a multilib system causes PackageKit to omit listing results for packages compiled for the architecture that runs in compatibility mode. For example, enabling this filter on a 64-bit system with an AMD64 CPU would cause all packages built for the 32-bit x86 CPU architecture not to be shown in the list of results, even though those packages are able to run on an AMD64 machine. Packages which are architecture-agnostic (i.e. noarch packages such as crontabs-1.10-32.1.el6.noarch.rpm) are never filtered out by checking Only Native Packages. This filter has no affect on non-multilib systems, such as x86 machines.

5.2.3. Installing and Removing Packages (and Dependencies)

With the two filters selected, Only Available and Only End User Files, search for the htop interactive process viewer and highlight the package. You now have access to some very useful information about it, including: a clickable link to the project homepage; the Yum package group it is found in, if any; the license of the package; a pointer to the GNOME menu location from where the application can be opened, if applicable; and the size of the package, which is relevant when we download and install it.
Viewing and installing a package with PackageKit's Add/Remove Software window
Viewing and installing a package with PackageKit's Add/Remove Software window
Figure 5.7. Viewing and installing a package with PackageKit's Add/Remove Software window

When the checkbox next to a package or group is checked, then that item is already installed on the system. Checking an unchecked box causes it to be marked for installation, which only occurs when the Apply button is clicked. In this way, you can search for and select multiple packages or package groups before performing the actual installation transactions. Additionally, you can remove installed packages by unchecking the checked box, and the removal will occur along with any pending installations when Apply is pressed. Dependency resolution , which may add additional packages to be installed or removed, is performed after pressing Apply. PackageKit will then display a window listing those additional packages to install or remove, and ask for confirmation to proceed.
Check htop and click the Apply button. You will then be prompted for the superuser password; enter it, and PackageKit will install htop. One nice feature of PackageKit is that, following installation, it sometimes presents you with a list of your newly-installed applications and offer you the choice of running them immediately. Alternatively, you will remember that finding a package and selecting it in the Add/Remove Software window shows you the Location of where in the GNOME menus its application shortcut is located, which is helpful when you want to run it.
Once it is installed, you can run htop, a colorful and enhanced version of the top process viewer, by opening a shell prompt and entering:
htop
htop is nifty, but we decide that top is good enough for us and we want to uninstall it. Remembering that we need to change the Only Available filter we recently used to install it to Only Installed in FiltersInstalled, we search for htop again and uncheck it. The program did not install any dependencies of its own; if it had, those would be automatically removed as well, as long as they were not also dependencies of any other packages still installed on our system.

Removing a package when other packages depend on it

Although PackageKit automatically resolves dependencies during package installation and removal, it is unable to remove a package without also removing packages which depend on it. This type of operation can only be performed by RPM, is not advised, and can potentially leave your system in a non-functioning state or cause applications to misbehave and/or crash.
Removing a package with PackageKit's Add/Remove Software window
Removing the htop package with PackageKit's Add/Remove Software window
Figure 5.8. Removing a package with PackageKit's Add/Remove Software window

5.2.4. Installing and Removing Package Groups

PackageKit also has the ability to install Yum package groups, which it calls Package collections. Clicking on Package collections in the top-left list of categories in the Software Updates window allows us to scroll through and find the package group we want to install. In this case, we want to install Czech language support (the Czech Support group). Checking the box and clicking Apply informs us how many additional packages must be installed in order to fulfill the dependencies of the package group.
Installing the Czech Support package group
Using PackageKit to install Czech language support with PackageKit's Add/Remove Software window
Figure 5.9. Installing the Czech Support package group

Similarly, installed package groups can be uninstalled by selecting Package collections, unchecking the appropriate checkbox, and applying.

5.2.5. Viewing the Transaction Log

PackageKit maintains a log of the transactions that it performs. To view the log, from the Add/Remove Software window, click SystemSoftware Log, or run the gpk-log command at the shell prompt.
The Software Log Viewer shows the Action, such as Updated Packages or Installed Packages, the Date on which that action was performed, the Username of the user who performed the action, and the front end Application the user used (such as Add/Remove Software, or Update System). The Details column provides the types of the transactions, such as Updated, Installed, or Removed, as well as the list of packages the transactions were performed on.
Viewing the log of package management transactions with the Software Log Viewer
Viewing the log of package management transactions with PackageKit's Software Log Viewer window
Figure 5.10. Viewing the log of package management transactions with the Software Log Viewer

Typing the name of a package in the top text entry field filters the list of transactions to those which affected that package.

5.3. PackageKit Architecture

Fedora provides the PackageKit suite of applications for viewing, updating, installing and uninstalling packages and package groups compatible with your system. Architecturally, PackageKit consists of several graphical front ends that communicate with the packagekitd daemon back end, which communicates with a package manager-specific back end that utilizes Yum to perform the actual transactions, such as installing and removing packages, etc.
Table 5.1, “PackageKit GUI windows, menu locations, and shell prompt commands” shows the name of the GUI window, how to start the window from the GNOME desktop or from the Add/Remove Software window, and the name of the command line application that opens that window.
Table 5.1. PackageKit GUI windows, menu locations, and shell prompt commands
Window Title Function How to Open Shell Command
Add/Remove Software Install, remove or view package info
From the GNOME panel: SystemAdministrationAdd/Remove Software
gpk-application
Software Update Perform package updates
From the GNOME panel: SystemAdministrationSoftware Update
gpk-update-viewer
Software Sources Enable and disable Yum repositories
From Add/Remove Software: SystemSoftware Sources
gpk-repo
Software Log Viewer View the transaction log
From Add/Remove Software: SystemSoftware Log
gpk-log
Software Update Preferences Set PackageKit preferences gpk-prefs
(Notification Area Alert) Alerts you when updates are available
From the GNOME panel: SystemPreferencesStartup Applications, Startup Programs tab
gpk-update-icon

The packagekitd daemon runs outside the user session and communicates with the various graphical front ends. The packagekitd daemon[1] communicates via the DBus system message bus with another back end, which utilizes Yum's Python API to perform queries and make changes to the system. On Linux systems other than Red Hat Enterprise Linux and Fedora, packagekitd can communicate with other back ends that are able to utilize the native package manager for that system. This modular architecture provides the abstraction necessary for the graphical interfaces to work with many different package managers to perform essentially the same types of package management tasks. Learning how to use the PackageKit front ends means that you can use the same familiar graphical interface across many different Linux distributions, even when they utilize a native package manager other than Yum.
In addition, PackageKit's separation of concerns provides reliability in that a crash of one of the GUI windows—or even the user's X Window session—will not affect any package management tasks being supervised by the packagekitd daemon, which runs outside of the user session.
All of the front end graphical applications discussed in this chapter are provided by the gnome-packagekit package instead of by PackageKit and its dependencies. Users working in a KDE environment may prefer to install the kpackagekit package, which provides a KDE interface for PackageKit.
Finally, PackageKit also comes with a console-based front end called pkcon.

5.4. Additional Resources

PackageKit home page — http://www.packagekit.org/index.html
Information about and mailing lists for PackageKit.
PackageKit FAQ — http://www.packagekit.org/pk-faq.html
An informative list of Frequently Asked Questions for the PackageKit software suite.
PackageKit Feature Matrix — http://www.packagekit.org/pk-matrix.html
Cross-reference PackageKit-provided features with the long list of package manager back ends.


[1] System daemons are typically long-running processes that provide services to the user or to other programs, and which are started, often at boot time. Daemons respond to the systemctl command and can be turned on or off permanently by using the systemctl enable or systemctl disablecommands. They can typically be recognized by a d appended to their name, such as the packagekitd daemon. Refer to Chapter 7, Services and Daemons for information about system services.

Part III. Networking

Chapter 6. Network Interfaces

Under Fedora, all network communications occur between configured software interfaces and physical networking devices connected to the system.
The configuration files for network interfaces are located in the /etc/sysconfig/network-scripts/ directory. The scripts used to activate and deactivate these network interfaces are also located here. Although the number and type of interface files can differ from system to system, there are three categories of files that exist in this directory:
  1. Interface configuration files
  2. Interface control scripts
  3. Network function files
The files in each of these categories work together to enable various network devices.
This chapter explores the relationship between these files and how they are used.

6.1. Network Configuration Files

Before delving into the interface configuration files, let us first itemize the primary configuration files used in network configuration. Understanding the role these files play in setting up the network stack can be helpful when customizing a Fedora system.
The primary network configuration files are as follows:
/etc/hosts
The main purpose of this file is to resolve hostnames that cannot be resolved any other way. It can also be used to resolve hostnames on small networks with no DNS server. Regardless of the type of network the computer is on, this file should contain a line specifying the IP address of the loopback device (127.0.0.1) as localhost.localdomain. For more information, refer to the hosts man page.
/etc/resolv.conf
This file specifies the IP addresses of DNS servers and the search domain. Unless configured to do otherwise, the network initialization scripts populate this file. For more information about this file, refer to the resolv.conf man page.
/etc/sysconfig/network
This file specifies routing and host information for all network interfaces. For more information about this file and the directives it accepts, refer to Section B.1.13, “ /etc/sysconfig/network ”.
/etc/sysconfig/network-scripts/ifcfg-interface-name
For each network interface, there is a corresponding interface configuration script. Each of these files provide information specific to a particular network interface. Refer to Section 6.2, “Interface Configuration Files” for more information on this type of file and the directives it accepts.

Warning

The /etc/sysconfig/networking/ directory is used by the Network Administration Tool (system-config-network) and its contents should not be edited manually. Using only one method for network configuration is strongly encouraged, due to the risk of configuration deletion.

6.2. Interface Configuration Files

Interface configuration files control the software interfaces for individual network devices. As the system boots, it uses these files to determine what interfaces to bring up and how to configure them. These files are usually named ifcfg-name , where name refers to the name of the device that the configuration file controls.

6.2.1. Ethernet Interfaces

One of the most common interface files is ifcfg-eth0, which controls the first Ethernet network interface card or NIC in the system. In a system with multiple NICs, there are multiple ifcfg-ethX files (where X is a unique number corresponding to a specific interface). Because each device has its own configuration file, an administrator can control how each interface functions individually.
The following is a sample ifcfg-eth0 file for a system using a fixed IP address:
DEVICE=eth0
BOOTPROTO=none
ONBOOT=yes
NETMASK=255.255.255.0
IPADDR=10.0.1.27
USERCTL=no
The values required in an interface configuration file can change based on other values. For example, the ifcfg-eth0 file for an interface using DHCP looks different because IP information is provided by the DHCP server:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
The Network Administration Tool (system-config-network) is an easy way to make changes to the various network interface configuration files.
However, it is also possible to manually edit the configuration files for a given network interface.
Below is a listing of the configurable parameters in an Ethernet interface configuration file:
BONDING_OPTS=parameters
sets the configuration parameters for the bonding device, and is used in /etc/sysconfig/network-scripts/ifcfg-bondN (see Section 6.2.2, “Channel Bonding Interfaces”). These parameters are identical to those used for bonding devices in /sys/class/net/bonding device/bonding, and the module parameters for the bonding driver as described in bonding Module Directives.
This configuration method is used so that multiple bonding devices can have different configurations. It is highly recommended to place all of your bonding options after the BONDING_OPTS directive in ifcfg-name. Do not specify options for the bonding device in /etc/modprobe.d/bonding.conf, or in the deprecated /etc/modprobe.conf file.
BOOTPROTO=protocol
where protocol is one of the following:
  • none — No boot-time protocol should be used.
  • bootp — The BOOTP protocol should be used.
  • dhcp — The DHCP protocol should be used.
BROADCAST=address
where address is the broadcast address. This directive is deprecated, as the value is calculated automatically with ipcalc.
DEVICE=name
where name is the name of the physical device (except for dynamically-allocated PPP devices where it is the logical name).
DHCP_HOSTNAME=name
where name is a short hostname to be sent to the DHCP server. Use this option only if the DHCP server requires the client to specify a hostname before receiving an IP address.
DNS{1,2}=address
where address is a name server address to be placed in /etc/resolv.conf if the PEERDNS directive is set to yes.
ETHTOOL_OPTS=options
where options are any device-specific options supported by ethtool. For example, if you wanted to force 100Mb, full duplex:
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
Instead of a custom initscript, use ETHTOOL_OPTS to set the interface speed and duplex settings. Custom initscripts run outside of the network init script lead to unpredictable results during a post-boot network service restart.

Set "autoneg off" before changing speed or duplex settings

Changing speed or duplex settings almost always requires disabling autonegotiation with the autoneg off option. This needs to be stated first, as the option entries are order-dependent.
GATEWAY=address
where address is the IP address of the network router or gateway device (if any).
HOTPLUG=answer
where answer is one of the following:
  • yes — This device should be activated when it is hot-plugged (this is the default option).
  • no — This device should not be activated when it is hot-plugged.
The HOTPLUG=no option can be used to prevent a channel bonding interface from being activated when a bonding kernel module is loaded.
Refer to Section 6.2.2, “Channel Bonding Interfaces” for more information about channel bonding interfaces.
HWADDR=MAC-address
where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive must be used in machines containing more than one NIC to ensure that the interfaces are assigned the correct device names regardless of the configured load order for each NIC's module. This directive should not be used in conjunction with MACADDR.
IPADDR=address
where address is the IP address.
LINKDELAY=time
where time is the number of seconds to wait for link negotiation before configuring the device.
MACADDR=MAC-address
where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive is used to assign a MAC address to an interface, overriding the one assigned to the physical NIC. This directive should not be used in conjunction with HWADDR.
MASTER=bond-interface
where bond-interface is the channel bonding interface to which the Ethernet interface is linked.
This directive is used in conjunction with the SLAVE directive.
Refer to Section 6.2.2, “Channel Bonding Interfaces” for more information about channel bonding interfaces.
NETMASK=mask
where mask is the netmask value.
NETWORK=address
where address is the network address. This directive is deprecated, as the value is calculated automatically with ipcalc.
ONBOOT=answer
where answer is one of the following:
  • yes — This device should be activated at boot-time.
  • no — This device should not be activated at boot-time.
PEERDNS=answer
where answer is one of the following:
  • yes — Modify /etc/resolv.conf if the DNS directive is set. If using DHCP, then yes is the default.
  • no — Do not modify /etc/resolv.conf.
SLAVE=answer
where answer is one of the following:
  • yes — This device is controlled by the channel bonding interface specified in the MASTER directive.
  • no — This device is not controlled by the channel bonding interface specified in the MASTER directive.
This directive is used in conjunction with the MASTER directive.
Refer to Section 6.2.2, “Channel Bonding Interfaces” for more about channel bonding interfaces.
SRCADDR=address
where address is the specified source IP address for outgoing packets.
USERCTL=answer
where answer is one of the following:
  • yes — Non-root users are allowed to control this device.
  • no — Non-root users are not allowed to control this device.

6.2.2. Channel Bonding Interfaces

Fedora allows administrators to bind multiple network interfaces together into a single channel using the bonding kernel module and a special network interface called a channel bonding interface. Channel bonding enables two or more network interfaces to act as one, simultaneously increasing the bandwidth and providing redundancy.
To create a channel bonding interface, create a file in the /etc/sysconfig/network-scripts/ directory called ifcfg-bondN , replacing N with the number for the interface, such as 0.
The contents of the file can be identical to whatever type of interface is getting bonded, such as an Ethernet interface. The only difference is that the DEVICE= directive must be bondN , replacing N with the number for the interface.
The following is a sample channel bonding configuration file:
Example 6.1. Sample ifcfg-bond0 interface configuration file
DEVICE=bond0
IPADDR=192.168.1.1
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=none
USERCTL=no
BONDING_OPTS="bonding parameters separated by spaces"

After the channel bonding interface is created, the network interfaces to be bound together must be configured by adding the MASTER= and SLAVE= directives to their configuration files. The configuration files for each of the channel-bonded interfaces can be nearly identical.
For example, if two Ethernet interfaces are being channel bonded, both eth0 and eth1 may look like the following example:
DEVICE=ethN
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
In this example, replace N with the numerical value for the interface.
For a channel bonding interface to be valid, the kernel module must be loaded. To ensure that the module is loaded when the channel bonding interface is brought up, create a new file as root named bonding.conf in the /etc/modprobe.d/ directory. Note that you can name this file anything you like as long as it ends with a .conf extension. Insert the following line in this new file:
alias bondN bonding
Replace N with the interface number, such as 0. For each configured channel bonding interface, there must be a corresponding entry in your new /etc/modprobe.d/bonding.conf file.

Put all bonding module parameters in ifcfg-bondN files

Parameters for the bonding kernel module must be specified as a space-separated list in the BONDING_OPTS="bonding parameters" directive in the ifcfg-bondN interface file. Do not specify options for the bonding device in /etc/modprobe.d/bonding.conf, or in the deprecated /etc/modprobe.conf file. For further instructions and advice on configuring the bonding module and to view the list of bonding parameters, refer to Section 21.7.2, “Using Channel Bonding”.

6.2.3. Alias and Clone Files

Two lesser-used types of interface configuration files are alias and clone files.
Alias interface configuration files, which are used to bind multiple addresses to a single interface, use the ifcfg-if-name:alias-value naming scheme.
For example, an ifcfg-eth0:0 file could be configured to specify DEVICE=eth0:0 and a static IP address of 10.0.0.2, serving as an alias of an Ethernet interface already configured to receive its IP information via DHCP in ifcfg-eth0. Under this configuration, eth0 is bound to a dynamic IP address, but the same physical network card can receive requests via the fixed, 10.0.0.2 IP address.

Caution

Alias interfaces do not support DHCP.
A clone interface configuration file should use the following naming convention: ifcfg-if-name-clone-name . While an alias file allows multiple addresses for an existing interface, a clone file is used to specify additional options for an interface. For example, a standard DHCP Ethernet interface called eth0, may look similar to this:
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
Since the default value for the USERCTL directive is no if it is not specified, users cannot bring this interface up and down. To give users the ability to control the interface, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the following line to ifcfg-eth0-user:
USERCTL=yes
This way a user can bring up the eth0 interface using the /sbin/ifup eth0-user command because the configuration options from ifcfg-eth0 and ifcfg-eth0-user are combined. While this is a very basic example, this method can be used with a variety of options and interfaces.
The easiest way to create alias and clone interface configuration files is to use the graphical Network Administration Tool.

6.2.4. Dialup Interfaces

If you are connecting to the Internet via a dialup connection, a configuration file is necessary for the interface.
PPP interface files are named using the following format:
ifcfg-pppX
where X is a unique number corresponding to a specific interface.
The PPP interface configuration file is created automatically when wvdial, the Network Administration Tool or Kppp is used to create a dialup account. It is also possible to create and edit this file manually.
The following is a typical ifcfg-ppp0 file:
DEVICE=ppp0
NAME=test
WVDIALSECT=test
MODEMPORT=/dev/modem
LINESPEED=115200
PAPNAME=test
USERCTL=true
ONBOOT=no
PERSIST=no
DEFROUTE=yes
PEERDNS=yes
DEMAND=no
IDLETIMEOUT=600
Serial Line Internet Protocol (SLIP) is another dialup interface, although it is used less frequently. SLIP files have interface configuration file names such as ifcfg-sl0.
Other options that may be used in these files include:
DEFROUTE=answer
where answer is one of the following:
  • yes — Set this interface as the default route.
  • no — Do not set this interface as the default route.
DEMAND=answer
where answer is one of the following:
  • yes — This interface allows pppd to initiate a connection when someone attempts to use it.
  • no — A connection must be manually established for this interface.
IDLETIMEOUT=value
where value is the number of seconds of idle activity before the interface disconnects itself.
INITSTRING=string
where string is the initialization string passed to the modem device. This option is primarily used in conjunction with SLIP interfaces.
LINESPEED=value
where value is the baud rate of the device. Possible standard values include 57600, 38400, 19200, and 9600.
MODEMPORT=device
where device is the name of the serial device that is used to establish the connection for the interface.
MTU=value
where value is the Maximum Transfer Unit (MTU) setting for the interface. The MTU refers to the largest number of bytes of data a frame can carry, not counting its header information. In some dialup situations, setting this to a value of 576 results in fewer packets dropped and a slight improvement to the throughput for a connection.
NAME=name
where name is the reference to the title given to a collection of dialup connection configurations.
PAPNAME=name
where name is the username given during the Password Authentication Protocol (PAP) exchange that occurs to allow connections to a remote system.
PERSIST=answer
where answer is one of the following:
  • yes — This interface should be kept active at all times, even if deactivated after a modem hang up.
  • no — This interface should not be kept active at all times.
REMIP=address
where address is the IP address of the remote system. This is usually left unspecified.
WVDIALSECT=name
where name associates this interface with a dialer configuration in /etc/wvdial.conf. This file contains the phone number to be dialed and other important information for the interface.

6.2.5. Other Interfaces

Other common interface configuration files include the following:
ifcfg-lo
A local loopback interface is often used in testing, as well as being used in a variety of applications that require an IP address pointing back to the same system. Any data sent to the loopback device is immediately returned to the host's network layer.

Do not manually edit the ifcfg-lo script

The loopback interface script, /etc/sysconfig/network-scripts/ifcfg-lo, should never be edited manually. Doing so can prevent the system from operating correctly.
ifcfg-irlan0
An infrared interface allows information between devices, such as a laptop and a printer, to flow over an infrared link. This works in a similar way to an Ethernet device except that it commonly occurs over a peer-to-peer connection.
ifcfg-plip0
A Parallel Line Interface Protocol (PLIP) connection works much the same way as an Ethernet device, except that it utilizes a parallel port.

6.3. Interface Control Scripts

The interface control scripts activate and deactivate system interfaces. There are two primary interface control scripts that call on control scripts located in the /etc/sysconfig/network-scripts/ directory: /sbin/ifdown and /sbin/ifup.
The ifup and ifdown interface scripts are symbolic links to scripts in the /sbin/ directory. When either of these scripts are called, they require the value of the interface to be specified, such as:
ifup eth0

Use the ifup and ifdown interface scripts

The ifup and ifdown interface scripts are the only scripts that the user should use to bring up and take down network interfaces.
The following scripts are described for reference purposes only.
Two files used to perform a variety of network initialization tasks during the process of bringing up a network interface are /etc/rc.d/init.d/functions and /etc/sysconfig/network-scripts/network-functions. Refer to Section 6.5, “Network Function Files” for more information.
After verifying that an interface has been specified and that the user executing the request is allowed to control the interface, the correct script brings the interface up or down. The following are common interface control scripts found within the /etc/sysconfig/network-scripts/ directory:
ifup-aliases
Configures IP aliases from interface configuration files when more than one IP address is associated with an interface.
ifup-ippp and ifdown-ippp
Brings ISDN interfaces up and down.
ifup-ipv6 and ifdown-ipv6
Brings IPv6 interfaces up and down.
ifup-plip
Brings up a PLIP interface.
ifup-plusb
Brings up a USB interface for network connections.
ifup-post and ifdown-post
Contains commands to be executed after an interface is brought up or down.
ifup-ppp and ifdown-ppp
Brings a PPP interface up or down.
ifup-routes
Adds static routes for a device as its interface is brought up.
ifdown-sit and ifup-sit
Contains function calls related to bringing up and down an IPv6 tunnel within an IPv4 connection.
ifup-wireless
Brings up a wireless interface.

Be careful when removing or modifying network scripts!

Removing or modifying any scripts in the /etc/sysconfig/network-scripts/ directory can cause interface connections to act irregularly or fail. Only advanced users should modify scripts related to a network interface.
The easiest way to manipulate all network scripts simultaneously is to use the systemctl command on the network service (/etc/rc.d/init.d/network), as illustrated the following command:
systemctl action network.service
Here, action can be either start, stop, or restart.
To view a list of configured devices and currently active network interfaces, use the following command:
service network status

6.4. Configuring Static Routes

If static routes are required, they can be configured for each interface. This can be useful if you have multiple interfaces in different subnets. Use the route command to display the IP routing table.
Static route configuration is stored in a /etc/sysconfig/network-scripts/route-interface file. For example, static routes for the eth0 interface would be stored in the /etc/sysconfig/network-scripts/route-eth0 file. The route-interface file has two formats: IP command arguments and network/netmask directives.
IP Command Arguments Format
Define a default gateway on the first line. This is only required if the default gateway is not set via DHCP:
default via X.X.X.X dev interface
X.X.X.X is the IP address of the default gateway. The interface is the interface that is connected to, or can reach, the default gateway.
Define a static route. Each line is parsed as an individual route:
X.X.X.X/X via X.X.X.X dev interface
X.X.X.X/X is the network number and netmask for the static route. X.X.X.X and interface are the IP address and interface for the default gateway respectively. The X.X.X.X address does not have to be the default gateway IP address. In most cases, X.X.X.X will be an IP address in a different subnet, and interface will be the interface that is connected to, or can reach, that subnet. Add as many static routes as required.
The following is a sample route-eth0 file using the IP command arguments format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks:
default via 192.168.0.1 dev eth0
10.10.10.0/24 via 192.168.0.1 dev eth0
172.16.1.0/24 via 192.168.0.1 dev eth0
Static routes should only be configured for other subnets. The above example is not necessary, since packets going to the 10.10.10.0/24 and 172.16.1.0/24 networks will use the default gateway anyway. Below is an example of setting static routes to a different subnet, on a machine in a 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:
10.10.10.0/24 via 10.10.10.1 dev eth1

Duplicate default gateways

If the default gateway is already assigned from DHCP, the IP command arguments format can cause one of two errors during start-up, or when bringing up an interface from the down state using the ifup command: "RTNETLINK answers: File exists" or 'Error: either "to" is a duplicate, or "X.X.X.X" is a garbage.', where X.X.X.X is the gateway, or a different IP address. These errors can also occur if you have another route to another network using the default gateway. Both of these errors are safe to ignore.
Network/Netmask Directives Format
You can also use the network/netmask directives format for route-interface files. The following is a template for the network/netmask format, with instructions following afterwards:
ADDRESS0=X.X.X.X
NETMASK0=X.X.X.X
GATEWAY0=X.X.X.X
  • ADDRESS0=X.X.X.X is the network number for the static route.
  • NETMASK0=X.X.X.X is the netmask for the network number defined with ADDRESS0=X.X.X.X .
  • GATEWAY0=X.X.X.X is the default gateway, or an IP address that can be used to reach ADDRESS0=X.X.X.X
The following is a sample route-eth0 file using the network/netmask directives format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks. However, as mentioned before, this example is not necessary as the 10.10.10.0/24 and 172.16.1.0/24 networks would use the default gateway anyway:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.0.1
ADDRESS1=172.16.1.0
NETMASK1=255.255.255.0
GATEWAY1=192.168.0.1
Subsequent static routes must be numbered sequentially, and must not skip any values. For example, ADDRESS0, ADDRESS1, ADDRESS2, and so on.
Below is an example of setting static routes to a different subnet, on a machine in the 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=10.10.10.1
Note that if DHCP is used, it can assign these settings automatically.

6.5. Network Function Files

Fedora makes use of several files that contain important common functions used to bring interfaces up and down. Rather than forcing each interface control file to contain these functions, they are grouped together in a few files that are called upon when necessary.
The /etc/sysconfig/network-scripts/network-functions file contains the most commonly used IPv4 functions, which are useful to many interface control scripts. These functions include contacting running programs that have requested information about changes in the status of an interface, setting hostnames, finding a gateway device, verifying whether or not a particular device is down, and adding a default route.
As the functions required for IPv6 interfaces are different from IPv4 interfaces, a /etc/sysconfig/network-scripts/network-functions-ipv6 file exists specifically to hold this information. The functions in this file configure and delete static IPv6 routes, create and remove tunnels, add and remove IPv6 addresses to an interface, and test for the existence of an IPv6 address on an interface.

6.6. Additional Resources

The following are resources which explain more about network interfaces.

6.6.1. Installed Documentation

/usr/share/doc/initscripts-version/sysconfig.txt
A guide to available options for network configuration files, including IPv6 options not covered in this chapter.
/usr/share/doc/iproute-version/ip-cref.ps
This file contains a wealth of information about the ip command, which can be used to manipulate routing tables, among other things. Use the ggv or kghostview application to view this file.

Part IV. Infrastructure Services

Chapter 7. Services and Daemons

Maintaining security on your system is extremely important, and one approach for this task is to manage access to system services carefully. Your system may need to provide open access to particular services (for example, httpd if you are running a web server). However, if you do not need to provide a service, you should turn it off to minimize your exposure to possible bug exploits.
This chapter covers the configuration of the services to be run when a system is started, and provides information on how to start, stop, and restart the services on the command line using the systemctl utility.

Keep the system secure

When you allow access for new services, always remember that both the firewall and SELinux need to be configured as well. One of the most common mistakes committed when configuring a new service is neglecting to implement the necessary firewall configuration and SELinux policies to allow access for it. Refer to the Fedora Security Guide (see Section 7.3, “Additional Resources”) for more information.

7.1. Configuring Services

To allow you to configure which services are started at boot time, Fedora is shipped with the systemctl command line tool.

Do not use the ntsysv and chkconfig utilities

Although it is still possible to use the ntsysv and chkconfig utilities to manage services that have init scripts installed in the /etc/rc.d/init.d/ directory, it is advised that you use the systemctl utility.

Enabling the irqbalance service

To ensure optimal performance on POWER architecture, it is recommended that the irqbalance service is enabled. In most cases, this service is installed and configured to run during the Fedora 15 installation. To verify that irqbalance is running, type the following at a shell prompt:
systemctl status irqbalance.service

7.1.1. Enabling the Service

To configure a service to be automatically started at boot time, use the systemctl command in the following form:
systemctl enable service_name.service
The service will be started the next time you boot the system. For information on how to start the service immediately, refer to Section 7.2.2, “Running the Service”.
Example 7.1. Enabling the httpd service
Imagine you want to run the Apache HTTP Server on your system. Provided that you have the httpd package installed, you can enable the httpd service by typing the following at a shell prompt as root:
~]# systemctl enable httpd.service

7.1.2. Disabling the Service

To disable starting a service at boot time, use the systemctl command in the following form:
systemctl disable service_name.service
The next time you boot the system, the service will not be started. For information on how to stop the service immediately, refer to Section 7.2.3, “Stopping the Service”.
Example 7.2. Disabling the telnet service
In order to secure the system, users are advised to disable insecure connection protocols such as Telnet. You can make sure that the telnet service is disabled by running the following command as root:
~]# systemctl disable telnet.service

7.2. Running Services

The systemctl utility also allows you to determine the status of a particular service, as well as to start, stop, or restart a service.

Do not use the service utility

Although it is still possible to use the service utility to manage services that have init scripts installed in the /etc/rc.d/init.d/ directory, it is advised that you use the systemctl utility.

7.2.1. Checking the Service Status

To determine the status of a particular service, use the systemctl command in the following form:
systemctl status service_name.service
This command provides detailed information on the service's status. However, if you merely need to verify that a service is running, you can use the systemctl command in the following form instead:
systemctl is-active service_name.service
Example 7.3. Checking the status of the httpd service
Example 7.1, “Enabling the httpd service” illustrated how to enable starting the httpd service at boot time. Imagine that the system has been restarted and you need to verify that the service is really running. You can do so by typing the following at a shell prompt:
~]$ systemctl is-active httpd.service
active
You can also display detailed information about the service by running the following command:
~]$ systemctl status httpd.service
httpd.service - LSB: start and stop Apache HTTP Server
          Loaded: loaded (/etc/rc.d/init.d/httpd)
          Active: active (running) since Mon, 23 May 2011 21:38:57 +0200; 27s ago
         Process: 2997 ExecStart=/etc/rc.d/init.d/httpd start (code=exited, status=0/SUCCESS)
        Main PID: 3002 (httpd)
          CGroup: name=systemd:/system/httpd.service
                  ├ 3002 /usr/sbin/httpd
                  ├ 3004 /usr/sbin/httpd
                  ├ 3005 /usr/sbin/httpd
                  ├ 3006 /usr/sbin/httpd
                  ├ 3007 /usr/sbin/httpd
                  ├ 3008 /usr/sbin/httpd
                  ├ 3009 /usr/sbin/httpd
                  ├ 3010 /usr/sbin/httpd
                  └ 3011 /usr/sbin/httpd

To display a list of all active system services, use the following command:
systemctl list-units --type=service
This command provides a tabular output with each line consisting of the following columns:
  • UNIT — A systemd unit name. In this case, a service name.
  • LOAD — Information whether the systemd unit was properly loaded.
  • ACTIVE — A high-level unit activation state.
  • SUB — A low-level unit activation state.
  • JOB — A pending job for the unit.
  • DESCRIPTION — A brief description of the unit.
Example 7.4. Listing all active services
You can list all active services by using the following command:
~]$ systemctl list-units --type=service
UNIT                      LOAD   ACTIVE SUB     JOB DESCRIPTION
abrt-ccpp.service         loaded active exited      LSB: Installs coredump handler which saves segfault data
abrt-oops.service         loaded active running     LSB: Watches system log for oops messages, creates ABRT dump directories for each oops
abrtd.service             loaded active running     ABRT Automated Bug Reporting Tool
accounts-daemon.service   loaded active running     Accounts Service
atd.service               loaded active running     Job spooling tools
[output truncated]
In the example above, the abrtd service is loaded, active, and running, and it does not have any pending jobs.

7.2.2. Running the Service

To run a service, use the systemctl command in the following form:
systemctl start service_name.service
This will start the service in the current session. To configure the service to be started at boot time, refer to Section 7.1.1, “Enabling the Service”.
Example 7.5. Running the httpd service
Example 7.1, “Enabling the httpd service” illustrated how to run the httpd service at boot time. You can start the service immediately by typing the following at a shell prompt as root:
~]# systemctl start httpd.service

7.2.3. Stopping the Service

To stop a service, use the systemctl command in the following form:
systemctl stop service_name.service
This will stop the service in the current session. To disable starting the service at boot time, refer to Section 7.1.1, “Enabling the Service”.
Example 7.6. Stopping the telnet service
Example 7.2, “Disabling the telnet service” illustrated how to disable starting the telnet service at boot time. You can stop the service immediately by running the following command as root:
~]# systemctl stop telnet.service

7.2.4. Restarting the Service

To restart a service, use the systemctl command in the following form:
systemctl restart service_name.service
Example 7.7. Restarting the sshd service
For any changes in the /etc/ssh/sshd_config configuration file to take effect, it is required that you restart the sshd service. You can do so by typing the following at a shell prompt as root:
~]# systemctl restart httpd.service

7.3. Additional Resources

7.3.1. Installed Documentation

  • systemctl(1) — The manual page for the systemctl utility.

7.3.2. Related Books

Security Guide
A guide to securing Fedora. It contains valuable information on how to set up the firewall, as well as the configuration of SELinux.

Chapter 8. Configuring Authentication

8.1. The Authentication Configuration Tool

When a user logs in to a Fedora system, the username and password combination must be verified, or authenticated, as a valid and active user. Sometimes the information to verify the user is located on the local system, and other times the system defers the authentication to a user database on a remote system.
The Authentication Configuration Tool provides a graphical interface for configuring user information retrieval from Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind user account databases. This tool also allows you to configure Kerberos to be used as the authentication protocol when using LDAP or NIS.

Using a high or medium security level

If you configured a medium or high security level during installation (or with the Security Level Configuration Tool), then the firewall will prevent NIS authentication. For more information about firewalls, refer to the "Firewalls" section of the Fedora Security Guide.
To start the graphical version of the Authentication Configuration tool from the desktop, select ApplicationsOtherAuthentication form the Activities menu or type the command system-config-authentication at a shell prompt (for example, in an XTerm or a GNOME terminal).

Your changes are immediately applied

After exiting the authentication program, any changes you made take effect immediately.

8.1.1. Identity & Authentication

The Identity & Authentication tab allows you to configure how users should be authenticated, and has several options for each method of authentication. To select which user account database should be used, select an option from the drop-down list.
Identity & Authentication; changing the option in the User Account Database drop-down list changes the contents of the tab
Figure 8.1. Identity & Authentication; changing the option in the User Account Database drop-down list changes the contents of the tab

The following list explains what each option configures:

LDAP

The LDAP option instructs the system to retrieve user information via LDAP. It contains the following specifications:
  • LDAP Search Base DN — Specifies that user information should be retrieved using the listed Distinguished Name (DN).
  • LDAP Server — Specifies the address of the LDAP server.
  • Use TLS to encrypt connections — When enabled, Transport Layer Security (TLC) will be used to encrypt passwords sent to the LDAP server. The Download CA Certificate option allows you to specify a URL from which to download a valid Certificate Authority certificate (CA). A valid CA certificate must be in the Privacy Enhanced Mail (PEM) format.

    Using ldaps:// in the LDAP Server field

    The Use TLS to encrypt connections option must not be ticked if an ldaps:// server address is specified in the LDAP Server field.
    For more information about CA Certificates, refer to Section 12.1.8.1, “An Overview of Certificates and Security”.
The openldap-clients package must be installed for this option to work.
For more information about LDAP, refer to Section 14.1, “OpenLDAP”.
LDAP provides the following methods of authentication:
  • Kerberos password — This option enables Kerberos authentication. It contains the following specifications:
    • Realm — Configures the realm for the Kerberos server. The realm is the network that uses Kerberos, composed of one or more KDCs and a potentially large number of clients.
    • KDCs — Defines the Key Distribution Centers (KDC), which are servers that issue Kerberos tickets.
    • Admin Servers — Specifies the administration server(s) running kadmind.
    The Kerberos Settings dialog also allows you to use DNS to resolve hosts to realms and locate KDCs for realms.
    The krb5-libs and krb5-workstation packages must be installed for this option to work. For more information about Kerberos, refer to section Using Kerberos of the Fedora 15 Managing Single Sign-On and Smart Cards guide.
  • LDAP password — This option instructs standard PAM-enabled applications to use LDAP authentication with options specified in the User Account Configuration of LDAP. When using this option, you must provide an ldaps:// server address or use TLS for LDAP authentication.

Setting up the SSSD service

The SSSD service is used as a client for LDAP and Kerberos servers. Thus, offline login is enabled and supported by default. No user interaction is needed to set up the SSSD service with the Authentication Configuration Tool. For more information about the SSSD service, refer to Section 8.2, “The System Security Services Daemon (SSSD)”

NIS

The NIS option configures the system to connect to a NIS server (as an NIS client) for user and password authentication. To configure this option, specify the NIS domain and NIS server. If the NIS server is not specified, the daemon attempts to find it via broadcast.
The ypbind package must be installed for this option to work. If the NIS user account database is used, the portmap and ypbind services are started and are also enabled to start at boot time.
For more information about NIS, refer to section "Securing NIS" of the Fedora Security Guide.
NIS provides the following methods of authentication:
  • Kerberos password — This option enables Kerberos authentication. For more information about configuration of the Kerberos authentication method, refer to the previous section on LDAP.
  • NIS password — This option enables NIS authentication. NIS can provide authentication information to outside processes to authenticate users.

Winbind

The Winbind option configures the system to connect to a Windows Active Directory or a Windows domain controller. User information from the specified directory or domain controller can then be accessed, and server authentication options can be configured. It contains the following specifications:
  • Winbind Domain — Specifies the Windows Active Directory or domain controller to connect to.
  • Security Model — Allows you to select a security model, which configures the Samba client mode of operation. The drop-down list allows you to select any of the following:
    • ads — This mode instructs Samba to act as a domain member in an Active Directory Server (ADS) realm. To operate in this mode, the krb5-server package must be installed, and Kerberos must be configured properly.
    • domain — In this mode, Samba will attempt to validate the username/password by authenticating it through a Windows NT Primary or Backup Domain Controller, similar to how a Windows NT Server would.
    • server — In this mode, Samba will attempt to validate the username/password by authenticating it through another SMB server (for example, a Windows NT Server). If the attempt fails, the user mode will take effect instead.
    • user — This is the default mode. With this level of security, a client must first log in with a valid username and password. Encrypted passwords can also be used in this security mode.
  • Winbind ADS Realm — When the ads Security Model is selected, this allows you to specify the ADS Realm the Samba server should act as a domain member of.
  • Winbind Domain Controllers — Use this option to specify which domain controller winbind should use. For more information about domain controllers, please refer to Section 15.1.6.3, “Domain Controller”.
  • Template Shell — When filling out the user information for a Windows NT user, the winbindd daemon uses the value chosen here to specify the login shell for that user.
  • Allow offline login — By checking this option, you allow authentication information to be stored in a local cache (provided by SSSD). This information is then used when a user attempts to authenticate while offline.
For more information about the winbindd service, refer to Section 15.1.2, “Samba Daemons and Related Services”.
Winbind provides only one method of authentication, Winbind password. This method of authentication uses the options specified in the User Account Configuration of Winbind to connect to a Windows Active Directory or a Windows domain controller.

8.1.2. Advanced Options

This tab allows you to configure advanced options, as listed below.
Advanced Options
Figure 8.2. Advanced Options

Local Authentication Options

  • Enable fingerprint reader support — By checking this option, you enable fingerprint authentication to log in by scanning your finger with the fingerprint reader.
  • Enable local access control — When enabled, /etc/security/access.conf is consulted for authorization of a user.
  • Password Hashing Algorithm — This option lets you specify which hashing or cryptographic algorithm should be used to encrypt locally stored passwords.

Other Authentication Options

Create home directories on the first login — When enabled, the user's home directory is automatically created when they log in if it does not already exist.

Smart Card Authentication Options

Enable smart card support — This option enables smart card authentication. Smart card authentication allows you to log in using a certificate and a key associated with a smart card.
When the Enable smart card support option is checked, the following options can be specified:
  • Card Removal Action — This option defines what action the system performs when the card is removed from the card reader during an active session. Two alternatives are available:
    • Ignore — The card removal is ignored and the system continues to function as normal.
    • Lock — The current session is immediately locked.
  • Require smart card login — Requires the user to login and authenticate with a smart card. It essentially disables any other type of password authentication. This option should not be selected until after you have successfully logged in using a smart card.
The pam_pkcs11 and the coolkey packages must be installed for this option to work. For more information about smart cards, refer to the Red Hat Enterprise Linux 6 Managing Single Sign-On and Smart Cards Guide.

Click Revert to restore the previous configuration

You can restore all of the options specified in the Authentication Configuration Tool to the previous configuration setup by clicking Revert.

8.1.3. Command Line Version

The Authentication Configuration tool also supports a command line interface. The command line version can be used in a configuration script or a kickstart script. The authentication options are summarized in Table 8.1, “Command line options”.

Getting the list of supported authentication options

These options can also be found in the authconfig man page or by typing authconfig --help at the shell prompt.
Table 8.1. Command line options
Option Description
--enableshadow, --useshadow Enable shadow passwords
--disableshadow Disable shadow passwords
--passalgo=descrypt|bigcrypt|md5|sha256|sha512 Hash/crypt algorithm to be used
--enablenis Enable NIS for user account configuration
--disablenis Disable NIS for user account configuration
--nisdomain=domain Specify an NIS domain
--nisserver=server Specify an NIS server
--enableldap Enable LDAP for user account configuration
--disableldap Disable LDAP for user account configuration
--enableldaptls Enable use of TLS with LDAP
--disableldaptls Disable use of TLS with LDAP
--enablerfc2307bis Enable use of RFC-2307bis schema for LDAP user information lookups
--disablerfc2307bis Disable use of RFC-2307bis schema for LDAP user information lookups
--enableldapauth Enable LDAP for authentication
--disableldapauth Disable LDAP for authentication
--ldapserver=server Specify an LDAP server
--ldapbasedn=dn Specify an LDAP base DN (Distinguished Name)
--ldaploadcacert=URL Load a CA certificate from the specified URL
--enablekrb5 Enable Kerberos for authentication
--disablekrb5 Disable Kerberos for authentication
--krb5kdc=server Specify Kerberos KDC server
--krb5adminserver=server Specify Kerberos administration server
--krb5realm=realm Specify Kerberos realm
--enablekrb5kdcdns Enable use of DNS to find Kerberos KDCs
--disablekrb5kdcdns Disable use of DNS to find Kerberos KDCs
--enablekrb5realmdns Enable use of DNS to find Kerberos realms
--disablekrb5realmdns Disable use of DNS to find Kerberos realms
--enablewinbind Enable winbind for user account configuration
--disablewinbind Disable winbind for user account configuration
--enablewinbindauth Enable winbindauth for authentication
--disablewinbindauth Disable winbindauth for authentication
--winbindseparator=\ Character used to separate the domain and user part of winbind usernames if winbindusedefaultdomain is not enabled
--winbindtemplatehomedir=/home/%D/%U Directory that winbind users have as their home
--winbindtemplateprimarygroup=nobody Group that winbind users have as their primary group
--winbindtemplateshell=/bin/false Shell that winbind users have as their default login shell
--enablewinbindusedefaultdomain Configures winbind to assume that users with no domain in their usernames are domain users
--disablewinbindusedefaultdomain Configures winbind to assume that users with no domain in their usernames are not domain users
--winbindjoin=Administrator Joins the winbind domain or ADS realm as the specified administrator
--enablewinbindoffline Configures winbind to allow offline login
--disablewinbindoffline Configures winbind to prevent offline login
--smbsecurity=user|server|domain|ads Security mode to use for the Samba and Winbind services
--smbrealm=realm Default realm for Samba and Winbind services when security is set to ads
--enablewins Enable Wins for hostname resolution
--disablewins Disable Wins for hostname resolution
--enablesssd Enable SSSD for user information
--disablesssd Disable SSSD for user information
--enablecache Enable nscd
--disablecache Disable nscd
--enablelocauthorize Local authorization is sufficient for local users
--disablelocauthorize Local users are also authorized through a remote service
--enablesysnetauth Authenticate system accounts with network services
--disablesysnetauth Authenticate system accounts with local files only
--enablepamaccess Check /etc/security/access.conf during account authorization
--disablepamaccess Do not check /etc/security/access.conf during account authorization
--enablemkhomedir Create a home directory for a user on the first login
--disablemkhomedir Do not create a home directory for a user on the first login
--enablesmartcard Enable authentication with a smart card
--disablesmartcard Disable authentication with a smart card
--enablerequiresmartcard Require smart card for authentication
--disablerequiresmartcard Do not require smart card for authentication
--smartcardmodule=module Default smart card module to use
--smartcardaction=0=Lock|1=Ignore Action to be taken when smart card removal is detected
--enablefingerprint Enable fingerprint authentication
--disablefingerprint Disable fingerprint authentication
--nostart Do not start or stop the portmap, ypbind, or nscd services even if they are configured
--test Do not update the configuration files, only print the new settings
--update, --kickstart Opposite of --test, update configuration files with changed settings
--updateall Update all configuration files
--probe Probe and display network defaults
--savebackup=name Save a backup of all configuration files
--restorebackup=name Restore a backup of all configuration files
--restorelastbackup Restore the backup of configuration files saved before the previous configuration change

8.2. The System Security Services Daemon (SSSD)

This section provides an introduction to the System Security Services Daemon (SSSD), the main features that it provides, and discusses the requirements and any limitations of a typical SSSD deployment.
This section also describes how to configure SSSD, and how to use the features that it provides. It provides information on the types of services that it supports and how to configure them, and introduces and describes the most important configuration options. Sample configuration files are also provided to help you optimize your deployment.

8.2.1. What is SSSD?

The System Security Services Daemon (SSSD) is a service which provides access to different identity and authentication providers. You can configure SSSD to use a native LDAP domain (that is, an LDAP identity provider with LDAP authentication), or an LDAP identity provider with Kerberos authentication. It provides an NSS and PAM interface to the system, and a pluggable back-end system to connect to multiple different account sources.
SSSD is also extensible; you can configure it to use new identity sources and authentication mechanisms should they arise. In addition, SSSD is fully IPv6-compatible, provided that it is built against c-ares 1.7.1 or later and krb5-libs 1.8.1 or later.

8.2.2. SSSD Features

8.2.2.1. Offline Authentication

One of the primary benefits of SSSD is offline authentication. This solves the case of users having a separate corporate account and a local machine account because of the common requirement to implement a Virtual Private Network (VPN).
SSSD can cache remote identities and authentication credentials. This means that you can still authenticate with these remote identities even when a machine is offline. In an SSSD system, you only need to manage one account.

8.2.2.2. Server Load Reduction

The use of SSSD also helps to reduce the load on identification servers. For example, using nss_ldap, every client application that needs to request user information opens its own connection to the LDAP server. Managing these multiple connections can lead to a heavy load on the LDAP server. In an SSSD system, only the SSSD Data Provider process actually communicates with the LDAP server, reducing the load to one connection per client system.

8.2.2.3. Support for Multiple Domains

You can use SSSD to specify multiple domains of the same type. Compare this to an nsswitch.conf file configuration, with which you can only request user information from a single server of any particular type (LDAP, NIS, etc.). With SSSD, you can create multiple domains of the same, or of different types of identity provider.
Beginning with version 0.6.0, SSSD maintains a separate database file for each domain. This means that each domain has its own cache, and in the event that problems occur and maintenance is necessary, it is very easy to purge the cache for a single domain, by stopping sssd and deleting the corresponding cache file. These cache files are stored in the /var/lib/sss/db/ directory.
All cache files are named according to the domain that they represent, for example cache_DOMAINNAME.ldb.
Considerations Associated with Deleting Cache Files
Deleting a domain's cache file can have some unexpected side effects. You should be aware of the following before you proceed:
  • Deleting the cache file also deletes all user data (both identification and cached credentials). Consequently, you should not proceed unless you are online and can authenticate with your username against the domain's servers, because offline authentication will fail.
  • If you are online and change your configuration to reference a different identity provider, SSSD will recognize users from both providers until the cached entries from the original provider time out.
    To avoid this situation, you can either purge the cache or use a different domain name for the new provider (this is the recommended practice). Changing the domain name means that when you restart SSSD it will create a new cache file (with the new name) and the old file will be ignored.

8.2.2.4. Support for LDAP Referrals

SSSD supports two types of LDAP referrals: object-level referrals and subtree referrals. These referral types and the extent of SSSD support is outlined below.
8.2.2.4.1. Object-level Referrals
SSSD provides full support for object-level referrals within the same LDAP server, correctly handling any differences in the distinguished name (DN) that might exist as part of the LDAP server referral configuration.
SSSD provides partial support for object-level referrals between different LDAP servers, and requires that the full DN of an LDAP request be identical on each server. SSSD does not support referrals to different DN paths on other servers.
8.2.2.4.2. Subtree Referrals
SSSD provides a similar level of support for subtree referrals as it does for object-level referrals. That is, it supports referrals to a changed DN on the local system or to an identical DN on a remote system. The difference with subtree referrals, however, is the ability to set up identical subtrees on each LDAP server and to then configure referrals between these subtrees.
8.2.2.4.3. Enabling LDAP Referrals
To take advantage of the SSSD LDAP referral functionality, you need to set the ldap_referrals option to TRUE in the LDAP domain configuration section of the /etc/sssd/sssd.conf file. This will enable anonymous access to the second LDAP server.

Make sure SSSD is compiled with OpenLDAP version 2.4.13 or later

SSSD only supports LDAP referrals when it is compiled with OpenLDAP version 2.4.13 or later.

8.2.2.5. Differentiating Like-named Users

SSSD supports the differentiation of like-named users in different domains. For example, you can differentiate the user kate in the ldap.example.com domain from the user kate in the ldap.myhome.com domain. You can use SSSD to make requests using fully-qualified usernames. If you request information for kate, you will receive the information from whichever domain is listed first in the look-up order. If you request information for kate@ldap.myhome.com, however, you will receive the correct user information.
SSSD also provides a filter_users option, which you can use to exclude certain users from being fetched from the database. Refer to the sssd.conf(5) manual page for full details about this option.

8.2.2.6. Integration with IPA

Beyond the offline authentication, multiple domain management and other features already described, SSSD is also designed to integrate with and enhance the functionality of IPA clients. In an environment with the latest version of IPA installed, SSSD provides additional functionality, including support for dynamic DNS updates, host-based access control, and password migration from an LDAP-only environment into the LDAP/Kerberos 5 environment employed by IPA.
8.2.2.6.1. Support for Dynamic DNS Updates
Because the IP address of IPA clients can change, SSSD provides the ability to dynamically update the client's DNS entry on the IPA server. Using a combination of Kerberos and GSS-TSIG (Generic Security Service Algorithm for Secret Key Transaction), IPA can determine the identity of the host machine, authenticate it, and allow that machine to edit its own DNS record. These changes are then stored in the LDAP back end.

Each IPA client can only edit its own DNS record

Using this authentication system means that each IPA client can only edit its own DNS record; it cannot edit the DNS record of any other client.
Setting up Dynamic DNS Updates
The SSSD configuration file provides two options used for setting up dynamic DNS updates: ipa_dyndns_update, used to enable dynamic DNS updates; and ipa_dyndns_iface, which specifies the interface whose IP address should be used for dynamic DNS updates.
Refer to the sssd-ipa manual page for more information about these options, and how to configure dynamic DNS updates.

Support for dynamic DNS updates

Support for dynamic DNS updates is only available on IPA version 2 or later, and with DNS correctly configured.

8.2.3. Setting Up SSSD

This section describes how to install SSSD, how to run the service, and how to configure it for each type of supported information provider.

8.2.3.1. Installing SSSD

Run the following command to install SSSD and any dependencies, including the SSSD client:
# yum install sssd
SSSD requires very few dependencies and should install very quickly, depending on the speed of your network connection.
8.2.3.1.1. Upgrading from a Previous Version
Upgrading Using RPM Packages
If you are upgrading using RPM packages, the script will run automatically when you upgrade to the new version. This will upgrade the /etc/sssd/sssd.conf file to the new format, and copy the existing version to /etc/sssd/sssd.conf.bak.
Upgrading Manually
It may be necessary to run the upgrade script manually, either because you built SSSD from source files, or because you are using a platform that does not support the use of RPM packages. The synopsis for the script is as follows:

upgrade_config.py [ -f INFILE ] [ -o OUTFILE ] [ -verbose ] [ --no-backup ]

  • -f INFILE — the configuration file to upgrade. If not specified, this defaults to /etc/sssd/sssd.conf
  • -o OUTFILE — the name of the upgraded configuration file. If not specified, this defaults to /etc/sssd/sssd.conf
  • -verbose — produce more verbose output during the upgrade process
  • --no-backup — do not produce a back-up file. If not specified, this defaults to INFILE.bak
8.2.3.1.2. Starting and Stopping SSSD

Starting SSSD for the first time

Before you start SSSD for the first time, you need to configure at least one domain. Refer to Section 8.2.5, “Configuring Domains” for information on how to configure an SSSD domain.
You can use either the service command or the /etc/init.d/sssd script to control SSSD. For example, run the following command to start sssd:
# systemctl start sssd.service
By default, SSSD is configured not to start automatically. There are two ways to change this behavior; if you use the Authentication Configuration tool to configure SSSD, it will reconfigure the default behavior so that SSSD starts when the machine boots. Alternatively, you can use the systemctl command, as follows:
# systemctl enable sssd.service

8.2.3.2. Configuring SSSD

The global configuration of SSSD is stored in the /etc/sssd/sssd.conf file. This file consists of various sections, each of which contains a number of key/value pairs. Some keys accept multiple values; use commas to separate multiple values for such keys. This configuration file uses data types of string (no quotes required), integer and Boolean (with values of TRUE or FALSE). Comments are indicated by either a hash sign (#) or a semicolon (;) in the first column. The following example illustrates some of this syntax:
[section]
# Keys with single values
key1 = value
key2 = val2

# Keys with multiple values
key10 = val10,val11

Specifying a different configuration file

You can use the -c (or --config) parameter on the command line to specify a different configuration file for SSSD.
The format of the configuration file is described in Section 8.2.9, “SSSD Configuration File Format”
Refer to the sssd.conf(5) manual page for more information on global SSSD configuration options.
8.2.3.2.1. Configuring NSS
SSSD provides a new NSS module, sssd_nss, so that you can configure your system to use SSSD to retrieve user information. Edit the /etc/nsswitch.conf file for your system to use the sss name database. For example:
passwd: files sss
group: files sss
8.2.3.2.2. Configuring PAM

Be careful when changing your PAM configuration

Use extreme care when changing your PAM configuration. A mistake in the PAM configuration file can lock you out of the system completely. Always back up your configuration files before performing any changes, and keep a session open so that you can revert any changes you make should the need arise.
To enable your system to use SSSD for PAM, you need to edit the default PAM configuration file. On Fedora—based systems, this is the /etc/pam.d/system-auth file. Edit this file to reflect the following example, and then restart sssd:
#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth        required      pam_env.so
auth        sufficient    pam_unix.so nullok try_first_pass
auth        requisite     pam_succeed_if.so uid >= 500 quiet
auth        sufficient    pam_sss.so use_first_pass
auth        required      pam_deny.so

account     required      pam_unix.so broken_shadow
account     sufficient    pam_localuser.so
account     sufficient    pam_succeed_if.so uid < 500 quiet
account [default=bad success=ok user_unknown=ignore] pam_sss.so
account     required      pam_permit.so

password    requisite     pam_cracklib.so try_first_pass retry=3
password    sufficient    pam_unix.so sha512 shadow nullok try_first_pass use_authtok
password    sufficient    pam_sss.so use_authtok
password    required      pam_deny.so

session     required      pam_mkhomedir.so umask=0022 skel=/etc/skel/
session     optional      pam_keyinit.so revoke
session     required      pam_limits.so
session     [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid
session     sufficient    pam_sss.so
session     required      pam_unix.so
8.2.3.2.2.1. Using Custom Home Directories with SSSD
If your LDAP users have home directories that are not in /home, and if your system is configured to create home directories the first time your users log in, then these directories will be created with the wrong permissions. For example, instead of a typical home directory such as /home/<username>, your users might have home directories that include their locale, such as /home/<locale>/<username>. If this is true for your system, the following steps need to be taken (preemptively):
  1. Apply the correct SELinux context and permissions from the /home directory to the home directory that you use on your system. In the example above, the following command would achieve this result (replace the directory names with those that apply to your system):
    # semanage fcontext -a -e /home /home/locale
  2. Ensure the oddjob-mkhomedir package is installed on your system and then re-run the Authentication Configuration tool.
    This package provides the pam_oddjob_mkhomedir.so library, which the Authentication Configuration tool will then use to create your custom home directories. You need to use this library to create your home directories, and not the default pam_mkhomedir.so library, because the latter cannot create SELinux labels.

    The pam_oddjob_mkhomedir and pam_mkhomedir libraries

    The Authentication Configuration tool will automatically use the pam_oddjob_mkhomedir.so library if it is available. Otherwise, it will default to using pam_mkhomedir.so.
If the preceding steps were not performed before the custom home directories were created, you can use the following commands to correct the permissions and SELinux contexts (again, replace the directory names with those that apply to your system):
# semanage fcontext -a -e /home /home/locale
# restorecon -R -v /home/locale
8.2.3.2.2.2. Using "include" Statements in PAM Configurations
Recent PAM implementations allow you to use include statements in PAM configurations. For example:
...
session     include      system-auth
session     optional     pam_console.so
...
In the preceding example, if a sufficient condition from system-auth returns PAM_SUCCESS, pam_console.so will not be executed.
8.2.3.2.3. Configuring Access Control
SSSD provides a rudimentary access control mechanism, offering two solutions based on the value of the access_provider option in the [domain/<NAME>] section in the /etc/sssd/sssd.conf file.
8.2.3.2.3.1. The Simple Access Provider
The first of these solutions is known as the Simple Access Provider, and is based on the implementation of access or deny lists of usernames. To enable the Simple Access Provider, you need to set the access_provider option to simple, and then add usernames as a comma-separated list to either the simple_allow_users or simple_deny_users options.
Using the Simple Access Provider
By using the Simple Access Provider, you can continue to support a number of network logins to maintain common network accounts on company or department laptops, but you might want to restrict the use of a particular laptop to one or two users. This means that even if a different user authenticated successfully against the same authentication provider, the Simple Access Provider would prevent that user from gaining access.
The following example demonstrates the use of the Simple Access Provider to grant access to two users. This example assumes that SSSD is correctly configured and example.com is one of the domains specified in the [sssd] section, and only shows the Simple Access Provider-specific options.
[domain/example.com]
access_provider = simple
simple_allow_users = user1, user2

Using the Local ID provider

The Local ID provider does not support simple as an access provider.
Access Control Rules
The Simple Access Provider adheres to the following three rules to determine which users should or should not be granted access:
  • If both lists are empty, access is granted.
  • If simple_allow_users is set, only users from this list are allowed access. This setting supersedes the simple_deny_users list (which would be redundant).
  • If the simple_allow_users list is empty, users are allowed access unless they appear in the simple_deny_users list.

Do not define both simple_allow_users and simple_deny_users

Defining both simple_allow_users and simple_deny_users is a configuration error. If this occurs, SSSD will output an error to the /var/log/sssd/sssd_default.log log file when loading the back end, but continue to start normally. Future versions of SSSD will output an error and fail to start.
8.2.3.2.3.2. The LDAP Access Provider
The second access control solution uses the LDAP server itself as the access provider (access_provider=ldap) and the associated filter option (ldap_access_filter) to specify which users are granted access to the specified host. Note that these two options are codependent; if you use LDAP as your access provider then you must specify a value for the ldap_access_filter option, otherwise all users will be denied access. If you are not using LDAP as your access provider, then the ldap_access_filter option has no effect.
Using the LDAP Access Provider
The following example demonstrates the use of the LDAP Access Provider to grant access to members of the "allowedusers" group in LDAP. This example assumes that SSSD is correctly configured and example.com is one of the domains specified in the [sssd] section, and only shows the LDAP Access Provider-specific options.
[domain/example.com]
access_provider = ldap
ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com

Using offline caching

Offline caching for this feature is limited to determining whether or not the user's last online login attempt was successful. If they were granted access during their last login, they will continue to be granted access while offline, and vice-versa.
Refer to the sssd-ldap manual page for more information about using the LDAP Access Provider.
8.2.3.2.4. Configuring Failover
The failover feature allows back ends to automatically switch to a different server if the primary server fails. These servers are entered as a case-insensitive, comma-separated list in the [domain/<NAME>] sections of the /etc/sssd/sssd.conf file, and listed in order of preference. This list can contain any number of servers.
For example, if you have configured a native LDAP domain, you could specify the following as your ldap_uri values:
ldap_uri = ldap://ldap0.mydomain.org, ldap://ldap1.mydomain.org, ldap://ldap2.mydomain.org
In this configuration, ldap://ldap0.mydomain.org functions as the primary server. If this server fails, the SSSD failover mechanism first attempts to connect to ldap1.mydomain.org, and if that server is unavailable, it then attempts to connect to ldap2.mydomain.org.
If the parameter that specifies which server to connect to for the specific domain (for example, ldap_uri, krb5_server, …) is not specified, the back end defaults to using Use service discovery. Refer to Section 8.2.3.2.4.1, “Using SRV Records with Failover” for more information on service discovery.

Do not use multiple ldap_uri parameters

Do not use multiple ldap_uri parameters to specify your failover servers. The failover servers must be entered as a comma-separated list of values for a single ldap_uri parameter. If you enter multiple ldap_uri parameters, SSSD only recognizes the last entry.
Future versions of SSSD will throw an error upon receiving additional ldap_uri entries.
8.2.3.2.4.1. Using SRV Records with Failover
SSSD also supports the use of SRV records in its failover configuration. This means that you can specify a server that is later resolved into a list of specific servers using SRV requests. The priority and weight attributes of SRV records provide further opportunity for specifying which servers should be contacted first in the event that the primary server fails.
For every service with which you want to use service discovery, you need to add a special DNS record to your DNS server using the following form:
_service._protocol._domain TTL priority weight port hostname
A typical configuration would contain multiple such records, each with a different priority (for failover) and different weights (for load balancing).
The client then makes an SRV DNS query to retrieve a list of host names, their priorities, and weights. These queries are of the form _service._protocol._domain, for example, _ldap._tcp._redhat.com. The client then sorts this list according to the priorities and weights, and connects to the first server in this sorted list.
For more information on SRV records, refer to RFC 2782.
8.2.3.2.4.2. How the Failover Mechanism Works
The failover mechanism distinguishes between machines and services. The back end first tries to resolve the hostname of a given machine; if this resolution attempt fails, the machine is considered offline. No further attempts are made to connect to this machine for any other service. If the resolution attempt succeeds, the back end tries to connect to a service on this machine. If the service connection attempt fails, then only this particular service is considered offline and the back end automatically switches over to the next service. The machine is still considered online and might still be tried for another service.
The failover mechanism does not handle DNS A records with multiple IP addresses; instead it only uses the first address. DNS round-robin cannot be used for failover. Further, providing multiple A records does not provide failover. Only the first A record is used, and if a lookup attempt on the first record fails then the system attempts no further lookups. To find multiple servers with a single request, and thus implementing failover, SSSD relies on SRV resource records, as explained in Section 8.2.3.2.4.1, “Using SRV Records with Failover”.
Further connection attempts are made to machines or services marked as offline after a specified period of time; this is currently hard coded to 30 seconds. If there are no more machines to try, the back end as a whole switches to offline mode, and then attempts to reconnect every 30 seconds.

8.2.4. Configuring Services

Individual pieces of SSSD functionality are provided by special SSSD services that are started and stopped together with SSSD. The services provided by SSSD have their own configuration sections. The [sssd] section also lists the services that are active and should be started when sssd starts within the services directive.
SSSD currently provides several services:
  • NSS — An NSS provider service that answers NSS requests from the sssd_nss module.
  • PAM — A PAM provider service that manages a PAM conversation through the sssd_pam PAM module.
  • monitor — A special service that monitors all other SSSD services, and starts or restarts them as needed. Its options are specified in the [sssd] section of the /etc/sssd/sssd.conf configuration file.

8.2.4.1. Configuration Options

The following sections cover the most important SSSD configuration options. Refer to the sssd.conf(5) manual page that ships with SSSD for information on all the available configuration options.
8.2.4.1.1. General Configuration Options
  • debug_level (integer)
    Sets the debug level for a particular service. This is a per-service setting (that is, it can appear in any of the [service/<NAME>] sections in the SSSD configuration file).
  • reconnection_retries (integer)
    In the event of a data provider crash or restart, this specifies the number of times that a service should attempt to reconnect.

    DNS lookup of IPv6 addresses

    If a DNS lookup fails to return an IPv4 address for a hostname, SSSD attempts to look up an IPv6 address before returning a failure. Note that this only ensures that the async resolver identifies the correct address; there is currently a bug in the LDAP code that prevents SSSD from connecting to an LDAP server over IPv6. This is being investigated separately.
8.2.4.1.2. NSS Configuration Options
Use the following options to configure the Name Service Switch (NSS) service. Refer to the sssd.conf(5) manual page for full details about each option.
  • enum_cache_timeout (integer)
    Specifies for how long (in seconds) sssd_nss should cache enumerations (requests for information about all users).
  • entry_cache_nowait_percentage (integer)
    Specifies for how long sssd_nss should return cached entries before initiating an out-of-band cache refresh (0 disables this feature).
    You can configure the entry cache to automatically update entries in the background if they are requested beyond a percentage of the entry_cache_timeout value for the domain.
    Valid values for this option are 0-99, and represent a percentage of the entry_cache_timeout value for each domain.
  • entry_negative_timeout (integer)
    Specifies for how long (in seconds) sssd_nss should cache negative cache hits (that is, queries for invalid database entries, such as nonexistent ones) before asking the back end again.
  • filter_users, filter_groups (string)
    Exclude certain users from being fetched from the sss NSS database. This is particularly useful for system accounts such as root.
  • filter_users_in_groups (Boolean)
    If set to TRUE, specifies that users listed in the filter_users list do not appear in group memberships when performing group lookups. If set to FALSE, group lookups return all users that are members of that group. If not specified, defaults to TRUE.
8.2.4.1.3. PAM Configuration Options
Use the following options to configure the Pluggable Authentication Module (PAM) service.
  • offline_credentials_expiration (integer)
    If the authentication provider is offline, specifies for how long to allow cached log-ins (in days). This value is measured from the last successful online log-in. If not specified, defaults to 0 (no limit).
  • offline_failed_login_attempts (integer)
    If the authentication provider is offline, specifies how many failed log in attempts are allowed. If not specified, defaults to 0 (no limit).
  • offline_failed_login_delay (integer)
    Specifies the time in minutes after the value of offline_failed_login_attempts has been reached before a new log in attempt is possible.
    If set to 0, the user cannot authenticate offline if the value of offline_failed_login_attempts has been reached. Only a successful online authentication can re-enable offline authentication. If not specified, defaults to 5.

8.2.5. Configuring Domains

A domain is a database of user information. SSSD can use more than one domain at the same time, but at least one must be configured for SSSD to start. Using SSSD domains, it is possible to use several LDAP servers providing several unique namespaces. You can specify not only where users' identity information is stored, but how users authenticate against each of the specified domains.
SSSD supports the following identity and authentication combinations:
LDAP/LDAP
This combination uses an LDAP back end as both the identity and authentication provider. For more information, refer to Section 8.2.5.2, “Configuring an LDAP Domain”.
LDAP/KRB5
This combination uses an LDAP back end as the identity provider, and uses Kerberos to provide authentication. For more information, refer to Section 8.2.6, “Setting Up Kerberos Authentication”.
proxy
Specifying a proxy identity or an authentication provider uses an existing NSS library or a customized PAM stack, but takes advantage of the SSSD caching mechanism. For more information, refer to Section 8.2.7, “Configuring a Proxy Domain”.
The following example assumes that SSSD is correctly configured and FOO is one of the domains in the [sssd] section. This example shows only the configuration of Kerberos authentication; it does not include any identity provider.
[domain/FOO]
auth_provider = krb5
krb5_server = 192.168.1.1
krb5_realm = EXAMPLE.COM

8.2.5.1. Domain Configuration Options

You can add new domain configurations to the [domain/<NAME>] sections of the /etc/sssd/sssd.conf file, and then add the list of domains to the domains attribute of the [sssd] section, in the order you want them to be queried.
8.2.5.1.1. General Domain Configuration Options
You can use the following configuration options in a domain configuration section:
  • min_id,max_id (integer)
    Specifies the UID and GID limits for the domain. If a domain contains entries that are outside these limits, they are ignored.
    The default value for min_id is 1; the default value for max_id is 0 (unbounded).

    Avoid conflicts with users in /etc/passwd

    If min_id is unspecified, it defaults to 1 for any back end. This default was chosen to provide compatibility with existing systems and to ease any migration attempts. LDAP administrators should be aware that granting identities in this range may conflict with users in the local /etc/passwd file. To avoid these conflicts, min_id should be set to 1000 or higher wherever possible.
    The min_id option determines the minimum acceptable value for both UID and GID numbers. Accounts with either UID or GID values below the min_id value are filtered out and not made available on the client.
  • enumerate (Boolean)
    Specifies whether or not to enumerate (list) the users and groups of a domain.
    Enumeration means that the entire set of available users and groups on the remote source is cached on the local machine. When enumeration is disabled, users and groups are only cached as they are requested.

    Disable enumeration for domains with many users and groups

    If a client has enumeration enabled, reinitialization of the client results in a complete refresh of the entire set of available users and groups from the remote source. Similarly, when SSSD is connected to a new server, the entire set of available users and groups from the remote source is pulled and cached on the local machine. In a domain with a large amount of clients connected to a remote source, both aforementioned cases can affect the network performance due to frequent queries from the clients. If the set of available users and groups is large enough, it will affect the performance of clients as well. For performance reasons, it is recommended that you disable enumeration for domains with many users and groups.
    The default value for this parameter is FALSE. Set this value to TRUE to enable enumeration of users and groups of a domain.
  • timeout (integer)
    Specifies the timeout in seconds for this particular domain.
    This is used to ensure that the back end process is alive and capable of answering requests. The default value for this parameter is 10 seconds. Raising this timeout might prove useful for slower back ends, such as distant LDAP servers.

    Changing the timeout value to 0

    If you set timeout = 0, SSSD reverts to the default value; you cannot force a timeout value of zero, because this would force the sssd daemon into a loop.
  • cache_credentials (Boolean)
    Specifies whether or not to store user credentials in the local SSSD domain database cache.
    The default value for this parameter is FALSE. You should set this value to TRUE for domains other than local if you want to enable offline authentication.
  • id_provider (string)
    Specifies the data provider identity back end to use for this domain. Currently supported identity back ends are:
    • proxy — Support a legacy NSS provider (for example, nss_nis).

      Changing the id_provider value to proxy

      SSSD needs to know which legacy NSS library to load in order to start successfully. If you set id_provider to proxy, ensure that you also specify a value for proxy_lib_name. Refer to Section 8.2.7, “Configuring a Proxy Domain” for information on this attribute.
    • local — SSSD internal local provider.
    • ldap — LDAP provider.
  • entry_cache_timeout (integer)
    Specifies for how long the domain's data provider should cache positive cache hits (that is, queries for valid database entries) before asking the database again.
  • use_fully_qualified_names (Boolean)
    Specifies whether or not requests to this domain require fully-qualified domain names.
    If set to TRUE, all requests to this domain must use fully-qualified domain names. It also means that the output from the request displays the fully-qualified name.
    The ability to restrict requests in this way means that if you know you have multiple domains with conflicting usernames, then there is no doubt about which username the query will resolve.
    Consider the following examples, in which the IPA domain database contains a user named ipauser01, and the use_fully_qualified_names attribute is set to TRUE:
    # getent passwd ipauser01
    [no output]
    # getent passwd ipauser01@IPA
    ipauser01@IPA:x:937315651:937315651:ipauser01:/home/ipauser01:/bin/sh
    
    In the following examples, using the same IPA domain and user, the use_fully_qualified_names attribute is set to FALSE:
    # getent passwd ipauser01
    ipauser01:x:937315651:937315651:ipauser01:/home/ipauser01:/bin/sh
    # getent passwd ipauser01@IPA
    ipauser01:x:937315651:937315651:ipauser01:/home/ipauser01:/bin/sh
    

    Changing the use_fully_qualified_names value to FALSE

    If use_fully_qualified_names is set to FALSE, you can continue to use the fully-qualified name in your requests, but only the simplified version is displayed in the output.
    SSSD can only parse name@domain, not name@realm. You can, however, use the same name for both your domain and your realm.
  • auth_provider (string)
    The authentication provider used for the domain. The default value for this option is the value of id_provider if it is set and can handle authentication requests.
    Currently supported authentication providers are:
    • ldap — for native LDAP authentication. Refer to the sssd-ldap(5) manual page for more information on configuring LDAP.
    • krb5 — for Kerberos authentication. Refer to the sssd-krb5(5) manual page for more information on configuring Kerberos.
    • proxy — for relaying authentication to some other PAM target.
    • none — explicitly disables authentication.
8.2.5.1.2. Proxy Configuration Options
  • proxy_pam_target (string)
    This option is only used when the auth_provider option is set to proxy, and specifies the target to which PAM must proxy.
    This option has no default value. If proxy authentication is required, you need to specify your own PAM target. This corresponds to a file containing PAM stack information in the system's default PAM configuration directory. On Fedora-based systems, this is the /etc/pam.d/ directory.

    Avoid recursive inclusion of pam_sss

    Ensure that your proxy PAM stack does not recursively include pam_sss.so.
  • proxy_lib_name (string)
    This option is only used when the id_provider option is set to proxy, and specifies which existing NSS library to proxy identity requests through.
    This option has no default value. You need to manually specify an existing library to take advantage of this option. For example, set this value to nis to use the existing libnss_nis.so file.

8.2.5.2. Configuring an LDAP Domain

An LDAP domain is one where the id_provider option is set to ldap (id_provider = ldap). Such a domain requires a running LDAP server against which to authenticate. This can be an open source LDAP server such as OpenLDAP or Microsoft Active Directory. SSSD currently supports Microsoft Active Directory 2003 (+Services for UNIX) and Active Directory 2008 (+Subsystem for UNIX-based Applications). In all cases, the client configuration is stored in the /etc/sssd/sssd.conf file.
How to Authenticate Against an LDAP Server
SSSD does not support authentication over an unencrypted channel. Consequently, if you want to authenticate against an LDAP server, either TLS/SSL or LDAPS is required. If the LDAP server is used only as an identity provider, an encrypted channel is not needed.
Edit your /etc/sssd/sssd.conf file to include the following settings:
# A native LDAP domain
[domain/LDAP]
enumerate = false
cache_credentials = TRUE

id_provider = ldap
auth_provider = ldap
ldap_schema = rfc2307
chpass_provider = ldap

ldap_uri = ldap://ldap.mydomain.org
ldap_search_base = dc=mydomain,dc=org
ldap_tls_reqcert = demand
ldap_tls_cacert = /etc/pki/tls/certs/ca-bundle.crt

Creating a certificate with an IP address instead of the server name

If you wish to use an IP address in the ldap_uri option instead of the server name, for example, if GSSAPI is used to avoid time consuming DNS lookups, the TSL/SSL setup might fail. This is due to the fact that TSL/SSL certificates contain the server name only. However, a special field in the certificate, called Subject Alternative Name (subjectAltName), can be used to additionally set the IP address of the server.
The following steps show how to create a certificate with a Subject Alternative Name being the IP address of your server:
  1. Using your command line, execute the following command to convert an existing certificate (previously signed by the key.pem key) into a certificate request:
    openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey key.pem
    Alternatively, if you are using a self-signed certificate(for example, one created by the Fedora OpenLDAP package in /etc/pki/tls/certs/slapd.pem), execute the following command:
    openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey old_cert.pem
  2. Edit your /etc/pki/tls/openssl.cnf configuration file to include the following line under the [ v3_ca ] section:
    subjectAltName = IP:10.0.0.10
    Replace the IP address with one of your choice.
  3. By executing the following command, use the previously generated certificate request to generate a new self-signed certificate that will contain your desired IP address:
    openssl x509 -req -in req.pem -out new_cert.pem -extfile ./openssl.cnf -extensions v3_ca -signkey old_cert.pem
    where:
    • The openssl x509 command creates the new certificate.
    • The -req option tells the command to expect a certificate request as an input.
    • The -in and -out options specify the input and output files.
    • The -extfile option expects a file containing certificate extensions to use (in our case the subjectAltName extension).
    • The -extensions option specifies the section of the openssl.cnf file to add certificate extensions from (in this case, the [ v3_ca ] section).
    • The -signkey option tells the command to self-sign the input file using the supplied private key.
    For more information on the x509 utility and its parameters, refer to man x509.
  4. Lastly, copy the private key block from the old_cert.pem file into the new_cert.pem file to keep all relevant information in one file.
When creating a certificate through the certutil utility provided by the nss-utils package, note that certutil supports DNS subject alternative names for certificate creation only.
It is advisable to use a Certificate Authority to issue your certificate. Consider using the Red Hat Certificate System; for more information on managing subject names and subject alternative names in your certificate, refer to the Red Hat Certificate System Admin Guide.
Selecting an LDAP Schema
You can set the ldap_schema attribute to either rfc2307 or rfc2307bis. These schema define how groups in LDAP are specified. In RFC 2307, group objects use a multi-valued attribute, memberuid, which lists the names of the users that belong to that group. In RFC 2307bis, instead of the memberuid, group objects use the member attribute. Rather than just the name of the user, this attribute contains the full Distinguished Name (DN) of another object in the LDAP database. This means that groups can have other groups as members. That is, it adds support for nested groups.
SSSD assumes that your LDAP server is using RFC 2307. If your LDAP server is using RFC 2307bis, and you do not update the /etc/sssd/sssd.conf file accordingly, this can impact how your users and groups are displayed. It also means that some groups will not be available and network resources may be inaccessible even though you have permissions to use them.
For example, when using RFC 2307bis and you have configured both primary and secondary groups, or are using nested groups, you can use the id command to display these groups:
[f12server@ipaserver ~]$ id
uid=500(f12server) gid=500(f12server) groups=500(f12server),510(f12tester)
If instead you have configured your client to use RFC 2307 then only the primary group is displayed.
Changes to this setting only affect how SSSD determines the groups to which a user belongs; there is no negative effect on the actual user data. If you do not know the correct value for this attribute, consult your System Administrator.
Specifying Timeout Values
SSSD supports a number of timeout values that apply when configuring an LDAP domain. These are described below.
  • ldap_search_timeout (integer) — Specifies the timeout (in seconds) that LDAP searches are allowed to run before they are canceled and cached results are returned (and offline mode is entered). If not specified:
    Defaults to five when enumerate = False
    Defaults to 30 when enumerate = True. This option is forced to a minimum of 30 in this case.

    The ldap_network_timeout option is going to be changed

    This option is subject to change in future versions of SSSD, where it may be replaced by a series of timeouts for specific look-up types.
  • ldap_network_timeout (integer) — Specifies the timeout (in seconds) after which the poll(2)/select(2) following a connect(2) returns in case of no activity.
    If not specified, defaults to five.
  • ldap_opt_timeout (integer) — Specifies the timeout (in seconds) after which calls to synchronous LDAP APIs will abort if no response is received. This option also controls the timeout when communicating with the KDC in case of a SASL bind.
    If not specified, defaults to five.

DNS Service Discovery

The DNS service discovery feature allows the LDAP back end to automatically find the appropriate DNS servers to connect to using a special DNS query. For more information on the DNS service discovery feature, refer to Section 8.2.3.2.4.1, “Using SRV Records with Failover”.

8.2.5.3. Configuring a Microsoft Active Directory Domain

You can configure SSSD to use Microsoft Active Directory as an LDAP back end, providing both identity and authentication services. If you are using Active Directory 2003, SSSD requires that you install Windows Services for UNIX (SFU) on the machine where Active Directory is installed. If instead you are using Active Directory 2008, you need to install the Subsystem for UNIX-based Applications (SUA) on the Active Directory machine.

SFU is not supported on 64-bit systems

SFU is not supported on 64-bit operating systems. Refer to http://support.microsoft.com/kb/920751 for more information about which Windows systems can provide a suitable platform for an SSSD LDAP back end.
8.2.5.3.1. Configuring Active Directory 2003 as an LDAP Back End
The example /etc/sssd/sssd.conf file that ships with SSSD contains the following sample configuration for Active Directory 2003:
# Example LDAP domain where the LDAP server is an Active Directory 2003 server.

[domain/AD]
description = LDAP domain with AD server
enumerate = false
min_id = 1000
;
id_provider = ldap
auth_provider = ldap
ldap_uri = ldap://your.ad.server.com
ldap_schema = rfc2307bis
ldap_search_base = dc=example,dc=com
ldap_default_bind_dn = cn=Administrator,cn=Users,dc=example,dc=com
ldap_default_authtok_type = password
ldap_default_authtok = YOUR_PASSWORD
ldap_user_object_class = person
ldap_user_name = msSFU30Name
ldap_user_uid_number = msSFU30UidNumber
ldap_user_gid_number = msSFU30GidNumber
ldap_user_home_directory = msSFU30HomeDirectory
ldap_user_shell = msSFU30LoginShell
ldap_user_principal = userPrincipalName
ldap_group_object_class = group
ldap_group_name = msSFU30Name
ldap_group_gid_number = msSFU30GidNumber
This configuration is specific to Windows Active Directory 2003. Refer to Section 8.2.5.3.2, “Configuring Active Directory 2003 R2 and 2008 as LDAP Back Ends” for information on how to configure Active Directory 2003 R2 and Active Directory 2008.
Note that the above configuration assumes that the certificates are stored in the default location (that is, in /etc/openldap/cacerts) and that the c_rehash function has been used to create the appropriate symlinks.
More Information
Refer to the sssd-ldap(5) manual page for a full description of all the options that apply to LDAP domains.
8.2.5.3.2. Configuring Active Directory 2003 R2 and 2008 as LDAP Back Ends
The configuration of /etc/sssd/sssd.conf to support Active Directory 2003 R2 or Active Directory 2008 as a back end is similar to that for AD 2003. The following example configuration highlights the necessary changes.
# Example LDAP domain where the LDAP server is an Active Directory 2003 R2 or an Active Directory 2008 server.

[domain/AD]
description = LDAP domain with AD server
; debug_level = 9
enumerate = false

id_provider = ldap
auth_provider = ldap
chpass_provider = ldap

ldap_uri = ldap://your.ad.server.com
ldap_tls_cacertdir = /etc/openldap/cacerts
ldap_tls_cacert = /etc/openldap/cacerts/test.cer
ldap_search_base = dc=example,dc=com
ldap_default_bind_dn = cn=Administrator,cn=Users,dc=example,dc=com
ldap_default_authtok_type = password
ldap_default_authtok = YOUR_PASSWORD
ldap_pwd_policy = none
ldap_user_object_class = user
ldap_group_object_class = group
Note that the above configuration assumes that the certificates are stored in the default location (that is, in /etc/openldap/cacerts) and that the c_rehash function has been used to create the appropriate symlinks.

8.2.6. Setting Up Kerberos Authentication

In order to set up Kerberos authentication, you need to know the address of your key distribution center (KDC) and the Kerberos domain. The client configuration is then stored in the /etc/sssd/sssd.conf file.
The Kerberos 5 authentication back end does not contain an identity provider and must be paired with one in order to function properly (for example, id_provider = ldap). Some information required by the Kerberos 5 authentication back end must be supplied by the identity provider, such as the user's Kerberos Principal Name (UPN). The identity provider configuration should contain an entry to specify this UPN. Refer to the manual page for the applicable identity provider for details on how to configure the UPN.
If the UPN is not available in the identity back end, SSSD will construct a UPN using the format username@krb5_realm.
SSSD assumes that the Kerberos KDC is also a Kerberos kadmin server. However, it is very common for production environments to have multiple, read-only replicas of the KDC, but only a single kadmin server (because password changes and similar procedures are comparatively rare). To manage this type of configuration, you can use the krb5_kpasswd option to specify where your password changing service is running, or if it is running on a non-default port. If the krb5_kpasswd option is not defined, SSSD tries to use the Kerberos KDC in order to change the password. Refer to the sssd-krb5(5) manual page for more information about this and all Kerberos configuration options.
How to Set Up Kerberos Authentication
Edit your /etc/sssd/sssd.conf file to include the following settings:
# A domain with identities provided by LDAP and authentication by Kerberos
[domain/KRBDOMAIN]
enumerate = false
id_provider = ldap
chpass_provider = krb5
ldap_uri = ldap://ldap.mydomain.org
ldap_search_base = dc=mydomain,dc=org
tls_reqcert = demand
ldap_tls_cacert = /etc/pki/tls/certs/ca-bundle.crt

auth_provider = krb5
krb5_server = 192.168.1.1
krb5_realm = EXAMPLE.COM
krb5_changepw_principal = kadmin/changepw
krb5_ccachedir = /tmp
krb5_ccname_template = FILE:%d/krb5cc_%U_XXXXXX
krb5_auth_timeout = 15
This example describes the minimum options that must be configured when using Kerberos authentication. Refer to the sssd-krb5(5) manual page for a full description of all the options that apply to configuring Kerberos authentication.

DNS Service Discovery

The DNS service discovery feature allows the Kerberos 5 authentication back end to automatically find the appropriate DNS servers to connect to using a special DNS query. For more information on the DNS service discovery feature, refer to Section 8.2.3.2.4.1, “Using SRV Records with Failover”.

8.2.6.1. Setting up SASL/GSSAPI Authentication

GSSAPI (Generic Security Services Application Programming Interface) is a supported SASL (Simple Authentication and Security Layer) authentication method. Kerberos is currently the only commonly used GSSAPI implementation. An LDAP client and an LDAP server use SASL to take advantage of GSSAPI as the authentication method (an alternative to plain text passwords, etc.). The GSSAPI plug-in for SASL is then invoked on the client and server side to use Kerberos to communicate.
Using GSSAPI protected communication for LDAP is an advanced configuration not supported by the Authentication Configuration tool; the following steps show how to manually configure it.

Setting up the SASL/GSSAPI authentication on Fedora 6.0

The following setup works correctly on all Fedora 6.1 systems and any systems released after it. However, when using Fedora 6.0, you must correctly configure the default_realm option in the [libdefaults] section and kdc option for your realm in the [realms] section in the /etc/krb5.conf configuration file not only on the directory server and the KDC but also on the client running SSSD. For more information on various /etc/krb5.conf options, refer to man krb5.conf
On the KDC
  1. Using kadmin, set up a Kerberos service principal for the directory server. Use the -randkey option for the kadmin's addprinc command to create the principal and assign it a random key:
    kadmin: addprinc -randkey ldap/server.example.com
  2. Use the ktadd command to write the service principal to a file:
    kadmin: ktadd -k /root/ldap.keytab ldap/server.example.com
  3. Using kadmin, set up a Kerberos host principal for the client running SSSD. Use the -randkey option for the kadmin's addprinc command to create the principal and assign it a random key:
    kadmin: addprinc -randkey host/client.example.com
  4. Use the ktadd command to write the host principal to a file:
    kadmin: ktadd -k /root/client.keytab host/client.example.com
On the Directory Server
Complete the following steps for a directory server of your choice:
OpenLDAP
  1. Copy the previously created /root/ldap.keytab file from the KDC to the /etc/openldap/ directory and name it ldap.keytab.
  2. Make the /etc/openldap/ldap.keytab file read-writable for the ldap user and readable for the ldap group only.
Red Hat Directory Server
  1. Copy the previously created /root/ldap.keytab file from the KDC to the /etc/dirsrv/ directory and name it ldap.keytab.
  2. Uncomment the KRB5_KTNAME line in the /etc/sysconfig/dirsrv (or instance-specific) file, and set the keytab location for the KRB5_KTNAME variable. For example:
    # In order to use SASL/GSSAPI the directory
    # server needs to know where to find its keytab
    # file - uncomment the following line and set
    # the path and filename appropriately
    KRB5_KTNAME=/etc/dirsrv/ldap.keytab; export KRB5_KTNAME
On the Client
  1. Copy the previously created /root/client.keytab file from the KDC to the /etc/ directory and name it krb5.keytab. If the /etc/krb5.keytab file exists already, use the ktutil utility to merge both files properly. For more information on the ktutil utility, refer to man ktutil.
  2. Modify your /etc/sssd/sssd.conf file to include the following settings:
    ldap_sasl_mech = gssapi
    ldap_sasl_authid = host/client.example.com@EXAMPLE.COM
    ldap_krb5_keytab = /etc/krb5.keytab (default)
    ldap_krb5_init_creds = true (default)
    ldap_krb5_ticket_lifetime = 86400 (default)
    krb5_realm = EXAMPLE.COM
    

8.2.7. Configuring a Proxy Domain

SSSD currently only supports LDAP and Kerberos as authentication providers. If you prefer to use SSSD (for example, to take advantage of its caching functionality), but SSSD does not support your authentication method, you can set up a proxy authentication provider. This could be the case if you use fingerprint scanners or smart cards as part of your authentication process. Similarly, you can set up proxy to serve as an identity provider.
The following sections cover combinations of identity and authentication providers in which the proxy server takes the role of one.

8.2.7.1. proxy/KRB5

The following configuration is an example of a combination of a proxy identity provider used with Kerberos authentication:
Edit the /etc/sssd/sssd.conf configuration file to include the following settings:
[domain/PROXY_KRB5]
auth_provider = krb5
krb5_server = 192.168.1.1
krb5_realm = EXAMPLE.COM

id_provider = proxy
proxy_lib_name = nis
enumerate = true
cache_credentials = true
For more information on various Kerberos configuration options, refer to Section 8.2.6, “Setting Up Kerberos Authentication”.

8.2.7.2. LDAP/proxy

An example of a combination of an LDAP identity provider and a proxy authentication provider is the use of the LDAP with a custom PAM stack. To enable authentication via the PAM stack, complete the following steps:
  1. Edit the /etc/sssd/sssd.conf configuration file to include the following settings:
    [domain/LDAP_PROXY]
    id_provider = ldap
    ldap_uri = ldap://example.com
    ldap_search_base = dc=example,dc=com
    
    auth_provider = proxy
    proxy_pam_target = sssdpamproxy
    enumerate = true
    cache_credentials = true
    
    By specifying the options above, authentication requests will be proxied via the /etc/pam.d/sssdpamproxy file which provides the needed module interfaces. Note that the pam_ldap.so file can be substituted with a PAM module of your choice.
    For more information on various LDAP configuration options, refer to Section 8.2.5.2, “Configuring an LDAP Domain”.
  2. Create a /etc/pam.d/sssdpamproxy file (if not already created) and specify the following settings in it:
    auth          required      pam_ldap.so
    account       required      pam_ldap.so
    password      required      pam_ldap.so
    session       required      pam_ldap.so

8.2.7.3. proxy/proxy

An example of a combination of an proxy identity provider and a proxy authentication provider is the use of the proxy identity provider with a custom PAM stack. To enable authentication via the PAM stack, complete the following steps:

Make sure the nss-pam-ldapd package is installed

In order to use the proxy identity provider, you must have the nss-pam-ldapd package installed.
  1. Edit the /etc/sssd/sssd.conf configuration file to include the following settings:
    [domain/PROXY_PROXY]
    auth_provider = proxy
    id_provider = proxy
    proxy_lib_name = ldap
    proxy_pam_target = sssdproxyldap
    enumerate = true 
    cache_credentials = true
    
    By specifying the options above, authentication requests will be proxied via the /etc/pam.d/sssdproxyldap file which provides the needed module interfaces.
    For more information on the options used in the configuration example above, refer to man sssd.conf
  2. Create a /etc/pam.d/sssdproxyldap file (if not already created) and specify the following settings in it:
    auth          required      pam_ldap.so
    account       required      pam_ldap.so
    password      required      pam_ldap.so
    session       required      pam_ldap.so
  3. Edit the /etc/nslcd.conf file (the default configuration file for the LDAP name service daemon) to include the following settings:
    uid nslcd
    gid ldap
    uri ldaps://ldap.mydomain.org:636
    base dc=mydomain,dc=org
    ssl on
    tls_cacertdir /etc/openldap/cacerts
    For more information on the options used in the configuration example above, refer to man nslcd.conf

8.2.8. Troubleshooting

This section lists some of the issues you may encounter when implementing SSSD, the possible causes of these issues, and how to resolve them. If you find further issues that are not covered here, refer to the We Need Feedback section in the Preface for information on how to file a bug report.

8.2.8.1. Using SSSD Log Files

SSSD uses a number of log files to report information about its operation, and this information can help to resolve issues in the event of SSSD failure or unexpected behavior. The default location for these log files on Fedora—based systems is the /var/log/sssd/ directory.
SSSD produces a log file for each back end (that is, one log file for each domain specified in the /etc/sssd/sssd.conf file), as well as an sssd_pam.log and an sssd_nss.log file. This level of granularity can help you to quickly isolate and resolve any errors or issues you might experience with SSSD.
You should also examine the /var/log/secure file, which logs authentication failures and the reason for the failure. For example, if you see Reason 4: System Error reported against any failure, you should increase the debug level of the log files.
Producing More Verbose Log Files
If you are unable to identify and resolve any problems with SSSD after inspection of the default log files, you can configure SSSD to produce more verbose files. You can set the debug_level option in the /etc/sssd/sssd.conf for the domain that is causing concern, and then restart SSSD. Refer to the sssd.conf(5) manual page for more information on how to set the debug_level for a specific domain.
All log files include timestamps on debug messages by default. This can make it easier to understand any errors that may occur, why they occurred, and how to address them. If necessary, you can disable these timestamps by setting the appropriate parameter to FALSE in the /etc/sssd/sssd.conf file:
--debug-timestamps=FALSE

8.2.8.2. Problems with SSSD Configuration

  • SSSD fails to start
    • SSSD requires at least one properly configured domain before the service will start. Without such a domain, you might see the following error message when trying to start SSSD with the following command:
      # sssd -d4
      [sssd] [ldb] (3): server_sort:Unable to register control with rootdse!
      
      [sssd] [confdb_get_domains] (0): No domains configured, fatal error!
      [sssd] [get_monitor_config] (0): No domains configured.
      
      You can ignore the "Unable to register control with rootdse!" message, as it is erroneous. The other messages, however, indicate that SSSD is unable to locate any properly configured domains.
      Edit your /etc/sssd/sssd.conf file and ensure you have at least one properly configured domain, and then try to start SSSD.
    • SSSD requires at least one available service provider before it will start. With no available service providers, you might see the following error message when trying to start SSSD with the following command:
      # sssd -d4
      [sssd] [ldb] (3): server_sort:Unable to register control with rootdse!
      
      [sssd] [get_monitor_config] (0): No services configured!
      
      You can ignore the "Unable to register control with rootdse!" message, as it is erroneous. The other message, however, indicates that SSSD is unable to locate any available service providers.
      Edit your /etc/sssd/sssd.conf file and ensure you have at least one available service providers, and then try to start SSSD.

      Configuring the service providers

      SSSD requires that service providers be configured as a comma-separated list in a single services entry in the /etc/sssd/sssd.conf file. If services are listed in multiple entries, only the last entry is recognized by SSSD.
    • Refer to the sssd.conf(5) manual page for more options that might assist in troubleshooting issues with SSSD.

8.2.8.3. Problems with SSSD Service Configuration

8.2.8.3.1. Problems with NSS
This section describes some common problems with NSS, their symptoms, and how to resolve them.
  • NSS fails to return user information
    • Ensure that NSS is running
      # systemctl is-active sssd.service
      This command should return results similar to the following:
      sssd (pid 21762) is running...
      
    • Ensure that you have correctly configured the [nss] section of the /etc/sssd/sssd.conf file. For example, ensure that you have not misconfigured the filter_users or filter_groups attributes. Refer to the NSS configuration options section of the sssd.conf(5) manual page for information on how to configure these attributes.
    • Ensure that you have included nss in the list of services that sssd should start
    • Ensure that you have correctly configured the /etc/nsswitch.conf file. Refer to the section Section 8.2.3.2.1, “Configuring NSS” for information on how to correctly configure this file.
8.2.8.3.2. Problems with PAM
This section describes some common problems with PAM, their symptoms, and how to resolve them.
  • Setting the password for the local SSSD user prompts twice for the password
    When attempting to change a local SSSD user's password, you might see output similar to the following:
    [root@clientF11 tmp]# passwd user1000
    Changing password for user user1000.
    New password:
    Retype new password:
    New Password:
    Reenter new Password:
    passwd: all authentication tokens updated successfully.
    
    This is the result of an incorrect PAM configuration. Refer to Section 8.2.3.2.2, “Configuring PAM”, and ensure that the use_authtok option is correctly configured in your /etc/pam.d/system-auth file.
8.2.8.3.3. Problems with NFS and NSCD
SSSD is not designed to be used with the nscd daemon, and will likely generate warnings in the SSSD log files. Even though SSSD does not directly conflict with nscd, the use of both at the same time can result in unexpected behavior (specifically with how long entries are being cached).
If you are using Network Manager to manage your network connections, it may take several minutes for the network interface to come up. During this time, various services will attempt to start. If these services start before the network is up (that is, the DNS servers cannot yet be reached) they will fail to identify the forward or reverse DNS entries they might need. These services will be reading an incorrect or possibly empty resolv.conf file. This file is typically only read once, and so any changes made to this file are not automatically applied.
This can result in the failure of some system services, and in particular can cause NFS locking to fail on the machine where the nscd service is running, unless that service is manually restarted.
One method of working around this problem is to enable caching for hosts and services in the /etc/nscd.conf file, and to rely on the SSSD cache for the passwd and group entries. With nscd answering hosts and services requests, these entries would have been cached and returned by nscd during the boot process.

NSCD and later versions of SSSD

Later versions of SSSD should negate any need for NSCD.

8.2.8.4. Problems with SSSD Domain Configuration

  • NSS returns incorrect user information
    • If your search for user information returns incorrect data, ensure that you do not have conflicting usernames in separate domains. If you use multiple domains, it is recommended that you set the use_fully_qualified_domains attribute to TRUE in the /etc/sssd/sssd.conf file.

8.2.8.5. Additional Resources

8.2.8.5.1. Manual Pages
SSSD ships with a number of manual pages, all of which provide additional information about specific aspects of SSSD, such as configuration files, commands, and available options. SSSD currently provides the following manual pages:
  • sssd.conf(5)
  • sssd-ipa(5)
  • sssd-krb5(5)
  • sssd-ldap(5)
  • sssd(8)
  • sssd_krb5_locator_plugin(8)
  • pam_sss(8)
You should refer to these manual pages for detailed information about all aspects of SSSD, its configuration, and associated tools and commands.
8.2.8.5.2. Mailing Lists
You can subscribe to the SSSD mailing list to follow and become involved in the development of SSSD, or to ask questions about any issues you may be experiencing with your SSSD deployment.
Visit https://fedorahosted.org/mailman/listinfo/sssd-devel to subscribe to this mailing list.

8.2.9. SSSD Configuration File Format

The following listing describes the current version (Version 2) of the SSSD configuration file format.
[sssd]
config_file_version = 2
services = nss, pam
domains = mybox.example.com, ldap.example.com, ipa.example.com, nis.example.com
# sbus_timeout = 300

[nss]
nss_filter_groups = root
nss_filter_users = root
nss_entry_cache_timeout = 30
nss_enum_cache_timeout = 30

[domain/mybox.example.com]
domain_type = local
enumerate = true
min_id = 1000
# max_id = 2000

local_default_shell = /bin/bash
local_default_homedir = /home

# Possible overrides
# id_provider = local
# auth_provider = local
# authz_provider = local
# passwd_provider = local

[domain/ldap.example.com]
domain_type = ldap
server = ldap.example.com, ldap3.example.com, 10.0.0.2
# ldap_uri = ldaps://ldap.example.com:9093
# ldap_use_tls = ssl
ldap_search_base = dc=ldap,dc=example,dc=com
enumerate = false

# Possible overrides
# id_provider = ldap
# id_server = ldap2.example.com
# auth_provider = krb5
# auth_server = krb5.example.com
# krb5_realm = KRB5.EXAMPLE.COM

[domain/ipa.example.com]
domain_type = ipa
server = ipa.example.com, ipa2.example.com
enumerate = false

# Possible overrides
# id_provider = ldap
# id_server = ldap2.example.com
# auth_provider = krb5
# auth_server = krb5.example.com
# krb5_realm = KRB5.EXAMPLE.COM

[domain/nis.example.com]
id_provider = proxy
proxy_lib = nis
auth_provider = proxy
proxy_auth_target = nis_pam_proxy

Chapter 9. OpenSSH

SSH (Secure Shell) is a protocol which facilitates secure communications between two systems using a client/server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, rendering the connection difficult for intruders to collect unencrypted passwords.
The ssh program is designed to replace older, less secure terminal applications used to log into remote hosts, such as telnet or rsh. A related program called scp replaces older programs designed to copy files between hosts, such as rcp. Because these older applications do not encrypt passwords transmitted between the client and the server, avoid them whenever possible. Using secure methods to log into remote systems decreases the risks for both the client system and the remote host.
Fedora includes the general OpenSSH package (openssh) as well as the OpenSSH server (openssh-server) and client (openssh-clients) packages. Note that the OpenSSH packages require the OpenSSL package (openssl), which installs several important cryptographic libraries, enabling OpenSSH to provide encrypted communications.

9.1. The SSH Protocol

9.1.1. Why Use SSH?

Potential intruders have a variety of tools at their disposal enabling them to disrupt, intercept, and re-route network traffic in an effort to gain access to a system. In general terms, these threats can be categorized as follows:
Interception of communication between two systems
The attacker can be somewhere on the network between the communicating parties, copying any information passed between them. He may intercept and keep the information, or alter the information and send it on to the intended recipient.
This attack is usually performed using a packet sniffer, a rather common network utility that captures each packet flowing through the network, and analyzes its content.
Impersonation of a particular host
Attacker's system is configured to pose as the intended recipient of a transmission. If this strategy works, the user's system remains unaware that it is communicating with the wrong host.
This attack can be performed using a technique known as DNS poisoning, or via so-called IP spoofing. In the first case, the intruder uses a cracked DNS server to point client systems to a maliciously duplicated host. In the second case, the intruder sends falsified network packets that appear to be from a trusted host.
Both techniques intercept potentially sensitive information and, if the interception is made for hostile reasons, the results can be disastrous. If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity. Additionally, all communication between the client and server systems is encrypted. Attempts to spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.

9.1.2. Main Features

The SSH protocol provides the following safeguards:
No one can pose as the intended server
After an initial connection, the client can verify that it is connecting to the same server it had connected to previously.
No one can capture the authentication information
The client transmits its authentication information to the server using strong, 128-bit encryption.
No one can intercept the communication
All data sent and received during a session is transferred using 128-bit encryption, making intercepted transmissions extremely difficult to decrypt and read.
Additionally, it also offers the following options:
It provides secure means to use graphical applications over a network
Using a technique called X11 forwarding, the client can forward X11 (X Window System) applications from the server.
It provides a way to secure otherwise insecure protocols
The SSH protocol encrypts everything it sends and receives. Using a technique called port forwarding, an SSH server can become a conduit to securing otherwise insecure protocols, like POP, and increasing overall system and data security.
It can be used to create a secure channel
The OpenSSH server and client can be configured to create a tunnel similar to a virtual private network for traffic between server and client machines.
It supports the Kerberos authentication
OpenSSH servers and clients can be configured to authenticate using the GSSAPI (Generic Security Services Application Program Interface) implementation of the Kerberos network authentication protocol.

9.1.3. Protocol Versions

Two varieties of SSH currently exist: version 1, and newer version 2. The OpenSSH suite under Fedora uses SSH version 2, which has an enhanced key exchange algorithm not vulnerable to the known exploit in version 1. However, for compatibility reasons, the OpenSSH suite does support version 1 connections as well.

Avoid using SSH version 1

To ensure maximum security for your connection, it is recommended that only SSH version 2-compatible servers and clients are used whenever possible.

9.1.4. Event Sequence of an SSH Connection

The following series of events help protect the integrity of SSH communication between two hosts.
  1. A cryptographic handshake is made so that the client can verify that it is communicating with the correct server.
  2. The transport layer of the connection between the client and remote host is encrypted using a symmetric cipher.
  3. The client authenticates itself to the server.
  4. The remote client interacts with the remote host over the encrypted connection.

9.1.4.1. Transport Layer

The primary role of the transport layer is to facilitate safe and secure communication between the two hosts at the time of authentication and during subsequent communication. The transport layer accomplishes this by handling the encryption and decryption of data, and by providing integrity protection of data packets as they are sent and received. The transport layer also provides compression, speeding the transfer of information.
Once an SSH client contacts a server, key information is exchanged so that the two systems can correctly construct the transport layer. The following steps occur during this exchange:
  • Keys are exchanged
  • The public key encryption algorithm is determined
  • The symmetric encryption algorithm is determined
  • The message authentication algorithm is determined
  • The hash algorithm is determined
During the key exchange, the server identifies itself to the client with a unique host key. If the client has never communicated with this particular server before, the server's host key is unknown to the client and it does not connect. OpenSSH gets around this problem by accepting the server's host key. This is done after the user is notified and has both accepted and verified the new host key. In subsequent connections, the server's host key is checked against the saved version on the client, providing confidence that the client is indeed communicating with the intended server. If, in the future, the host key no longer matches, the user must remove the client's saved version before a connection can occur.

Always verify the integrity of a new SSH server

It is possible for an attacker to masquerade as an SSH server during the initial contact since the local system does not know the difference between the intended server and a false one set up by an attacker. To help prevent this, verify the integrity of a new SSH server by contacting the server administrator before connecting for the first time or in the event of a host key mismatch.
SSH is designed to work with almost any kind of public key algorithm or encoding format. After an initial key exchange creates a hash value used for exchanges and a shared secret value, the two systems immediately begin calculating new keys and algorithms to protect authentication and future data sent over the connection.
After a certain amount of data has been transmitted using a given key and algorithm (the exact amount depends on the SSH implementation), another key exchange occurs, generating another set of hash values and a new shared secret value. Even if an attacker is able to determine the hash and shared secret value, this information is only useful for a limited period of time.

9.1.4.2. Authentication

Once the transport layer has constructed a secure tunnel to pass information between the two systems, the server tells the client the different authentication methods supported, such as using a private key-encoded signature or typing a password. The client then tries to authenticate itself to the server using one of these supported methods.
SSH servers and clients can be configured to allow different types of authentication, which gives each side the optimal amount of control. The server can decide which encryption methods it supports based on its security model, and the client can choose the order of authentication methods to attempt from the available options.

9.1.4.3. Channels

After a successful authentication over the SSH transport layer, multiple channels are opened via a technique called multiplexing[2]. Each of these channels handles communication for different terminal sessions and for forwarded X11 sessions.
Both clients and servers can create a new channel. Each channel is then assigned a different number on each end of the connection. When the client attempts to open a new channel, the clients sends the channel number along with the request. This information is stored by the server and is used to direct communication to that channel. This is done so that different types of sessions do not affect one another and so that when a given session ends, its channel can be closed without disrupting the primary SSH connection.
Channels also support flow-control, which allows them to send and receive data in an orderly fashion. In this way, data is not sent over the channel until the client receives a message that the channel is open.
The client and server negotiate the characteristics of each channel automatically, depending on the type of service the client requests and the way the user is connected to the network. This allows great flexibility in handling different types of remote connections without having to change the basic infrastructure of the protocol.

9.2. An OpenSSH Configuration

In order to perform tasks described in this section, you must have superuser privileges. To obtain them, log in as root by typing:
su -

9.2.1. Configuration Files

There are two different sets of configuration files: those for client programs (that is, ssh, scp, and sftp), and those for the server (the sshd daemon).
System-wide SSH configuration information is stored in the /etc/ssh/ directory. See Table 9.1, “System-wide configuration files” for a description of its content.
Table 9.1. System-wide configuration files
Configuration File Description
/etc/ssh/moduli Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is critical for constructing a secure transport layer. When keys are exchanged at the beginning of an SSH session, a shared, secret value is created which cannot be determined by either party alone. This value is then used to provide host authentication.
/etc/ssh/ssh_config The default SSH client configuration file. Note that it is overridden by ~/.ssh/config if it exists.
/etc/ssh/sshd_config The configuration file for the sshd daemon.
/etc/ssh/ssh_host_dsa_key The DSA private key used by the sshd daemon.
/etc/ssh/ssh_host_dsa_key.pub The DSA public key used by the sshd daemon.
/etc/ssh/ssh_host_key The RSA private key used by the sshd daemon for version 1 of the SSH protocol.
/etc/ssh/ssh_host_key.pub The RSA public key used by the sshd daemon for version 1 of the SSH protocol.
/etc/ssh/ssh_host_rsa_key The RSA private key used by the sshd daemon for version 2 of the SSH protocol.
/etc/ssh/ssh_host_rsa_key.pub The RSA public key used by the sshd for version 2 of the SSH protocol.

User-specific SSH configuration information is stored in the user's home directory within the ~/.ssh/ directory. See Table 9.2, “User-specific configuration files” for a description of its content.
Table 9.2. User-specific configuration files
Configuration File Description
~/.ssh/authorized_keys Holds a list of authorized public keys for servers. When the client connects to a server, the server authenticates the client by checking its signed public key stored within this file.
~/.ssh/id_dsa Contains the DSA private key of the user.
~/.ssh/id_dsa.pub The DSA public key of the user.
~/.ssh/id_rsa The RSA private key used by ssh for version 2 of the SSH protocol.
~/.ssh/id_rsa.pub The RSA public key used by ssh for version 2 of the SSH protocol
~/.ssh/identity The RSA private key used by ssh for version 1 of the SSH protocol.
~/.ssh/identity.pub The RSA public key used by ssh for version 1 of the SSH protocol.
~/.ssh/known_hosts Contains DSA host keys of SSH servers accessed by the user. This file is very important for ensuring that the SSH client is connecting the correct SSH server.

Refer to the ssh_config and sshd_config man pages for information concerning the various directives available in the SSH configuration files.

9.2.2. Starting an OpenSSH Server

Make sure you have relevant packages installed

To run an OpenSSH server, you must have the openssh-server and openssh packages installed. Refer to Section 4.2.4, “Installing Packages” for more information on how to install new packages in Fedora.
To start the sshd daemon, type the following at a shell prompt:
systemctl start sshd.service
To stop the running sshd daemon, use the following command:
systemctl stop sshd.service
If you want the daemon to start automatically at the boot time, type:
systemctl enable sshd.service
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.
Note that if you reinstall the system, a new set of identification keys will be created. As a result, clients who had connected to the system with any of the OpenSSH tools before the reinstall will see the following message:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
To prevent this, you can back up the relevant files from the /etc/ssh/ directory (see Table 9.1, “System-wide configuration files” for a complete list), and restore them whenever you reinstall the system.

9.2.3. Requiring SSH for Remote Connections

For SSH to be truly effective, using insecure connection protocols should be prohibited. Otherwise, a user's password may be protected using SSH for one session, only to be captured later while logging in using Telnet. Some services to disable include telnet, rsh, rlogin, and vsftpd.
To make sure these services are not running, type the following commands at a shell prompt:
systemctl stop telnet.service
systemctl stop rsh.service
systemctl stop rlogin.service
systemctl stop vsftpd.service
To disable running these services at startup, type:
systemctl disable telnet.service
systemctl disable rsh.service
systemctl disable rlogin.service
systemctl disable vsftpd.service
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.

9.2.4. Using a Key-Based Authentication

To improve the system security even further, you can enforce the key-based authentication by disabling the standard password authentication. To do so, open the /etc/ssh/sshd_config configuration file in a text editor, and change the PasswordAuthentication option as follows:
PasswordAuthentication no
To be able to use ssh, scp, or sftp to connect to the server from a client machine, generate an authorization key pair by following the steps below. Note that keys must be generated for each user separately.
Fedora 15 uses SSH Protocol 2 and RSA keys by default (see Section 9.1.3, “Protocol Versions” for more information).

Do not generate key pairs as root

If you complete the steps as root, only root will be able to use the keys.

Backup your ~/.ssh/ directory

If you reinstall your system and want to keep previously generated key pair, backup the ~/.ssh/ directory. After reinstalling, copy it back to your home directory. This process can be done for all users on your system, including root.

9.2.4.1. Generating Key Pairs

To generate an RSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa
    Generating public/private rsa key pair.
    Enter file in which to save the key (/home/john/.ssh/id_rsa):
  2. Press Enter to confirm the default location (that is, ~/.ssh/id_rsa) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/id_rsa.
    Your public key has been saved in /home/john/.ssh/id_rsa.pub.
    The key fingerprint is:
    e7:97:c7:e2:0e:f9:0e:fc:c4:d7:cb:e5:31:11:92:14 john@penguin.example.com
    The key's randomart image is:
    +--[ RSA 2048]----+
    |             E.  |
    |            . .  |
    |             o . |
    |              . .|
    |        S .    . |
    |         + o o ..|
    |          * * +oo|
    |           O +..=|
    |           o*  o.|
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 755 ~/.ssh
  5. Copy the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 644 ~/.ssh/authorized_keys
To generate a DSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate a DSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t dsa
    Generating public/private dsa key pair.
    Enter file in which to save the key (/home/john/.ssh/id_dsa):
  2. Press Enter to confirm the default location (that is, ~/.ssh/id_dsa) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/id_dsa.
    Your public key has been saved in /home/john/.ssh/id_dsa.pub.
    The key fingerprint is:
    81:a1:91:a8:9f:e8:c5:66:0d:54:f5:90:cc:bc:cc:27 john@penguin.example.com
    The key's randomart image is:
    +--[ DSA 1024]----+
    |   .oo*o.        |
    |  ...o Bo        |
    | .. . + o.       |
    |.  .   E o       |
    | o..o   S        |
    |. o= .           |
    |. +              |
    | .               |
    |                 |
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 775 ~/.ssh
  5. Copy the content of ~/.ssh/id_dsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 644 ~/.ssh/authorized_keys
To generate an RSA key pair for version 1 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa1
    Generating public/private rsa1 key pair.
    Enter file in which to save the key (/home/john/.ssh/identity):
  2. Press Enter to confirm the default location (that is, ~/.ssh/identity) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log into your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/identity.
    Your public key has been saved in /home/john/.ssh/identity.pub.
    The key fingerprint is:
    cb:f6:d5:cb:6e:5f:2b:28:ac:17:0c:e4:62:e4:6f:59 john@penguin.example.com
    The key's randomart image is:
    +--[RSA1 2048]----+
    |                 |
    |     . .         |
    |    o o          |
    |     + o E       |
    |    . o S        |
    |       = +   .   |
    |      . = . o . .|
    |       . = o o..o|
    |       .o o  o=o.|
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 755 ~/.ssh
  5. Copy the content of ~/.ssh/identity.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 644 ~/.ssh/authorized_keys
Refer to Section 9.2.4.2, “Configuring ssh-agent” for information on how to set up your system to remember the passphrase.

Never share your private key

The private key is for your personal use only, and it is important that you never give it to anyone.

9.2.4.2. Configuring ssh-agent

To store your passphrase so that you do not have to enter it each time you initiate a connection with a remote machine, you can use the ssh-agent authentication agent. To save your passphrase for a certain shell prompt, use the following command:
~]$ ssh-add
Enter passphrase for /home/john/.ssh/id_rsa:
Note that when you log out, your passphrase will be forgotten. You must execute the command each time you log in to a virtual console or a terminal window.

9.3. OpenSSH Clients

Make sure you have relevant packages installed

To connect to an OpenSSH server from a client machine, you must have the openssh-clients and openssh packages installed. Refer to Section 4.2.4, “Installing Packages” for more information on how to install new packages in Fedora.

9.3.1. Using the ssh Utility

ssh allows you to log in to a remote machine and execute commands there. It is a secure replacement for the rlogin, rsh, and telnet programs.
Similarly to telnet, to log in to a remote machine named penguin.example.com, type the following command at a shell prompt:
~]$ ssh penguin.example.com
This will log you in with the same username you are using on a local machine. If you want to specify a different one, use a command in the ssh username@hostname form. For example, to log in as john, type:
~]$ ssh john@penguin.example.com
The first time you initiate a connection, you will be presented with a message similar to this:
The authenticity of host 'penguin.example.com' can't be established.
RSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c.
Are you sure you want to continue connecting (yes/no)?
Type yes to confirm. You will see a notice that the server has been added to the list of known hosts, and a prompt asking for your password:
Warning: Permanently added 'penguin.example.com' (RSA) to the list of known hosts.
john@penguin.example.com's password:

Updating the host key of an SSH server

If the SSH server's host key changes, the client notifies the user that the connection cannot proceed until the server's host key is deleted from the ~/.ssh/known_hosts file. To do so, open the file in a text editor, and remove a line containing the remote machine name at the beginning. Before doing this, however, contact the system administrator of the SSH server to verify the server is not compromised.
After entering the password, you will be provided with a shell prompt for the remote machine.
Alternatively, the ssh program can be used to execute a command on the remote machine without logging in to a shell prompt. The syntax for that is ssh [username@]hostname command. For example, if you want to execute the whoami command on penguin.example.com, type:
~]$ ssh john@penguin.example.com whoami
john@penguin.example.com's password:
john
After you enter the correct password, the username will be displayed, and you will return to your local shell prompt.

9.3.2. Using the scp Utility

scp can be used to transfer files between machines over a secure, encrypted connection. In its design, it is very similar to rcp.
To transfer a local file to a remote system, use a command in the following form:
scp localfile username@hostname:remotefile
For example, if you want to transfer taglist.vim to a remote machine named penguin.example.com, type the following at a shell prompt:
~]$ scp taglist.vim john@penguin.example.com:.vim/plugin/taglist.vim
john@penguin.example.com's password:
taglist.vim                                   100%  144KB 144.5KB/s   00:00
Multiple files can be specified at once. To transfer the contents of .vim/plugin/ to the same directory on the remote machine penguin.example.com, type the following command:
~]$ scp .vim/plugin/* john@penguin.example.com:.vim/plugin/
john@penguin.example.com's password:
closetag.vim                                  100%   13KB  12.6KB/s   00:00    
snippetsEmu.vim                               100%   33KB  33.1KB/s   00:00    
taglist.vim                                   100%  144KB 144.5KB/s   00:00
To transfer a remote file to the local system, use the following syntax:
scp username@hostname:remotefile localfile
For instance, to download the .vimrc configuration file from the remote machine, type:
~]$ scp john@penguin.example.com:.vimrc .vimrc
john@penguin.example.com's password:
.vimrc                                        100% 2233     2.2KB/s   00:00

9.3.3. Using the sftp Utility

The sftp utility can be used to open a secure, interactive FTP session. In its design, it is similar to ftp except that it uses a secure, encrypted connection.
To connect to a remote system, use a command in the following form:
sftp username@hostname
For example, to log in to a remote machine named penguin.example.com with john as a username, type:
~]$ sftp john@penguin.example.com
john@penguin.example.com's password:
Connected to penguin.example.com.
sftp>
After you enter the correct password, you will be presented with a prompt. The sftp utility accepts a set of commands similar to those used by ftp (see Table 9.3, “A selection of available sftp commands”).
Table 9.3. A selection of available sftp commands
Command Description
ls [directory] List the content of a remote directory. If none is supplied, a current working directory is used by default.
cd directory Change the remote working directory to directory.
mkdir directory Create a remote directory.
rmdir path Remove a remote directory.
put localfile [remotefile] Transfer localfile to a remote machine.
get remotefile [localfile] Transfer remotefile from a remote machine.

For a complete list of available commands, refer to the sftp man page.

9.4. More Than a Secure Shell

A secure command line interface is just the beginning of the many ways SSH can be used. Given the proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by using TCP/IP forwarding, previously insecure port connections between systems can be mapped to specific SSH channels.

9.4.1. X11 Forwarding

To open an X11 session over an SSH connection, use a command in the following form:
ssh -Y username@hostname
For example, to log in to a remote machine named penguin.example.com with john as a username, type:
~]$ ssh -Y john@penguin.example.com
john@penguin.example.com's password:
When an X program is run from the secure shell prompt, the SSH client and server create a new secure channel, and the X program data is sent over that channel to the client machine transparently.
X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive session of the Printer Configuration utility. To do this, connect to the server using ssh and type:
~]$ system-config-printer &
The Printer Configuration Tool will appear, allowing the remote user to safely configure printing on the remote system.

9.4.2. Port Forwarding

SSH can secure otherwise insecure TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.
Port forwarding works by mapping a local port on the client to a remote port on the server. SSH can map any port from the server to any port on the client. Port numbers do not need to match for this technique to work.

Using reserved port numbers

Setting up port forwarding to listen on ports below 1024 requires root level access.
To create a TCP/IP port forwarding channel which listens for connections on the localhost, use a command in the following form:
ssh -L local-port:remote-hostname:remote-port username@hostname
For example, to check email on a server called mail.example.com using POP3 through an encrypted connection, use the following command:
~]$ ssh -L 1100:mail.example.com:110 mail.example.com
Once the port forwarding channel is in place between the client machine and the mail server, direct a POP3 mail client to use port 1100 on the localhost to check for new email. Any requests sent to port 1100 on the client system will be directed securely to the mail.example.com server.
If mail.example.com is not running an SSH server, but another machine on the same network is, SSH can still be used to secure part of the connection. However, a slightly different command is necessary:
~]$ ssh -L 1100:mail.example.com:110 other.example.com
In this example, POP3 requests from port 1100 on the client machine are forwarded through the SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com connects to port 110 on mail.example.com to check for new email. Note that when using this technique, only the connection between the client system and other.example.com SSH server is secure.
Port forwarding can also be used to get information securely through network firewalls. If the firewall is configured to allow SSH traffic via its standard port (that is, port 22) but blocks access to other ports, a connection between two hosts using the blocked ports is still possible by redirecting their communication over an established SSH connection.

A connection is only as secure as a client system

Using port forwarding to forward connections in this manner allows any user on the client system to connect to that service. If the client system becomes compromised, the attacker also has access to forwarded services.
System administrators concerned about port forwarding can disable this functionality on the server by specifying a No parameter for the AllowTcpForwarding line in /etc/ssh/sshd_config and restarting the sshd service.

9.5. Additional Resources

The OpenSSH and OpenSSL projects are in constant development, and the most up-to-date information for them is available from their websites. The man pages for OpenSSH and OpenSSL tools are also good sources of detailed information.

9.5.1. Installed Documentation

man ssh
The manual page for ssh containing the full documentation on its usage.
man scp
The manual page for scp containing the full documentation on its usage.
man sftp
The manual page for sftp containing the full documentation on its usage.
man sshd
The manual page for sshd containing the full documentation on its usage.
man ssh-keygen
The manual page for ssh-keygen containing the full documentation on its usage.
man ssh_config
The manual page with full description of available SSH client configuration options.
man sshd_config
The manual page with full description of available SSH daemon configuration options.

9.5.2. Useful Websites

http://www.openssh.com/
The OpenSSH home page containing further documentation, frequently asked questions, links to the mailing lists, bug reports, and other useful resources.
http://www.openssl.org/
The OpenSSL home page containing further documentation, frequently asked questions, links to the mailing lists, and other useful resources.
http://www.freesshd.com/
Another implementation of an SSH server.


[2] A multiplexed connection consists of several signals being sent over a shared, common medium. With SSH, different channels are sent over a common secure connection.

Part V. Servers

This part discusses various topics related to servers such as how to set up a Web server or share files and directories over the network.

Table of Contents

10. DHCP Servers
10.1. Why Use DHCP?
10.2. Configuring a DHCP Server
10.2.1. Configuration File
10.2.2. Lease Database
10.2.3. Starting and Stopping the Server
10.2.4. DHCP Relay Agent
10.3. Configuring a DHCP Client
10.4. Configuring a Multihomed DHCP Server
10.4.1. Host Configuration
10.5. DHCP for IPv6 (DHCPv6)
10.6. Additional Resources
10.6.1. Installed Documentation
11. DNS Servers
11.1. Introduction to DNS
11.1.1. Nameserver Zones
11.1.2. Nameserver Types
11.1.3. BIND as a Nameserver
11.2. BIND
11.2.1. Configuring the named Service
11.2.2. Editing Zone Files
11.2.3. Using the rndc Utility
11.2.4. Using the dig Utility
11.2.5. Advanced Features of BIND
11.2.6. Common Mistakes to Avoid
11.2.7. Additional Resources
12. Web Servers
12.1. The Apache HTTP Server
12.1.1. New Features
12.1.2. Notable Changes
12.1.3. Updating the Configuration
12.1.4. Running the httpd Service
12.1.5. Editing the Configuration Files
12.1.6. Working with Modules
12.1.7. Setting Up Virtual Hosts
12.1.8. Setting Up an SSL Server
12.1.9. Additional Resources
13. Mail Servers
13.1. Email Protocols
13.1.1. Mail Transport Protocols
13.1.2. Mail Access Protocols
13.2. Email Program Classifications
13.2.1. Mail Transport Agent
13.2.2. Mail Delivery Agent
13.2.3. Mail User Agent
13.3. Mail Transport Agents
13.3.1. Postfix
13.3.2. Sendmail
13.3.3. Fetchmail
13.3.4. Mail Transport Agent (MTA) Configuration
13.4. Mail Delivery Agents
13.4.1. Procmail Configuration
13.4.2. Procmail Recipes
13.5. Mail User Agents
13.5.1. Securing Communication
13.6. Additional Resources
13.6.1. Installed Documentation
13.6.2. Useful Websites
13.6.3. Related Books
14. Directory Servers
14.1. OpenLDAP
14.1.1. Introduction to LDAP
14.1.2. Installing the OpenLDAP Suite
14.1.3. Configuring an OpenLDAP Server
14.1.4. Running an OpenLDAP Server
14.1.5. Configuring a System to Authenticate Using OpenLDAP
14.1.6. Additional Resources
15. File and Print Servers
15.1. Samba
15.1.1. Introduction to Samba
15.1.2. Samba Daemons and Related Services
15.1.3. Connecting to a Samba Share
15.1.4. Configuring a Samba Server
15.1.5. Starting and Stopping Samba
15.1.6. Samba Server Types and the smb.conf File
15.1.7. Samba Security Modes
15.1.8. Samba Account Information Databases
15.1.9. Samba Network Browsing
15.1.10. Samba with CUPS Printing Support
15.1.11. Samba Distribution Programs
15.1.12. Additional Resources
15.2. FTP
15.2.1. The File Transfer Protocol
15.2.2. FTP Servers
15.2.3. Files Installed with vsftpd
15.2.4. Starting and Stopping vsftpd
15.2.5. vsftpd Configuration Options
15.2.6. Additional Resources
15.3. Printer Configuration
15.3.1. Starting the Printer Configuration Tool
15.3.2. Starting Printer Setup
15.3.3. Adding a Local Printer
15.3.4. Adding an AppSocket/HP JetDirect printer
15.3.5. Adding an IPP Printer
15.3.6. Adding an LPD/LPR Host or Printer
15.3.7. Adding a Samba (SMB) printer
15.3.8. Selecting the Printer Model and Finishing
15.3.9. Printing a test page
15.3.10. Modifying Existing Printers
15.3.11. Additional Resources

Chapter 10. DHCP Servers

Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns TCP/IP information to client machines. Each DHCP client connects to the centrally located DHCP server, which returns that client's network configuration (including the IP address, gateway, and DNS servers).

10.1. Why Use DHCP?

DHCP is useful for automatic configuration of client network interfaces. When configuring the client system, the administrator chooses DHCP instead of specifying an IP address, netmask, gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is also useful if an administrator wants to change the IP addresses of a large number of systems. Instead of reconfiguring all the systems, he can just edit one DHCP configuration file on the server for the new set of IP addresses. If the DNS servers for an organization changes, the changes are made on the DHCP server, not on the DHCP clients. When the administrator restarts the network or reboots the clients, the changes will go into effect.
If an organization has a functional DHCP server properly connected to a network, laptops and other mobile computer users can move these devices from office to office.

10.2. Configuring a DHCP Server

The dhcp package contains an ISC DHCP server. First, install the package as root:
yum install dhcp
Installing the dhcp package creates a file, /etc/dhcp/dhcpd.conf, which is merely an empty configuration file:
#
# DHCP Server Configuration file.
#   see /usr/share/doc/dhcp*/dhcpd.conf.sample
#   see dhcpd.conf(5) man page
#
The sample configuration file can be found at /usr/share/doc/dhcp-version/dhcpd.conf.sample. You should use this file to help you configure /etc/dhcp/dhcpd.conf, which is explained in detail below.
DHCP also uses the file /var/lib/dhcpd/dhcpd.leases to store the client lease database. Refer to Section 10.2.2, “Lease Database” for more information.

10.2.1. Configuration File

The first step in configuring a DHCP server is to create the configuration file that stores the network information for the clients. Use this file to declare options and global options for client systems.
The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-insensitive and lines beginning with a hash sign (#) are considered comments.
There are two types of statements in the configuration file:
  • Parameters — State how to perform a task, whether to perform a task, or what network configuration options to send to the client.
  • Declarations — Describe the topology of the network, describe the clients, provide addresses for the clients, or apply a group of parameters to a group of declarations.
The parameters that start with the keyword option are referred to as options. These options control DHCP options; whereas, parameters configure values that are not optional or control how the DHCP server behaves.
Parameters (including options) declared before a section enclosed in curly brackets ({ }) are considered global parameters. Global parameters apply to all the sections below it.

Restart the DHCP daemon for the changes to take effect

If the configuration file is changed, the changes do not take effect until the DHCP daemon is restarted. To do so, type the following at a shell prompt as root:
systemctl restart dhcpd.service

Use the omshell command

Instead of changing a DHCP configuration file and restarting the service each time, using the omshell command provides an interactive way to connect to, query, and change the configuration of a DHCP server. By using omshell, all changes can be made while the server is running. For more information on omshell, refer to the omshell man page.
In Example 10.1, “Subnet declaration”, the routers, subnet-mask, domain-search, domain-name-servers, and time-offset options are used for any host statements declared below it.
Additionally, a subnet can be declared, a subnet declaration must be included for every subnet in the network. If it is not, the DHCP server fails to start.
In this example, there are global options for every DHCP client in the subnet and a range declared. Clients are assigned an IP address within the range.
Example 10.1. Subnet declaration
subnet 192.168.1.0 netmask 255.255.255.0 {
        option routers                  192.168.1.254;
        option subnet-mask              255.255.255.0;
        option domain-search              "example.com";
        option domain-name-servers       192.168.1.1;
        option time-offset              -18000;     # Eastern Standard Time
	range 192.168.1.10 192.168.1.100;
}

To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Example 10.2, “Range parameter” with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns IP addresses in the range 192.168.1.10 and 192.168.1.100 to client systems.
Example 10.2. Range parameter
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option domain-name-servers 192.168.1.1, 192.168.1.2;
option domain-search "example.com";
subnet 192.168.1.0 netmask 255.255.255.0 {
   range 192.168.1.10 192.168.1.100;
}

To assign an IP address to a client based on the MAC address of the network interface card, use the hardware ethernet parameter within a host declaration. As demonstrated in Example 10.3, “Static IP address using DHCP”, the host apex declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4.
Note that the optional parameter host-name can also be used to assign a host name to the client.
Example 10.3. Static IP address using DHCP
host apex {
   option host-name "apex.example.com";
   hardware ethernet 00:A0:78:8E:9E:AA;
   fixed-address 192.168.1.4;
}

All subnets that share the same physical network should be declared within a shared-network declaration as shown in Example 10.4, “Shared-network declaration”. Parameters within the shared-network, but outside the enclosed subnet declarations, are considered to be global parameters. The name of the shared-network must be a descriptive title for the network, such as using the title 'test-lab' to describe all the subnets in a test lab environment.
Example 10.4. Shared-network declaration
shared-network name {
    option domain-search              "test.redhat.com";
    option domain-name-servers      ns1.redhat.com, ns2.redhat.com;
    option routers                  192.168.0.254;
    more parameters for EXAMPLE shared-network
    subnet 192.168.1.0 netmask 255.255.252.0 {
        parameters for subnet
        range 192.168.1.1 192.168.1.254;
    }
    subnet 192.168.2.0 netmask 255.255.252.0 {
        parameters for subnet
        range 192.168.2.1 192.168.2.254;
    }
}

As demonstrated in Example 10.5, “Group declaration”, the group declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.
Example 10.5. Group declaration
group {
   option routers                  192.168.1.254;
   option subnet-mask              255.255.255.0;
   option domain-search              "example.com";
   option domain-name-servers       192.168.1.1;
   option time-offset              -18000;     # Eastern Standard Time
   host apex {
      option host-name "apex.example.com";
      hardware ethernet 00:A0:78:8E:9E:AA;
      fixed-address 192.168.1.4;
   }
   host raleigh {
      option host-name "raleigh.example.com";
      hardware ethernet 00:A1:DD:74:C3:F2;
      fixed-address 192.168.1.6;
   }
}

Using the sample configuration file

The sample configuration file provided can be used as a starting point and custom configuration options can be added to it. To copy it to the proper location, use the following command:
cp /usr/share/doc/dhcp-version-number/dhcpd.conf.sample /etc/dhcp/dhcpd.conf
... where version-number is the DHCP version number.
For a complete list of option statements and what they do, refer to the dhcp-options man page.

10.2.2. Lease Database

On the DHCP server, the file /var/lib/dhcpd/dhcpd.leases stores the DHCP client lease database. Do not change this file. DHCP lease information for each recently assigned IP address is automatically stored in the lease database. The information includes the length of the lease, to whom the IP address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.
All times in the lease database are in Coordinated Universal Time (UTC), not local time.
The lease database is recreated from time to time so that it is not too large. First, all known leases are saved in a temporary lease database. The dhcpd.leases file is renamed dhcpd.leases~ and the temporary lease database is written to dhcpd.leases.
The DHCP daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the dhcpd.leases file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.

10.2.3. Starting and Stopping the Server

Starting the DHCP server for the first time

When the DHCP server is started for the first time, it fails unless the dhcpd.leases file exists. Use the command touch /var/lib/dhcpd/dhcpd.leases to create the file if it does not exist.
If the same server is also running BIND as a DNS server, this step is not necessary, as starting the named service automatically checks for a dhcpd.leases file.
To start the DHCP service, use the following command:
systemctl start dhcpd.service
To stop the DHCP server, type:
systemctl stop dhcpd.service
By default, the DHCP service does not start at boot time. To configure the daemon to start automatically at boot time, run:
systemctl enable dhcpd.service
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.
If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interfaces, configure the DHCP server to start only on that device. In /etc/sysconfig/dhcpd, add the name of the interface to the list of DHCPDARGS:
# Command line options here
DHCPDARGS=eth0
This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can not connect to the daemon via the Internet.
Other command line options that can be specified in /etc/sysconfig/dhcpd include:
  • -p portnum — Specifies the UDP port number on which dhcpd should listen. The default is port 67. The DHCP server transmits responses to the DHCP clients at a port number one greater than the UDP port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responses to the client on port 68. If a port is specified here and the DHCP relay agent is used, the same port on which the DHCP relay agent should listen must be specified. Refer to Section 10.2.4, “DHCP Relay Agent” for details.
  • -f — Runs the daemon as a foreground process. This is mostly used for debugging.
  • -d — Logs the DHCP server daemon to the standard error descriptor. This is mostly used for debugging. If this is not specified, the log is written to /var/log/messages.
  • -cf filename — Specifies the location of the configuration file. The default location is /etc/dhcp/dhcpd.conf.
  • -lf filename — Specifies the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the DHCP server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is /var/lib/dhcpd/dhcpd.leases.
  • -q — Do not print the entire copyright message when starting the daemon.

10.2.4. DHCP Relay Agent

The DHCP Relay Agent (dhcrelay) allows for the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets.
When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified when the DHCP Relay Agent is started. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request.
The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are specified in /etc/sysconfig/dhcrelay with the INTERFACES directive.
To start the DHCP Relay Agent, use the following command:
systemctl start dhcrelay.service

10.3. Configuring a DHCP Client

To configure a DHCP client manually, modify the /etc/sysconfig/network file to enable networking and the configuration file for each network device in the /etc/sysconfig/network-scripts directory. In this directory, each device should have a configuration file named ifcfg-eth0, where eth0 is the network device name.
The /etc/sysconfig/network-scripts/ifcfg-eth0 file should contain the following lines:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
A configuration file is needed for each device to be configured to use DHCP.
Other options for the network script includes:
  • DHCP_HOSTNAME — Only use this option if the DHCP server requires the client to specify a hostname before receiving an IP address. (The DHCP server daemon in Fedora does not support this feature.)
  • PEERDNS=answer , where answer is one of the following:
    • yes — Modify /etc/resolv.conf with information from the server. If using DHCP, then yes is the default.
    • no — Do not modify /etc/resolv.conf.

Advanced configurations

For advanced configurations of client DHCP options such as protocol timing, lease requirements and requests, dynamic DNS support, aliases, as well as a wide variety of values to override, prepend, or append to client-side configurations, refer to the dhclient and dhclient.conf man pages.

10.4. Configuring a Multihomed DHCP Server

A multihomed DHCP server serves multiple networks, that is, multiple subnets. The examples in these sections detail how to configure a DHCP server to serve multiple networks, select which network interfaces to listen on, and how to define network settings for systems that move networks.
Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcp/dhcpd.conf files.
The DHCP daemon listens on all network interfaces unless otherwise specified. Use the /etc/sysconfig/dhcpd file to specify which network interfaces the DHCP daemon listens on. The following /etc/sysconfig/dhcpd example specifies that the DHCP daemon listens on the eth0 and eth1 interfaces:
DHCPDARGS="eth0 eth1";
If a system has three network interfaces cards -- eth0, eth1, and eth2 -- and it is only desired that the DHCP daemon listens on eth0, then only specify eth0 in /etc/sysconfig/dhcpd:
DHCPDARGS="eth0";
The following is a basic /etc/dhcp/dhcpd.conf file, for a server that has two network interfaces, eth0 in a 10.0.0.0/24 network, and eth1 in a 172.16.0.0/24 network. Multiple subnet declarations allow different settings to be defined for multiple networks:
default-lease-time 600;
max-lease-time 7200;
subnet 10.0.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 10.0.0.1;
	range 10.0.0.5 10.0.0.15;
}
subnet 172.16.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 172.16.0.1;
	range 172.16.0.5 172.16.0.15;
}
subnet 10.0.0.0 netmask 255.255.255.0;
A subnet declaration is required for every network your DHCP server is serving. Multiple subnets require multiple subnet declarations. If the DHCP server does not have a network interface in a range of a subnet declaration, the DHCP server does not serve that network.
If there is only one subnet declaration, and no network interfaces are in the range of that subnet, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
dhcpd: No subnet declaration for eth0 (0.0.0.0).
dhcpd: ** Ignoring requests on eth0.  If this is not what
dhcpd:    you want, please write a subnet declaration
dhcpd:    in your dhcpd.conf file for the network segment
dhcpd:    to which interface eth1 is attached. **
dhcpd:
dhcpd:
dhcpd: Not configured to listen on any interfaces!
option subnet-mask 255.255.255.0;
The option subnet-mask option defines a subnet mask, and overrides the netmask value in the subnet declaration. In simple cases, the subnet and netmask values are the same.
option routers 10.0.0.1;
The option routers option defines the default gateway for the subnet. This is required for systems to reach internal networks on a different subnet, as well as external networks.
range 10.0.0.5 10.0.0.15;
The range option specifies the pool of available IP addresses. Systems are assigned an address from the range of specified IP addresses.
For further information, refer to the dhcpd.conf(5) man page.

Do not use alias interfaces

Alias interfaces are not supported by DHCP. If an alias interface is the only interface, in the only subnet specified in /etc/dhcp/dhcpd.conf, the DHCP daemon fails to start.

10.4.1. Host Configuration

Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcp/dhcpd.conf files.
Configuring a single system for multiple networks
The following /etc/dhcp/dhcpd.conf example creates two subnets, and configures an IP address for the same system, depending on which network it connects to:
default-lease-time 600;
max-lease-time 7200;
subnet 10.0.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 10.0.0.1;
	range 10.0.0.5 10.0.0.15;
}
subnet 172.16.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 172.16.0.1;
	range 172.16.0.5 172.16.0.15;
}
host example0 {
	hardware ethernet 00:1A:6B:6A:2E:0B;
	fixed-address 10.0.0.20;
}
host example1 {
	hardware ethernet 00:1A:6B:6A:2E:0B;
	fixed-address 172.16.0.20;
}
host example0
The host declaration defines specific parameters for a single system, such as an IP address. To configure specific parameters for multiple hosts, use multiple host declarations.
Most DHCP clients ignore the name in host declarations, and as such, this name can anything, as long as it is unique to other host declarations. To configure the same system for multiple networks, use a different name for each host declaration, otherwise the DHCP daemon fails to start. Systems are identified by the hardware ethernet option, not the name in the host declaration.
hardware ethernet 00:1A:6B:6A:2E:0B;
The hardware ethernet option identifies the system. To find this address, run the ip link command.
fixed-address 10.0.0.20;
The fixed-address option assigns a valid IP address to the system specified by the hardware ethernet option. This address must be outside the IP address pool specified with the range option.
If option statements do not end with a semicolon, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
/etc/dhcp/dhcpd.conf line 20: semicolon expected.
dhcpd: }
dhcpd: ^
dhcpd: /etc/dhcp/dhcpd.conf line 38: unexpected end of file
dhcpd:
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting
Configuring systems with multiple network interfaces
The following host declarations configure a single system, that has multiple network interfaces, so that each interface receives the same IP address. This configuration will not work if both network interfaces are connected to the same network at the same time:
host interface0 {
	hardware ethernet 00:1a:6b:6a:2e:0b;
	fixed-address 10.0.0.18;
}
host interface1 {
	hardware ethernet 00:1A:6B:6A:27:3A;
	fixed-address 10.0.0.18;
}
For this example, interface0 is the first network interface, and interface1 is the second interface. The different hardware ethernet options identify each interface.
If such a system connects to another network, add more host declarations, remembering to:
  • assign a valid fixed-address for the network the host is connecting to.
  • make the name in the host declaration unique.
When a name given in a host declaration is not unique, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
dhcpd: /etc/dhcp/dhcpd.conf line 31: host interface0: already exists
dhcpd: }
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting
This error was caused by having multiple host interface0 declarations defined in /etc/dhcp/dhcpd.conf.

10.5. DHCP for IPv6 (DHCPv6)

The ISC DHCP includes support for IPv6 (DHCPv6) since the 4.x release with a DHCPv6 server, client and relay agent functionality. The server, client and relay agents support both IPv4 and IPv6. However, the client and the server can only manage one protocol at a time — for dual support they must be started separately for IPv4 and IPv6.
The DHCPv6 server configuration file can be found at /etc/dhcp/dhcpd6.conf.
The sample server configuration file can be found at /usr/share/doc/dhcp-version/dhcpd6.conf.sample.
To start the DHCPv6 service, use the following command:
systemctl start dhcpd6.service
A simple DHCPv6 server configuration file can look like this:
subnet6 2001:db8:0:1::/64 {
        range6 2001:db8:0:1::129 2001:db8:0:1::254;
        option dhcp6.name-servers fec0:0:0:1::1;
        option dhcp6.domain-search "domain.example";
}

10.6. Additional Resources

For additional information, refer to The DHCP Handbook; Ralph Droms and Ted Lemon; 2003 or the following resources.

10.6.1. Installed Documentation

  • dhcpd man page — Describes how the DHCP daemon works.
  • dhcpd.conf man page — Explains how to configure the DHCP configuration file; includes some examples.
  • dhcpd.leases man page — Describes a persistent database of leases.
  • dhcp-options man page — Explains the syntax for declaring DHCP options in dhcpd.conf; includes some examples.
  • dhcrelay man page — Explains the DHCP Relay Agent and its configuration options.
  • /usr/share/doc/dhcp-version/ — Contains sample files, README files, and release notes for current versions of the DHCP service.

Chapter 11. DNS Servers

DNS (Domain Name System), also known as a nameserver, is a network system that associates hostnames with their respective IP addresses. For users, this has the advantage that they can refer to machines on the network by names that are usually easier to remember than the numerical network addresses. For system administrators, using the nameserver allows them to change the IP address for a host without ever affecting the name-based queries, or to decide which machines handle these queries.

11.1. Introduction to DNS

DNS is usually implemented using one or more centralized servers that are authoritative for certain domains. When a client host requests information from a nameserver, it usually connects to port 53. The nameserver then attempts to resolve the name requested. If it does not have an authoritative answer, or does not already have the answer cached from an earlier query, it queries other nameservers, called root nameservers, to determine which nameservers are authoritative for the name in question, and then queries them to get the requested name.

11.1.1. Nameserver Zones

In a DNS server such as BIND, all information is stored in basic data elements called resource records (RR). The resource record is usually a fully qualified domain name (FQDN) of a host, and is broken down into multiple sections organized into a tree-like hierarchy. This hierarchy consists of a main trunk, primary branches, secondary branches, and so on. The following is an example of a resource record:
bob.sales.example.com
Each level of the hierarchy is divided by a period (that is, .). In the example above, com defines the top-level domain, example its subdomain, and sales the subdomain of example. In this case, bob identifies a resource record that is part of the sales.example.com domain. With the exception of the part furthest to the left (that is, bob), each of these sections is called a zone and defines a specific namespace.
Zones are defined on authoritative nameservers through the use of zone files, which contain definitions of the resource records in each zone. Zone files are stored on primary nameservers (also called master nameservers), where changes are made to the files, and secondary nameservers (also called slave nameservers), which receive zone definitions from the primary nameservers. Both primary and secondary nameservers are authoritative for the zone and look the same to clients. Depending on the configuration, any nameserver can also serve as a primary or secondary server for multiple zones at the same time.

11.1.2. Nameserver Types

There are two nameserver configuration types:
authoritative
Authoritative nameservers answer to resource records that are part of their zones only. This category includes both primary (master) and secondary (slave) nameservers.
recursive
Recursive nameservers offer resolution services, but they are not authoritative for any zone. Answers for all resolutions are cached in a memory for a fixed period of time, which is specified by the retrieved resource record.
Although a nameserver can be both authoritative and recursive at the same time, it is recommended not to combine the configuration types. To be able to perform their work, authoritative servers should be available to all clients all the time. On the other hand, since the recursive lookup takes far more time than authoritative responses, recursive servers should be available to a restricted number of clients only, otherwise they are prone to distributed denial of service (DDoS) attacks.

11.1.3. BIND as a Nameserver

BIND consists of a set of DNS-related programs. It contains a monolithic nameserver called named, an administration utility called rndc, and a debugging tool called dig. Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.

11.2. BIND

This chapter covers BIND (Berkeley Internet Name Domain), the DNS server included in Fedora. It focuses on the structure of its configuration files, and describes how to administer it both locally and remotely.

11.2.1. Configuring the named Service

When the named service is started, it reads the configuration from the files as described in Table 11.1, “The named service configuration files”.
Table 11.1. The named service configuration files
Path Description
/etc/named.conf The main configuration file.
/etc/named/ An auxiliary directory for configuration files that are included in the main configuration file.

The configuration file consists of a collection of statements with nested options surrounded by opening and closing curly brackets (that is, { and }). Note that when editing the file, you have to be careful not to make any syntax error, otherwise the named service will not start. A typical /etc/named.conf file is organized as follows:
statement-1 ["statement-1-name"] [statement-1-class] {
  option-1;
  option-2;
  option-N;
};
statement-2 ["statement-2-name"] [statement-2-class] {
  option-1;
  option-2;
  option-N;
};
statement-N ["statement-N-name"] [statement-N-class] {
  option-1;
  option-2;
  option-N;
};

Running BIND in a chroot environment

If you have installed the bind-chroot package, the BIND service will run in the /var/named/chroot environment. In that case, the initialization script will mount the above configuration files using the mount --bind command, so that you can manage the configuration outside this environment.

11.2.1.1. Common Statement Types

The following types of statements are commonly used in /etc/named.conf:
acl
The acl (Access Control List) statement allows you to define groups of hosts, so that they can be permitted or denied access to the nameserver. It takes the following form:
acl acl-name {
  match-element;
  ...
};
The acl-name statement name is the name of the access control list, and the match-element option is usually an individual IP address (such as 10.0.1.1) or a CIDR network notation (for example, 10.0.1.0/24). For a list of already defined keywords, see Table 11.2, “Predefined access control lists”.
Table 11.2. Predefined access control lists
Keyword Description
any Matches every IP address.
localhost Matches any IP address that is in use by the local system.
localnets Matches any IP address on any network to which the local system is connected.
none Does not match any IP address.

The acl statement can be especially useful with conjunction with other statements such as options. Example 11.1, “Using acl in conjunction with options” defines two access control lists, black-hats and red-hats, and adds black-hats on the blacklist while granting red-hats a normal access.
Example 11.1. Using acl in conjunction with options
acl black-hats {
  10.0.2.0/24;
  192.168.0.0/24;
  1234:5678::9abc/24;
};
acl red-hats {
  10.0.1.0/24;
};
options {
  blackhole { black-hats; };
  allow-query { red-hats; };
  allow-query-cache { red-hats; };
};

include
The include statement allows you to include files in the /etc/named.conf, so that potentially sensitive data can be placed in a separate file with restricted permissions. It takes the following form:
include "file-name"
The file-name statement name is an absolute path to a file.
Example 11.2. Including a file to /etc/named.conf
include "/etc/named.rfc1912.zones";

options
The options statement allows you to define global server configuration options as well as to set defaults for other statements. It can be used to specify the location of the named working directory, the types of queries allowed, and much more. It takes the following form:
options {
  option;
  ...
};
For a list of frequently used option directives, see Table 11.3, “Commonly used options” below.
Table 11.3. Commonly used options
Option Description
allow-query Specifies which hosts are allowed to query the nameserver for authoritative resource records. It accepts an access control lists, a collection of IP addresses, or networks in the CIDR notation. All hosts are allowed by default.
allow-query-cache Specifies which hosts are allowed to query the nameserver for non-authoritative data such as recursive queries. Only localhost and localnets are allowed by default.
blackhole Specifies which hosts are not allowed to query the nameserver. This option should be used when particular host or network floods the server with requests. The default option is none.
directory Specifies a working directory for the named service. The default option is /var/named/.
dnssec-enable Specifies whether to return DNSSEC related resource records. The default option is yes.
dnssec-validation Specifies whether to prove that resource records are authentic via DNSSEC. The default option is yes.
forwarders Specifies a list of valid IP addresses for nameservers to which the requests should be forwarded for resolution.
forward
Specifies the behavior of the forwarders directive. It accepts the following options:
  • first — The server will query the nameservers listed in the forwarders directive before attempting to resolve the name on its own.
  • only — When unable to query the nameservers listed in the forwarders directive, the server will not attempt to resolve the name on its own.
listen-on Specifies the IPv4 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv4 interfaces are used by default.
listen-on-v6 Specifies the IPv6 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv6 interfaces are used by default.
max-cache-size Specifies the maximum amount of memory to be used for server caches. When the limit is reached, the server causes records to expire prematurely so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the cache of each view. The default option is 32M.
notify
Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:
  • yes — The server will notify all secondary nameservers.
  • no — The server will not notify any secondary nameserver.
  • master-only — The server will notify primary server for the zone only.
  • explicit — The server will notify only the secondary servers that are specified in the also-notify list within a zone statement.
pid-file Specifies the location of the process ID file created by the named service.
recursion Specifies whether to act as a recursive server. The default option is yes.
statistics-file Specifies an alternate location for statistics files. The /var/named/named.stats file is used by default.

Restrict recursive servers to selected clients only

To prevent distributed denial of service (DDoS) attacks, it is recommended that you use the allow-query-cache option to restrict recursive DNS services for a particular subset of clients only.
Refer to the BIND 9 Administrator Reference Manual referenced in Section 11.2.7.1, “Installed Documentation”, and the named.conf manual page for a complete list of available options.
Example 11.3. Using the options statement
options {
  allow-query       { localhost; };
  listen-on port    53 { 127.0.0.1; };
  listen-on-v6 port 53 { ::1; };
  max-cache-size    256M;
  directory         "/var/named";
  statistics-file   "/var/named/data/named_stats.txt";

  recursion         yes;
  dnssec-enable     yes;
  dnssec-validation yes;
};

zone
The zone statement allows you to define the characteristics of a zone, such as the location of its configuration file and zone-specific options, and can be used to override the global options statements. It takes the following form:
zone zone-name [zone-class] {
  option;
  ...
};
The zone-name attribute is the name of the zone, zone-class is the optional class of the zone, and option is a zone statement option as described in Table 11.4, “Commonly used options”.
The zone-name attribute is particularly important, as it is the default value assigned for the $ORIGIN directive used within the corresponding zone file located in the /var/named/ directory. The named daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file. For example, if a zone statement defines the namespace for example.com, use example.com as the zone-name so that it is placed at the end of hostnames within the example.com zone file.
For more information about zone files, refer to Section 11.2.2, “Editing Zone Files”.
Table 11.4. Commonly used options
Option Description
allow-query Specifies which clients are allowed to request information about this zone. This option overrides global allow-query option. All query requests are allowed by default.
allow-transfer Specifies which secondary servers are allowed to request a transfer of the zone's information. All transfer requests are allowed by default.
allow-update
Specifies which hosts are allowed to dynamically update information in their zone. The default option is to deny all dynamic update requests.
Note that you should be careful when allowing hosts to update information about their zone. Do not set IP addresses in this option unless the server is in the trusted network. Instead, use TSIG key as described in Section 11.2.5.3, “Transaction SIGnatures (TSIG)”.
file Specifies the name of the file in the named working directory that contains the zone's configuration data.
masters Specifies from which IP addresses to request authoritative zone information. This option is used only if the zone is defined as type slave.
notify
Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:
  • yes — The server will notify all secondary nameservers.
  • no — The server will not notify any secondary nameserver.
  • master-only — The server will notify primary server for the zone only.
  • explicit — The server will notify only the secondary servers that are specified in the also-notify list within a zone statement.
type
Specifies the zone type. It accepts the following options:
  • delegation-only — Enforces the delegation status of infrastructure zones such as COM, NET, or ORG. Any answer that is received without an explicit or implicit delegation is treated as NXDOMAIN. This option is only applicable in TLDs or root zone files used in recursive or caching implementations.
  • forward — Forwards all requests for information about this zone to other nameservers.
  • hint — A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with a hint zone.
  • master — Designates the nameserver as authoritative for this zone. A zone should be set as the master if the zone's configuration files reside on the system.
  • slave — Designates the nameserver as a slave server for this zone. Master server is specified in masters directive.

Most changes to the /etc/named.conf file of a primary or secondary nameserver involve adding, modifying, or deleting zone statements, and only a small subset of zone statement options is usually needed for a nameserver to work efficiently.
In Example 11.4, “A zone statement for a primary nameserver”, the zone is identified as example.com, the type is set to master, and the named service is instructed to read the /var/named/example.com.zone file. It also allows only a secondary nameserver (192.168.0.2) to transfer the zone.
Example 11.4. A zone statement for a primary nameserver
zone "example.com" IN {
  type master;
  file "example.com.zone";
  allow-transfer { 192.168.0.2; };
};

A secondary server's zone statement is slightly different. The type is set to slave, and the masters directive is telling named the IP address of the master server.
In Example 11.5, “A zone statement for a secondary nameserver”, the named service is configured to query the primary server at the 192.168.0.1 IP address for information about the example.com zone. The received information is then saved to the /var/named/slaves/example.com.zone file. Note that you have to put all slave zones to /var/named/slaves directory, otherwise the service will fail to transfer the zone.
Example 11.5. A zone statement for a secondary nameserver
zone "example.com" {
  type slave;
  file "slaves/example.com.zone";
  masters { 192.168.0.1; };
};

11.2.1.2. Other Statement Types

The following types of statements are less commonly used in /etc/named.conf:
controls
The controls statement allows you to configure various security requirements necessary to use the rndc command to administer the named service.
Refer to Section 11.2.3, “Using the rndc Utility” for more information on the rndc utility and its usage.
key
The key statement allows you to define a particular key by name. Keys are used to authenticate various actions, such as secure updates or the use of the rndc command. Two options are used with key:
  • algorithm algorithm-name — The type of algorithm to be used (for example, hmac-md5).
  • secret "key-value" — The encrypted key.
Refer to Section 11.2.3, “Using the rndc Utility” for more information on the rndc utility and its usage.
logging
The logging statement allows you to use multiple types of logs, so called channels. By using the channel option within the statement, you can construct a customized type of log with its own file name (file), size limit (size), versioning (version), and level of importance (severity). Once a customized channel is defined, a category option is used to categorize the channel and begin logging when the named service is restarted.
By default, named sends standard messages to the rsyslog daemon, which places them in /var/log/messages. Several standard channels are built into BIND with various severity levels, such as default_syslog (which handles informational logging messages) and default_debug (which specifically handles debugging messages). A default category, called default, uses the built-in channels to do normal logging without any special configuration.
Customizing the logging process can be a very detailed process and is beyond the scope of this chapter. For information on creating custom BIND logs, refer to the BIND 9 Administrator Reference Manual referenced in Section 11.2.7.1, “Installed Documentation”.
server
The server statement allows you to specify options that affect how the named service should respond to remote nameservers, especially with regard to notifications and zone transfers.
The transfer-format option controls the number of resource records that are sent with each message. It can be either one-answer (only one resource record), or many-answers (multiple resource records). Note that while the many-answers option is more efficient, it is not supported by older versions of BIND.
trusted-keys
The trusted-keys statement allows you to specify assorted public keys used for secure DNS (DNSSEC). Refer to Section 11.2.5.4, “DNS Security Extensions (DNSSEC)” for more information on this topic.
view
The view statement allows you to create special views depending upon which network the host querying the nameserver is on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to particular trusted hosts while non-trusted hosts can only make queries for other zones.
Multiple views can be used as long as their names are unique. The match-clients option allows you to specify the IP addresses that apply to a particular view. If the options statement is used within a view, it overrides the already configured global options. Finally, most view statements contain multiple zone statements that apply to the match-clients list.
Note that the order in which the view statements are listed is important, as the first statement that matches a particular client's IP address is used. For more information on this topic, refer to Section 11.2.5.1, “Multiple Views”.

11.2.1.3. Comment Tags

Additionally to statements, the /etc/named.conf file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to a user. The following are valid comment tags:
//
Any text after the // characters to the end of the line is considered a comment. For example:
notify yes;  // notify all secondary nameservers
#
Any text after the # character to the end of the line is considered a comment. For example:
notify yes;  # notify all secondary nameservers
/* and */
Any block of text enclosed in /* and */ is considered a comment. For example:
notify yes;  /* notify all secondary nameservers */

11.2.2. Editing Zone Files

As outlined in Section 11.1.1, “Nameserver Zones”, zone files contain information about a namespace. They are stored in the named working directory located in /var/named/ by default, and each zone file is named according to the file option in the zone statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone.
Table 11.5. The named service zone files
Path Description
/var/named/ The working directory for the named service. The nameserver is not allowed to write to this directory.
/var/named/slaves/ The directory for secondary zones. This directory is writable by the named service.
/var/named/dynamic/ The directory for other files, such as dynamic DNS (DDNS) zones or managed DNSSEC keys. This directory is writable by the named service.
/var/named/data/ The directory for various statistics and debugging files. This directory is writable by the named service.

A zone file consists of directives and resource records. Directives tell the nameserver to perform tasks or apply special settings to the zone, resource records define the parameters of the zone and assign identities to individual hosts. While the directives are optional, the resource records are required in order to provide name service to a zone.
All directives and resource records should be entered on individual lines.

11.2.2.1. Common Directives

Directives begin with the dollar sign character (that is, $) followed by the name of the directive, and usually appear at the top of the file. The following directives are commonly used in zone files:
$INCLUDE
The $INCLUDE directive allows you to include another file at the place where it appears, so that other zone settings can be stored in a separate zone file.
Example 11.6. Using the $INCLUDE directive
$INCLUDE /var/named/penguin.example.com

$ORIGIN
The $ORIGIN directive allows you to append the domain name to unqualified records, such as those with the hostname only. Note that the use of this directive is not necessary if the zone is specified in /etc/named.conf, since the zone name is used by default.
In Example 11.7, “Using the $ORIGIN directive”, any names used in resource records that do not end in a trailing period (that is, the . character) are appended with example.com.
Example 11.7. Using the $ORIGIN directive
$ORIGIN example.com.

$TTL
The $TTL directive allows you to set the default Time to Live (TTL) value for the zone, that is, how long is a zone record valid. Each resource record can contain its own TTL value, which overrides this directive.
Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to proliferate resource record changes.
Example 11.8. Using the $TTL directive
$TTL 1D

11.2.2.2. Common Resource Records

The following resource records are commonly used in zone files:
A
The Address record specifies an IP address to be assigned to a name. It takes the following form:
hostname IN A IP-address
If the hostname value is omitted, the record will point to the last specified hostname.
In Example 11.9, “Using the A resource record”, the requests for server1.example.com are pointed to 10.0.1.3 or 10.0.1.5.
Example 11.9. Using the A resource record
server1  IN  A  10.0.1.3
         IN  A  10.0.1.5

CNAME
The Canonical Name record maps one name to another. Because of this, this type of record is sometimes referred to as an alias record. It takes the following form:
alias-name IN CNAME real-name
CNAME records are most commonly used to point to services that use a common naming scheme, such as www for Web servers. However, there are multiple restrictions for their usage:
  • CNAME records should not point to other CNAME records. This is mainly to avoid possible infinite loops.
  • CNAME records should not contain other resource record types (such as A, NS, MX, etc.). The only exception are DNSSEC related records (that is, RRSIG, NSEC, etc.) when the zone is signed.
  • Other resource record that point to the fully qualified domain name (FQDN) of a host (that is, NS, MX, PTR) should not point to a CNAME record.
In Example 11.10, “Using the CNAME resource record”, the A record binds a hostname to an IP address, while the CNAME record points the commonly used www hostname to it.
Example 11.10. Using the CNAME resource record
server1  IN  A      10.0.1.5
www      IN  CNAME  server1

MX
The Mail Exchange record specifies where the mail sent to a particular namespace controlled by this zone should go. It takes the following form:
IN MX preference-value email-server-name
The email-server-name is a fully qualified domain name (FQDN). The preference-value allows numerical ranking of the email servers for a namespace, giving preference to some email systems over others. The MX resource record with the lowest preference-value is preferred over the others. However, multiple email servers can possess the same value to distribute email traffic evenly among them.
In Example 11.11, “Using the MX resource record”, the first mail.example.com email server is preferred to the mail2.example.com email server when receiving email destined for the example.com domain.
Example 11.11. Using the MX resource record
example.com.  IN  MX  10  mail.example.com.
              IN  MX  20  mail2.example.com.

NS
The Nameserver record announces authoritative nameservers for a particular zone. It takes the following form:
IN NS nameserver-name
The nameserver-name should be a fully qualified domain name (FQDN). Note that when two nameservers are listed as authoritative for the domain, it is not important whether these nameservers are secondary nameservers, or if one of them is a primary server. They are both still considered authoritative.
Example 11.12. Using the NS resource record
IN  NS  dns1.example.com.
IN  NS  dns2.example.com.

PTR
The Pointer record points to another part of the namespace. It takes the following form:
last-IP-digit IN PTR FQDN-of-system
The last-IP-digit directive is the last number in an IP address, and the FQDN-of-system is a fully qualified domain name (FQDN).
PTR records are primarily used for reverse name resolution, as they point IP addresses back to a particular name. Refer to Section 11.2.2.4.2, “A Reverse Name Resolution Zone File” for more examples of PTR records in use.
SOA
The Start of Authority record announces important authoritative information about a namespace to the nameserver. Located after the directives, it is the first resource record in a zone file. It takes the following form:
@  IN  SOA  primary-name-server hostmaster-email (
       serial-number
       time-to-refresh
       time-to-retry
       time-to-expire
       minimum-TTL )
The directives are as follows:
  • The @ symbol places the $ORIGIN directive (or the zone's name if the $ORIGIN directive is not set) as the namespace being defined by this SOA resource record.
  • The primary-name-server directive is the hostname of the primary nameserver that is authoritative for this domain.
  • The hostmaster-email directive is the email of the person to contact about the namespace.
  • The serial-number directive is a numerical value incremented every time the zone file is altered to indicate it is time for the named service to reload the zone.
  • The time-to-refresh directive is the numerical value secondary nameservers use to determine how long to wait before asking the primary nameserver if any changes have been made to the zone.
  • The time-to-retry directive is a numerical value used by secondary nameservers to determine the length of time to wait before issuing a refresh request in the event that the primary nameserver is not answering. If the primary server has not replied to a refresh request before the amount of time specified in the time-to-expire directive elapses, the secondary servers stop responding as an authority for requests concerning that namespace.
  • In BIND 4 and 8, the minimum-TTL directive is the amount of time other nameservers cache the zone's information. In BIND 9, it defines how long negative answers are cached for. Caching of negative answers can be set to a maximum of 3 hours (that is, 3H).
When configuring BIND, all times are specified in seconds. However, it is possible to use abbreviations when specifying units of time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). Table 11.6, “Seconds compared to other time units” shows an amount of time in seconds and the equivalent time in another format.
Table 11.6. Seconds compared to other time units
Seconds Other Time Units
60 1M
1800 30M
3600 1H
10800 3H
21600 6H
43200 12H
86400 1D
259200 3D
604800 1W
31536000 365D

Example 11.13. Using the SOA resource record
@  IN  SOA  dns1.example.com.  hostmaster.example.com. (
       2001062501  ; serial
       21600       ; refresh after 6 hours
       3600        ; retry after 1 hour
       604800      ; expire after 1 week
       86400 )     ; minimum TTL of 1 day

11.2.2.3. Comment Tags

Additionally to resource records and directives, a zone file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to the user. Any text after the semicolon character (that is, ;) to the end of the line is considered a comment. For example:
   604800  ; expire after 1 week

11.2.2.4. Example Usage

The following examples show the basic usage of zone files.
11.2.2.4.1. A Simple Zone File
Example 11.14, “A simple zone file” demonstrates the use of standard directives and SOA values.
Example 11.14. A simple zone file
$ORIGIN example.com.
$TTL 86400
@         IN  SOA  dns1.example.com.  hostmaster.example.com. (
              2001062501  ; serial
              21600       ; refresh after 6 hours
              3600        ; retry after 1 hour
              604800      ; expire after 1 week
              86400 )     ; minimum TTL of 1 day
;
;
          IN  NS     dns1.example.com.
          IN  NS     dns2.example.com.
dns1      IN  A      10.0.1.1
          IN  AAAA   aaaa:bbbb::1
dns2      IN  A      10.0.1.2
          IN  AAAA   aaaa:bbbb::2
;
;
@         IN  MX     10  mail.example.com.
          IN  MX     20  mail2.example.com.
mail      IN  A      10.0.1.5
          IN  AAAA   aaaa:bbbb::5
mail2     IN  A      10.0.1.6
          IN  AAAA   aaaa:bbbb::6
;
;
; This sample zone file illustrates sharing the same IP addresses
; for multiple services:
;
services  IN  A      10.0.1.10
          IN  AAAA   aaaa:bbbb::10
          IN  A      10.0.1.11
          IN  AAAA   aaaa:bbbb::11

ftp       IN  CNAME  services.example.com.
www       IN  CNAME  services.example.com.
;
;

In this example, the authoritative nameservers are set as dns1.example.com and dns2.example.com, and are tied to the 10.0.1.1 and 10.0.1.2 IP addresses respectively using the A record.
The email servers configured with the MX records point to mail and mail2 via A records. Since these names do not end in a trailing period (that is, the . character), the $ORIGIN domain is placed after them, expanding them to mail.example.com and mail2.example.com.
Services available at the standard names, such as www.example.com (WWW), are pointed at the appropriate servers using the CNAME record.
This zone file would be called into service with a zone statement in the /etc/named.conf similar to the following:
zone "example.com" IN {
  type master;
  file "example.com.zone";
  allow-update { none; };
};
11.2.2.4.2. A Reverse Name Resolution Zone File
A reverse name resolution zone file is used to translate an IP address in a particular namespace into an fully qualified domain name (FQDN). It looks very similar to a standard zone file, except that the PTR resource records are used to link the IP addresses to a fully qualified domain name as shown in Example 11.15, “A reverse name resolution zone file”.
Example 11.15. A reverse name resolution zone file
$ORIGIN 1.0.10.in-addr.arpa.
$TTL 86400
@  IN  SOA  dns1.example.com.  hostmaster.example.com. (
       2001062501  ; serial
       21600       ; refresh after 6 hours
       3600        ; retry after 1 hour
       604800      ; expire after 1 week
       86400 )     ; minimum TTL of 1 day
;
@  IN  NS   dns1.example.com.
;
1  IN  PTR  dns1.example.com.
2  IN  PTR  dns2.example.com.
;
5  IN  PTR  server1.example.com.
6  IN  PTR  server2.example.com.
;
3  IN  PTR  ftp.example.com.
4  IN  PTR  ftp.example.com.

In this example, IP addresses 10.0.1.1 through 10.0.1.6 are pointed to the corresponding fully qualified domain name.
This zone file would be called into service with a zone statement in the /etc/named.conf file similar to the following:
zone "1.0.10.in-addr.arpa" IN {
  type master;
  file "example.com.rr.zone";
  allow-update { none; };
};
There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.

11.2.3. Using the rndc Utility

The rndc utility is a command line tool that allows you to administer the named service, both locally and from a remote machine. Its usage is as follows:
rndc [option...] command [command-option]

11.2.3.1. Configuring the Utility

To prevent unauthorized access to the service, named must be configured to listen on the selected port (that is, 953 by default), and an identical key must be used by both the service and the rndc utility.
Table 11.7. Relevant files
Path Description
/etc/named.conf The default configuration file for the named service.
/etc/rndc.conf The default configuration file for the rndc utility.
/etc/rndc.key The default key location.

The rndc configuration is located in /etc/rndc.conf. If the file does not exist, the utility will use the key located in /etc/rndc.key, which was generated automatically during the installation process using the rndc-confgen -a command.
The named service is configured using the controls statement in the /etc/named.conf configuration file as described in Section 11.2.1.2, “Other Statement Types”. Unless this statement is present, only the connections from the loopback address (that is, 127.0.0.1) will be allowed, and the key located in /etc/rndc.key will be used.
For more information on this topic, refer to manual pages and the BIND 9 Administrator Reference Manual listed in Section 11.2.7, “Additional Resources”.

Set the correct permissions

To prevent unprivileged users from sending control commands to the service, make sure only root is allowed to read the /etc/rndc.key file:
~]# chmod o-rwx /etc/rndc.key

11.2.3.2. Checking the Service Status

To check the current status of the named service, use the following command:
~]# rndc status
version: 9.7.0-P2-RedHat-9.7.0-5.P2.el6
CPUs found: 1
worker threads: 1
number of zones: 16
debug level: 0
xfers running: 0
xfers deferred: 0
soa queries in progress: 0
query logging is OFF
recursive clients: 0/0/1000
tcp clients: 0/100
server is up and running

11.2.3.3. Reloading the Configuration and Zones

To reload both the configuration file and zones, type the following at a shell prompt:
~]# rndc reload
server reload successful
This will reload the zones while keeping all previously cached responses, so that you can make changes to the zone files without losing all stored name resolutions.
To reload a single zone, specify its name after the reload command, for example:
~]# rndc reload localhost
zone reload up-to-date
Finally, to reload the configuration file and newly added zones only, type:
~]# rndc reconfig

Modifying zones with dynamic DNS

If you intend to manually modify a zone that uses Dynamic DNS (DDNS), make sure you run the freeze command first:
~]# rndc freeze localhost
Once you are finished, run the thaw command to allow the DDNS again and reload the zone:
~]# rndc thaw localhost
The zone reload and thaw was successful.

11.2.3.4. Updating Zone Keys

To update the DNSSEC keys and sign the zone, use the sign command. For example:
~]# rndc sign localhost
Note that to sign a zone with the above command, the auto-dnssec option has to be set to maintain in the zone statement. For instance:
zone "localhost" IN {
  type master;
  file "named.localhost";
  allow-update { none; };
  auto-dnssec maintain;
};

11.2.3.5. Enabling the DNSSEC Validation

To enable the DNSSEC validation, type the following at a shell prompt:
~]# rndc validation on
Similarly, to disable this option, type:
~]# rndc validation off
Refer to the options statement described in Section 11.2.1.1, “Common Statement Types” for information on how configure this option in /etc/named.conf.

11.2.3.6. Enabling the Query Logging

To enable (or disable in case it is currently enabled) the query logging, run the following command:
~]# rndc querylog
To check the current setting, use the status command as described in Section 11.2.3.2, “Checking the Service Status”.

11.2.4. Using the dig Utility

The dig utility is a command line tool that allows you to perform DNS lookups and debug a nameserver configuration. Its typical usage is as follows:
dig [@server] [option...] name type
Refer to Section 11.2.2.2, “Common Resource Records” for a list of common types.

11.2.4.1. Looking Up a Nameserver

To look up a nameserver for a particular domain, use the command in the following form:
dig name NS
In Example 11.16, “A sample nameserver lookup”, the dig utility is used to display nameservers for example.com.
Example 11.16. A sample nameserver lookup
~]$ dig example.com NS

; <<>> DiG 9.7.1-P2-RedHat-9.7.1-2.P2.fc13 <<>> example.com NS
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57883
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;example.com.                   IN      NS

;; ANSWER SECTION:
example.com.            99374   IN      NS      a.iana-servers.net.
example.com.            99374   IN      NS      b.iana-servers.net.

;; Query time: 1 msec
;; SERVER: 10.34.255.7#53(10.34.255.7)
;; WHEN: Wed Aug 18 18:04:06 2010
;; MSG SIZE  rcvd: 77

11.2.4.2. Looking Up an IP Address

To look up an IP address assigned to a particular domain, use the command in the following form:
dig name A
In Example 11.17, “A sample IP address lookup”, the dig utility is used to display the IP address of example.com.
Example 11.17. A sample IP address lookup
~]$ dig example.com A

; <<>> DiG 9.7.1-P2-RedHat-9.7.1-2.P2.fc13 <<>> example.com A
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4849
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;example.com.                   IN      A

;; ANSWER SECTION:
example.com.            155606  IN      A       192.0.32.10

;; AUTHORITY SECTION:
example.com.            99175   IN      NS      a.iana-servers.net.
example.com.            99175   IN      NS      b.iana-servers.net.

;; Query time: 1 msec
;; SERVER: 10.34.255.7#53(10.34.255.7)
;; WHEN: Wed Aug 18 18:07:25 2010
;; MSG SIZE  rcvd: 93

11.2.4.3. Looking Up a Hostname

To look up a hostname for a particular IP address, use the command in the following form:
dig -x address
In Example 11.18, “A sample hostname lookup”, the dig utility is used to display the hostname assigned to 192.0.32.10.
Example 11.18. A sample hostname lookup
~]$ dig -x 192.0.32.10

; <<>> DiG 9.7.1-P2-RedHat-9.7.1-2.P2.fc13 <<>> -x 192.0.32.10
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29683
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 6

;; QUESTION SECTION:
;10.32.0.192.in-addr.arpa.      IN      PTR

;; ANSWER SECTION:
10.32.0.192.in-addr.arpa. 21600 IN      PTR     www.example.com.

;; AUTHORITY SECTION:
32.0.192.in-addr.arpa.  21600   IN      NS      b.iana-servers.org.
32.0.192.in-addr.arpa.  21600   IN      NS      c.iana-servers.net.
32.0.192.in-addr.arpa.  21600   IN      NS      d.iana-servers.net.
32.0.192.in-addr.arpa.  21600   IN      NS      ns.icann.org.
32.0.192.in-addr.arpa.  21600   IN      NS      a.iana-servers.net.

;; ADDITIONAL SECTION:
a.iana-servers.net.     13688   IN      A       192.0.34.43
b.iana-servers.org.     5844    IN      A       193.0.0.236
b.iana-servers.org.     5844    IN      AAAA    2001:610:240:2::c100:ec
c.iana-servers.net.     12173   IN      A       139.91.1.10
c.iana-servers.net.     12173   IN      AAAA    2001:648:2c30::1:10
ns.icann.org.           12884   IN      A       192.0.34.126

;; Query time: 156 msec
;; SERVER: 10.34.255.7#53(10.34.255.7)
;; WHEN: Wed Aug 18 18:25:15 2010
;; MSG SIZE  rcvd: 310

11.2.5. Advanced Features of BIND

Most BIND implementations only use the named service to provide name resolution services or to act as an authority for a particular domain. However, BIND version 9 has a number of advanced features that allow for a more secure and efficient DNS service.

Make sure the feature is supported

Before attempting to use advanced features like DNSSEC, TSIG, or IXFR, make sure that the particular feature is supported by all nameservers in the network environment, especially when you use older versions of BIND or non-BIND servers.
All of the features mentioned are discussed in greater detail in the BIND 9 Administrator Reference Manual referenced in Section 11.2.7.1, “Installed Documentation”.

11.2.5.1. Multiple Views

Optionally, different information can be presented to a client depending on the network a request originates from. This is primarily used to deny sensitive DNS entries from clients outside of the local network, while allowing queries from clients inside the local network.
To configure multiple views, add the view statement to the /etc/named.conf configuration file. Use the match-clients option to match IP addresses or entire networks and give them special options and zone data.

11.2.5.2. Incremental Zone Transfers (IXFR)

Incremental Zone Transfers (IXFR) allow a secondary nameserver to only download the updated portions of a zone modified on a primary nameserver. Compared to the standard transfer process, this makes the notification and update process much more efficient.
Note that IXFR is only available when using dynamic updating to make changes to master zone records. If manually editing zone files to make changes, Automatic Zone Transfer (AXFR) is used.

11.2.5.3. Transaction SIGnatures (TSIG)

Transaction SIGnatures (TSIG) ensure that a shared secret key exists on both primary and secondary nameserver before allowing a transfer. This strengthens the standard IP address-based method of transfer authorization, since attackers would not only need to have access to the IP address to transfer the zone, but they would also need to know the secret key.
Since version 9, BIND also supports TKEY, which is another shared secret key method of authorizing zone transfers.

Secure the transfer

When communicating over an insecure network, do not rely on IP address-based authentication only.

11.2.5.4. DNS Security Extensions (DNSSEC)

Domain Name System Security Extensions (DNSSEC) provide origin authentication of DNS data, authenticated denial of existence, and data integrity. When a particular domain is marked as secure, the SERFVAIL response is returned for each resource record that fails the validation.
Note that to debug a DNSSEC-signed domain or a DNSSEC-aware resolver, you can use the dig utility as described in Section 11.2.4, “Using the dig Utility”. Useful options are +dnssec (requests DNSSEC-related resource records by setting the DNSSEC OK bit), +cd (tells recursive nameserver not to validate the response), and +bufsize=512 (changes the packet size to 512B to get through some firewalls).

11.2.5.5. Internet Protocol version 6 (IPv6)

Internet Protocol version 6 (IPv6) is supported through the use of AAAA resource records, and the listen-on-v6 directive as described in Table 11.3, “Commonly used options”.

11.2.6. Common Mistakes to Avoid

The following is a list of advices how to avoid common mistakes users make when configuring a nameserver:
Use semicolons and curly brackets correctly
An omitted semicolon or unmatched curly bracket in the /etc/named.conf file can prevent the named service from starting.
Use period (that is, the . character) correctly
In zone files, a period at the end of a domain name denotes a fully qualified domain name. If omitted, the named service will append the name of the zone or the value of $ORIGIN to complete it.
Increment the serial number when editing a zone file
If the serial number is not incremented, the primary nameserver will have the correct, new information, but the secondary nameservers will never be notified of the change, and will not attempt to refresh their data of that zone.
Configure the firewall
If a firewall is blocking connections from the named service to other nameservers, the recommended best practice is to change the firewall settings whenever possible.

Avoid using fixed UDP source ports

According to the recent research in DNS security, using a fixed UDP source port for DNS queries is a potential security vulnerability that could allow an attacker to conduct cache-poisoning attacks more easily. To prevent this, configure your firewall to allow queries from a random UDP source port.

11.2.7. Additional Resources

The following sources of information provide additional resources regarding BIND.

11.2.7.1. Installed Documentation

BIND features a full range of installed documentation covering many different topics, each placed in its own subject directory. For each item below, replace version with the version of the bind package installed on the system:
/usr/share/doc/bind-version/
The main directory containing the most recent documentation.
/usr/share/doc/bind-version/arm/
The directory containing the BIND 9 Administrator Reference Manual in HTML and SGML formats, which details BIND resource requirements, how to configure different types of nameservers, how to perform load balancing, and other advanced topics. For most new users of BIND, this is the best place to start.
/usr/share/doc/bind-version/draft/
The directory containing assorted technical documents that review issues related to the DNS service, and propose some methods to address them.
/usr/share/doc/bind-version/misc/
The directory designed to address specific advanced issues. Users of BIND version 8 should consult the migration document for specific changes they must make when moving to BIND 9. The options file lists all of the options implemented in BIND 9 that are used in /etc/named.conf.
/usr/share/doc/bind-version/rfc/
The directory providing every RFC document related to BIND.
There is also a number of man pages for the various applications and configuration files involved with BIND:
man rndc
The manual page for rndc containing the full documentation on its usage.
man named
The manual page for named containing the documentation on assorted arguments that can be used to control the BIND nameserver daemon.
man lwresd
The manual page for lwresd containing the full documentation on the lightweight resolver daemon and its usage.
man named.conf
The manual page with a comprehensive list of options available within the named configuration file.
man rndc.conf
The manual page with a comprehensive list of options available within the rndc configuration file.

11.2.7.2. Useful Websites

http://www.isc.org/software/bind
The home page of the BIND project containing information about current releases as well as a PDF version of the BIND 9 Administrator Reference Manual.

Chapter 12. Web Servers

HTTP (Hypertext Transfer Protocol) server, or a web server, is a network service that serves content to a client over the web. This typically means web pages, but any other documents can be served as well.

12.1. The Apache HTTP Server

This section focuses on the Apache HTTP Server 2.2, a robust, full-featured open source web server developed by the Apache Software Foundation, that is included in Fedora 15. It describes the basic configuration of the httpd service, and covers advanced topics such as adding server modules, setting up virtual hosts, or configuring the secure HTTP server.
There are important differences between the Apache HTTP Server 2.2 and version 2.0, and if you are upgrading from a previous release of Fedora, you will need to update the httpd service configuration accordingly. This section reviews some of the newly added features, outlines important changes, and guides you through the update of older configuration files.

12.1.1. New Features

The Apache HTTP Server version 2.2 introduces the following enhancements:
  • Improved caching modules, that is, mod_cache and mod_disk_cache.
  • Support for proxy load balancing, that is, the mod_proxy_balancer module.
  • Support for large files on 32-bit architectures, allowing the web server to handle files greater than 2GB.
  • A new structure for authentication and authorization support, replacing the authentication modules provided in previous versions.

12.1.2. Notable Changes

Since version 2.0, few changes have been made to the default httpd service configuration:
  • The following modules are no longer loaded by default: mod_cern_meta and mod_asis.
  • The following module is newly loaded by default: mod_ext_filter.

12.1.3. Updating the Configuration

To update the configuration files from the Apache HTTP Server version 2.0, take the following steps:
  1. Make sure all module names are correct, since they may have changed. Adjust the LoadModule directive for each module that has been renamed.
  2. Recompile all third party modules before attempting to load them. This typically means authentication and authorization modules.
  3. If you use the mod_userdir module, make sure the UserDir directive indicating a directory name (typically public_html) is provided.
  4. If you use the Apache HTTP Secure Server, edit the /etc/httpd/conf.d/ssl.conf to enable the Secure Sockets Layer (SSL) protocol.
Note that you can check the configuration for possible errors by using the following command:
service httpd configtest
For more information on upgrading the Apache HTTP Server configuration from version 2.0 to 2.2, refer to http://httpd.apache.org/docs/2.2/upgrading.html.

12.1.4. Running the httpd Service

This section describes how to start, stop, restart, and check the current status of the Apache HTTP Server. To be able to use the httpd service, make sure you have the httpd installed. You can do so by using the following command as root:
yum install httpd
For more information on the concept of runlevels and how to manage system services in Fedora in general, refer to Chapter 7, Services and Daemons.

12.1.4.1. Starting the Service

To run the httpd service, type the following at a shell prompt as root:
systemctl start httpd.service
If you want the service to start automatically at the boot time, use the following command:
systemctl enable httpd.service
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.

Using the secure server

If running the Apache HTTP Server as a secure server, a password may be required after the machine boots if using an encrypted private SSL key.

12.1.4.2. Stopping the Service

To stop the running httpd service, type the following at a shell prompt as root:
systemctl stop httpd.service
To prevent the service from starting automatically at the boot time, type:
systemctl disable httpd.service
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.

12.1.4.3. Restarting the Service

There are two different ways to restart the running httpd service:
  1. To restart the service completely, type the following at a shell prompt as root:
    systemctl restart httpd.service
    This will stop the running httpd service, and then start it again. Use this command after installing or removing a dynamically loaded module such as PHP.
  2. To only reload the configuration, as root, type:
    systemctl reload httpd.service
    This will cause the running httpd service to reload the configuration file. Note that any requests being currently processed will be interrupted, which may cause a client browser to display an error message or render a partial page.
  3. To reload the configuration without affecting active requests, run the following command as root:
    service httpd graceful
    This will cause the running httpd service to reload the configuration file. Note that any requests being currently processed will use the old configuration.
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.

12.1.4.4. Checking the Service Status

To check whether the service is running, type the following at a shell prompt:
systemctl is-active httpd.service
Refer to Chapter 7, Services and Daemons for more information on how to configure services in Fedora.

12.1.5. Editing the Configuration Files

When the httpd service is started, by default, it reads the configuration from locations that are listed in Table 12.1, “The httpd service configuration files”.
Table 12.1. The httpd service configuration files
Path Description
/etc/httpd/conf/httpd.conf The main configuration file.
/etc/httpd/conf.d/ An auxiliary directory for configuration files that are included in the main configuration file.

Although the default configuration should be suitable for most situations, it is a good idea to become at least familiar with some of the more important configuration options. Note that for any changes to take effect, the web server has to be restarted first. Refer to Section 12.1.4.3, “Restarting the Service” for more information on how to restart the httpd service.
To check the configuration for possible errors, type the following at a shell prompt:
service httpd configtest
To make the recovery from mistakes easier, it is recommended that you make a copy of the original file before editing it.

12.1.5.1. Common httpd.conf Directives

The following directives are commonly used in the /etc/httpd/conf/httpd.conf configuration file:
<Directory>
The <Directory> directive allows you to apply certain directives to a particular directory only. It takes the following form:
<Directory directory>
  directive
  …
</Directory>
The directory can be either a full path to an existing directory in the local file system, or a wildcard expression.
This directive can be used to configure additional cgi-bin directories for server-side scripts located outside the directory that is specified by ScriptAlias. In this case, the ExecCGI and AddHandler directives must be supplied, and the permissions on the target directory must be set correctly (that is, 0755).
Example 12.1. Using the <Directory> directive
<Directory /var/www/html>
  Options Indexes FollowSymLinks
  AllowOverride None
  Order allow,deny
  Allow from all
</Directory>

<IfDefine>
The IfDefine directive allows you to use certain directives only when a particular parameter is supplied on the command line. It takes the following form:
<IfDefine [!]parameter>
  directive
  …
</IfDefine>
The parameter can be supplied at a shell prompt using the -Dparameter command line option (for example, httpd -DEnableHome). If the optional exclamation mark (that is, !) is present, the enclosed directives are used only when the parameter is not specified.
Example 12.2. Using the <IfDefine> directive
<IfDefine EnableHome>
  UserDir public_html
</IfDefine>

<IfModule>
The <IfModule> directive allows you to use certain directive only when a particular module is loaded. It takes the following form:
<IfModule [!]module>
  directive
  …
</IfModule>
The module can be identified either by its name, or by the file name. If the optional exclamation mark (that is, !) is present, the enclosed directives are used only when the module is not loaded.
Example 12.3. Using the <IfModule> directive
<IfModule mod_disk_cache.c>
  CacheEnable disk /
  CacheRoot /var/cache/mod_proxy
</IfModule>

<Location>
The <Location> directive allows you to apply certain directives to a particular URL only. It takes the following form:
<Location url>
  directive
  …
</Location>
The url can be either a path relative to the directory specified by the DocumentRoot directive (for example, /server-info), or an external URL such as http://example.com/server-info.
Example 12.4. Using the <Location> directive
<Location /server-info>
  SetHandler server-info
  Order deny,allow
  Deny from all
  Allow from .example.com
</Location>

<Proxy>
The <Proxy> directive allows you to apply certain directives to the proxy server only. It takes the following form:
<Proxy pattern>
  directive
  …
</Proxy>
The pattern can be an external URL, or a wildcard expression (for example, http://example.com/*).
Example 12.5. Using the <Proxy> directive
<Proxy *>
  Order deny,allow
  Deny from all
  Allow from .example.com
</Proxy>

<VirtualHost>
The <VirtualHost> directive allows you apply certain directives to particular virtual hosts only. It takes the following form:
<VirtualHost address[:port]…>
  directive
  …
</VirtualHost>
The address can be an IP address, a fully qualified domain name, or a special form as described in Table 12.2, “Available <VirtualHost> options”.
Table 12.2. Available <VirtualHost> options
Option Description
* Represents all IP addresses.
_default_ Represents unmatched IP addresses.

Example 12.6. Using the <VirtualHost> directive
<VirtualHost *:80>
  ServerAdmin webmaster@penguin.example.com
  DocumentRoot /www/docs/penguin.example.com
  ServerName penguin.example.com
  ErrorLog logs/penguin.example.com-error_log
  CustomLog logs/penguin.example.com-access_log common
</VirtualHost>

AccessFileName
The AccessFileName directive allows you to specify the file to be used to customize access control information for each directory. It takes the following form:
AccessFileName filename
The filename is a name of the file to look for in the requested directory. By default, the server looks for .htaccess.
For security reasons, the directive is typically followed by the Files tag to prevent the files beginning with .ht from being accessed by web clients. This includes the .htaccess and .htpasswd files.
Example 12.7. Using the AccessFileName directive
AccessFileName .htaccess

<Files ~ "^\.ht">
  Order allow,deny
  Deny from all
  Satisfy All
</Files>

Action
The Action directive allows you to specify a CGI script to be executed when a certain media type is requested. It takes the following form:
Action content-type path
The content-type has to be a valid MIME type such as text/html, image/png, or application/pdf. The path refers to an existing CGI script, and must be relative to the directory specified by the DocumentRoot directive (for example, /cgi-bin/process-image.cgi).
Example 12.8. Using the Action directive
Action image/png /cgi-bin/process-image.cgi

AddDescription
The AddDescription directive allows you to specify a short description to be displayed in server-generated directory listings for a given file. It takes the following form:
AddDescription "description" filename
The description should be a short text enclosed in double quotes (that is, "). The filename can be a full file name, a file extension, or a wildcard expression.
Example 12.9. Using the AddDescription directive
AddDescription "GZIP compressed tar archive" .tgz

AddEncoding
The AddEncoding directive allows you to specify an encoding type for a particular file extension. It takes the following form:
AddEncoding encoding extension
The encoding has to be a valid MIME encoding such as x-compress, x-gzip, etc. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example, .gz).
This directive is typically used to instruct web browsers to decompress certain file types as they are downloaded.
Example 12.10. Using the AddEncoding directive
AddEncoding x-gzip .gz .tgz

AddHandler
The AddHandler directive allows you to map certain file extensions to a selected handler. It takes the following form:
AddHandler handler extension
The handler has to be a name of previously defined handler. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example, .cgi).
This directive is typically used to treat files with the .cgi extension as CGI scripts regardless of the directory they are in. Additionally, it is also commonly used to process server-parsed HTML and image-map files.
Example 12.11. Using the AddHandler option
AddHandler cgi-script .cgi

AddIcon
The AddIcon directive allows you to specify an icon to be displayed for a particular file in server-generated directory listings. It takes the following form:
AddIcon path pattern
The path refers to an existing icon file, and must be relative to the directory specified by the DocumentRoot directive (for example, /icons/folder.png). The pattern can be a file name, a file extension, a wildcard expression, or a special form as described in the following table:
Table 12.3. Available AddIcon options
Option Description
^^DIRECTORY^^ Represents a directory.
^^BLANKICON^^ Represents a blank line.

Example 12.12. Using the AddIcon directive
AddIcon /icons/text.png .txt README

AddIconByEncoding
The AddIconByEncoding directive allows you to specify an icon to be displayed for a particular encoding type in server-generated directory listings. It takes the following form:
AddIconByEncoding path encoding
The path refers to an existing icon file, and must be relative to the directory specified by the DocumentRoot directive (for example, /icons/compressed.png). The encoding has to be a valid MIME encoding such as x-compress, x-gzip, etc.
Example 12.13. Using the AddIconByEncoding directive
AddIconByEncoding /icons/compressed.png x-compress x-gzip

AddIconByType
The AddIconByType directive allows you to specify an icon to be displayed for a particular media type in server-generated directory listings. It takes the following form:
AddIconByType path content-type
The path refers to an existing icon file, and must be relative to the directory specified by the DocumentRoot directive (for example, /icons/text.png). The content-type has to be either a valid MIME type (for example, text/html or image/png), or a wildcard expression such as text/*, image/*, etc.
Example 12.14. Using the AddIconByType directive
AddIconByType /icons/video.png video/*

AddLanguage
The AddLanguage directive allows you to associate a file extension with a specific language. It takes the following form:
AddLanguage language extension
The language has to be a valid MIME language such as cs, en, or fr. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example, .cs).
This directive is especially useful for web servers that serve content in multiple languages based on the client's language settings.
Example 12.15. Using the AddLanguage directive
AddLanguage cs .cs .cz

AddType
The AddType directive allows you to define or override the media type for a particular file extension. It takes the following form:
AddType content-type extension
The content-type has to be a valid MIME type such as text/html, image/png, etc. The extension is a case sensitive file extension, and is conventionally written with a leading dot (for example, .cs).
Example 12.16. Using the AddType directive
AddType application/x-gzip .gz .tgz

Alias
The Alias directive allows you to refer to files and directories outside the default directory specified by the DocumentRoot directive. It takes the following form:
Alias url-path real-path
The url-path must be relative to the directory specified by the DocumentRoot directive (for example, /images/). The real-path is a full path to a file or directory in the local file system.
This directive is typically followed by the Directory tag with additional permissions to access the target directory. By default, the /icons/ alias is created so that the icons from /var/www/icons/ are displayed in server-generated directory listings.
Example 12.17. Using the Alias directive
Alias /icons/ /var/www/icons/

<Directory "/var/www/icons">
  Options Indexes MultiViews FollowSymLinks
  AllowOverride None
  Order allow,deny
  Allow from all
<Directory>

Allow
The Allow directive allows you to specify which clients have permission to access a given directory. It takes the following form:
Allow from client
The client can be a domain name, an IP address (both full and partial), a network/netmask pair, or all for all clients.
Example 12.18. Using the Allow directive
Allow from 192.168.1.0/255.255.255.0

AllowOverride
The AllowOverride directive allows you to specify which directives in a .htaccess file can override the default configuration. It takes the following form:
AllowOverride type
The type has to be one of the available grouping options as described in Table 12.4, “Available AllowOverride options”.
Table 12.4. Available AllowOverride options
Option Description
All All directives in .htaccess are allowed to override earlier configuration settings.
None No directive in .htaccess is allowed to override earlier configuration settings.
AuthConfig Allows the use of authorization directives such as AuthName, AuthType, or Require.
FileInfo Allows the use of file type, metadata, and mod_rewrite directives such as DefaultType, RequestHeader, or RewriteEngine, as well as the Action directive.
Indexes Allows the use of directory indexing directives such as AddDescription, AddIcon, or FancyIndexing.
Limit Allows the use of host access directives, that is, Allow, Deny, and Order.
Options[=option,…] Allows the use of the Options directive. Additionally, you can provide a comma-separated list of options to customize which options can be set using this directive.

Example 12.19. Using the AllowOverride directive
AllowOverride FileInfo AuthConfig Limit

BrowserMatch
The BrowserMatch directive allows you to modify the server behavior based on the client's web browser type. It takes the following form:
BrowserMatch pattern variable
The pattern is a regular expression to match the User-Agent HTTP header field. The variable is an environment variable that is set when the header field matches the pattern.
By default, this directive is used to deny connections to specific browsers with known issues, and to disable keepalives and HTTP header flushes for browsers that are known to have problems with these actions.
Example 12.20. Using the BrowserMatch directive
BrowserMatch "Mozilla/2" nokeepalive

CacheDefaultExpire
The CacheDefaultExpire option allows you to set how long to cache a document that does not have any expiration date or the date of its last modification specified. It takes the following form:
CacheDefaultExpire time
The time is specified in seconds. The default option is 3600 (that is, one hour).
Example 12.21. Using the CacheDefaultExpire directive
CacheDefaultExpire 3600

CacheDisable
The CacheDisable directive allows you to disable caching of certain URLs. It takes the following form:
CacheDisable path
The path must be relative to the directory specified by the DocumentRoot directive (for example, /files/).
Example 12.22. Using the CacheDisable directive
CacheDisable /temporary

CacheEnable
The CacheEnable directive allows you to specify a cache type to be used for certain URLs. It takes the following form:
CacheEnable type url
The type has to be a valid cache type as described in Table 12.5, “Available cache types”. The url can be a path relative to the directory specified by the DocumentRoot directive (for example, /images/), a protocol (for example, ftp://), or an external URL such as http://example.com/.
Table 12.5. Available cache types
Type Description
mem The memory-based storage manager.
disk The disk-based storage manager.
fd The file descriptor cache.

Example 12.23. Using the CacheEnable directive
CacheEnable disk /

CacheLastModifiedFactor
The CacheLastModifiedFactor directive allows you to customize how long to cache a document that does not have any expiration date specified, but that provides information about the date of its last modification. It takes the following form:
CacheLastModifiedFactor number
The number is a coefficient to be used to multiply the time that passed since the last modification of the document. The default option is 0.1 (that is, one tenth).
Example 12.24. Using the CacheLastModifiedFactor directive
CacheLastModifiedFactor 0.1

CacheMaxExpire
The CacheMaxExpire directive allows you to specify the maximum amount of time to cache a document. It takes the following form:
CacheMaxExpire time
The time is specified in seconds. The default option is 86400 (that is, one day).
Example 12.25. Using the CacheMaxExpire directive
CacheMaxExpire 86400

CacheNegotiatedDocs
The CacheNegotiatedDocs directive allows you to enable caching of the documents that were negotiated on the basis of content. It takes the following form:
CacheNegotiatedDocs option
The option has to be a valid keyword as described in Table 12.6, “Available CacheNegotiatedDocs options”. Since the content-negotiated documents may change over time or because of the input from the requester, the default option is Off.
Table 12.6. Available CacheNegotiatedDocs options
Option Description
On Enables caching the content-negotiated documents.
Off Disables caching the content-negotiated documents.

Example 12.26. Using the CacheNegotiatedDocs directive
CacheNegotiatedDocs On

CacheRoot
The CacheRoot directive allows you to specify the directory to store cache files in. It takes the following form:
CacheRoot directory
The directory must be a full path to an existing directory in the local file system. The default option is /var/cache/mod_proxy/.
Example 12.27. Using the CacheRoot directive
CacheRoot /var/cache/mod_proxy

CustomLog
The CustomLog directive allows you to specify the log file name and the log file format. It takes the following form:
CustomLog path format
The path refers to a log file, and must be relative to the directory that is specified by the ServerRoot directive (that is, /etc/httpd/ by default). The format has to be either an explicit format string, or a format name that was previously defined using the LogFormat directive.
Example 12.28. Using the CustomLog directive
CustomLog logs/access_log combined

DefaultIcon
The DefaultIcon directive allows you to specify an icon to be displayed for a file in server-generated directory listings when no other icon is associated with it. It takes the following form:
DefaultIcon path
The path refers to an existing icon file, and must be relative to the directory specified by the DocumentRoot directive (for example, /icons/unknown.png).
Example 12.29. Using the DefaultIcon directive
DefaultIcon /icons/unknown.png

DefaultType
The DefaultType directive allows you to specify a media type to be used in case the proper MIME type cannot be determined by the server. It takes the following form:
DefaultType content-type
The content-type has to be a valid MIME type such as text/html, image/png, application/pdf, etc.
Example 12.30. Using the DefaultType directive
DefaultType text/plain

Deny
The Deny directive allows you to specify which clients are denied access to a given directory. It takes the following form:
Deny from client
The client can be a domain name, an IP address (both full and partial), a network/netmask pair, or all for all clients.
Example 12.31. Using the Deny directive
Deny from 192.168.1.1

DirectoryIndex
The DirectoryIndex directive allows you to specify a document to be served to a client when a directory is requested (that is, when the URL ends with the / character). It takes the following form:
DirectoryIndex filename
The filename is a name of the file to look for in the requested directory. By default, the server looks for index.html, and index.html.var.
Example 12.32. Using the DirectoryIndex directive
DirectoryIndex index.html index.html.var

DocumentRoot
The DocumentRoot directive allows you to specify the main directory from which the content is served. It takes the following form:
DocumentRoot directory
The directory must be a full path to an existing directory in the local file system. The default option is /var/www/html/.
Example 12.33. Using the DocumentRoot directive
DocumentRoot /var/www/html

ErrorDocument
The ErrorDocument directive allows you to specify a document or a message to be displayed as a response to a particular error. It takes the following form:
ErrorDocument error-code action
The error-code has to be a valid code such as 403 (Forbidden), 404 (Not Found), or 500 (Internal Server Error). The action can be either a URL (both local and external), or a message string enclosed in double quotes (that is, ").
Example 12.34. Using the ErrorDocument directive
ErrorDocument 403 "Access Denied"
ErrorDocument 404 /404-not_found.html

ErrorLog
The ErrorLog directive allows you to specify a file to which the server errors are logged. It takes the following form:
ErrorLog path
The path refers to a log file, and can be either absolute, or relative to the directory that is specified by the ServerRoot directive (that is, /etc/httpd/ by default). The default option is logs/error_log
Example 12.35. Using the ErrorLog directive
ErrorLog logs/error_log

ExtendedStatus
The ExtendedStatus directive allows you to enable detailed server status information. It takes the following form:
ExtendedStatus option
The option has to be a valid keyword as described in Table 12.7, “Available ExtendedStatus options”. The default option is Off.
Table 12.7. Available ExtendedStatus options
Option Description
On Enables generating the detailed server status.
Off Disables generating the detailed server status.

Example 12.36. Using the ExtendedStatus directive
ExtendedStatus On

Group
The Group directive allows you to specify the group under which the httpd service will run. It takes the following form:
Group group
The group has to be an existing UNIX group. The default option is apache.
Note that Group is no longer supported inside <VirtualHost>, and has been replaced by the SuexecUserGroup directive.
Example 12.37. Using the Group directive
Group apache

HeaderName
The HeaderName directive allows you to specify a file to be prepended to the beginning of the server-generated directory listing. It takes the following form:
HeaderName filename
The filename is a name of the file to look for in the requested directory. By default, the server looks for HEADER.html.
Example 12.38. Using the HeaderName directive
HeaderName HEADER.html

HostnameLookups
The HostnameLookups directive allows you to enable automatic resolving of IP addresses. It takes the following form:
HostnameLookups option
The option has to be a valid keyword as described in Table 12.8, “Available HostnameLookups options”. To conserve resources on the server, the default option is Off.
Table 12.8. Available HostnameLookups options
Option Description
On Enables resolving the IP address for each connection so that the hostname can be logged. However, this also adds a significant processing overhead.
Double Enables performing the double-reverse DNS lookup. In comparison to the above option, this adds even more processing overhead.
Off Disables resolving the IP address for each connection.

Note that when the presence of hostnames is required in server log files, it is often possible to use one of the many log analyzer tools that perform the DNS lookups more efficiently.
Example 12.39. Using the HostnameLookups directive
HostnameLookups Off

Include
The Include directive allows you to include other configuration files. It takes the following form:
Include filename
The filename can be an absolute path, a path relative to the directory specified by the ServerRoot directive, or a wildcard expression. All configuration files from the /etc/httpd/conf.d/ directory are loaded by default.
Example 12.40. Using the Include directive
Include conf.d/*.conf

IndexIgnore
The IndexIgnore directive allows you to specify a list of file names to be omitted from the server-generated directory listings. It takes the following form:
IndexIgnore filename
The filename option can be either a full file name, or a wildcard expression.
Example 12.41. Using the IndexIgnore directive
IndexIgnore .??* *~ *# HEADER* README* RCS CVS *,v *,t

IndexOptions
The IndexOptions directive allows you to customize the behavior of server-generated directory listings. It takes the following form:
IndexOptions option
The option has to be a valid keyword as described in Table 12.9, “Available directory listing options”. The default options are Charset=UTF-8, FancyIndexing, HTMLTable, NameWidth=*, and VersionSort.
Table 12.9. Available directory listing options
Option Description
Charset=encoding Specifies the character set of a generated web page. The encoding has to be a valid character set such as UTF-8 or ISO-8859-2.
Type=content-type Specifies the media type of a generated web page. The content-type has to be a valid MIME type such as text/html or text/plain.
DescriptionWidth=value Specifies the width of the description column. The value can be either a number of characters, or an asterisk (that is, *) to adjust the width automatically.
FancyIndexing Enables advanced features such as different icons for certain files or possibility to re-sort a directory listing by clicking on a column header.
FolderFirst Enables listing directories first, always placing them above files.
HTMLTable Enables the use of HTML tables for directory listings.
IconsAreLinks Enables using the icons as links.
IconHeight=value Specifies an icon height. The value is a number of pixels.
IconWidth=value Specifies an icon width. The value is a number of pixels.
IgnoreCase Enables sorting files and directories in a case-sensitive manner.
IgnoreClient Disables accepting query variables from a client.
NameWidth=value Specifies the width of the file name column. The value can be either a number of characters, or an asterisk (that is, *) to adjust the width automatically.
ScanHTMLTitles Enables parsing the file for a description (that is, the title element) in case it is not provided by the AddDescription directive.
ShowForbidden Enables listing the files with otherwise restricted access.
SuppressColumnSorting Disables re-sorting a directory listing by clicking on a column header.
SuppressDescription Disables reserving a space for file descriptions.
SuppressHTMLPreamble Disables the use of standard HTML preamble when a file specified by the HeaderName directive is present.
SuppressIcon Disables the use of icons in directory listings.
SuppressLastModified Disables displaying the date of the last modification field in directory listings.
SuppressRules Disables the use of horizontal lines in directory listings.
SuppressSize Disables displaying the file size field in directory listings.
TrackModified Enables returning the Last-Modified and ETag values in the HTTP header.
VersionSort Enables sorting files that contain a version number in the expected manner.
XHTML Enables the use of XHTML 1.0 instead of the default HTML 3.2.

Example 12.42. Using the IndexOptions directive
IndexOptions FancyIndexing VersionSort NameWidth=* HTMLTable Charset=UTF-8

KeepAlive
The KeepAlive directive allows you to enable persistent connections. It takes the following form:
KeepAlive option
The option has to be a valid keyword as described in Table 12.10, “Available KeepAlive options”. The default option is Off.
Table 12.10. Available KeepAlive options
Option Description
On Enables the persistent connections. In this case, the server will accept more than one request per connection.
Off Disables the keep-alive connections.

Note that when the persistent connections are enabled, on a busy server, the number of child processes can increase rapidly and eventually reach the maximum limit, slowing down the server significantly. To reduce the risk, it is recommended that you set KeepAliveTimeout to a low number, and monitor the /var/log/httpd/logs/error_log log file carefully.
Example 12.43. Using the KeepAlive directive
KeepAlive Off

KeepAliveTimeout
The KeepAliveTimeout directive allows you to specify the amount of time to wait for another request before closing the connection. It takes the following form:
KeepAliveTimeout time
The time is specified in seconds. The default option is 15.
Example 12.44. Using the KeepAliveTimeout directive
KeepAliveTimeout 15

LanguagePriority
The LanguagePriority directive allows you to customize the precedence of languages. It takes the following form:
LanguagePriority language
The language has to be a valid MIME language such as cs, en, or fr.
This directive is especially useful for web servers that serve content in multiple languages based on the client's language settings.
Example 12.45. Using the LanguagePriority directive
LanguagePriority sk cs en

Listen
The Listen directive allows you to specify IP addresses or ports to listen to. It takes the following form:
Listen [ip-address:]port [protocol]
The ip-address is optional and unless supplied, the server will accept incoming requests on a given port from all IP addresses. Since the protocol is determined automatically from the port number, it can be usually omitted. The default option is to listen to port 80.
Note that if the server is configured to listen to a port under 1024, only superuser will be able to start the httpd service.
Example 12.46. Using the Listen directive
Listen 80

LoadModule
The LoadModule directive allows you to load a Dynamic Shared Object (DSO) module. It takes the following form:
LoadModule name path
The name has to be a valid identifier of the required module. The path refers to an existing module file, and must be relative to the directory in which the libraries are placed (that is, /usr/lib/httpd/ on 32-bit and /usr/lib64/httpd/ on 64-bit systems by default).
Refer to Section 12.1.6, “Working with Modules” for more information on the Apache HTTP Server's DSO support.
Example 12.47. Using the LoadModule directive
LoadModule php5_module modules/libphp5.so

LogFormat
The LogFormat directive allows you to specify a log file format. It takes the following form:
LogFormat format name
The format is a string consisting of options as described in Table 12.11, “Common LogFormat options”. The name can be used instead of the format string in the CustomLog directive.
Table 12.11. Common LogFormat options
Option Description
%b Represents the size of the response in bytes.
%h Represents the IP address or hostname of a remote client.
%l Represents the remote log name if supplied. If not, a hyphen (that is, -) is used instead.
%r Represents the first line of the request string as it came from the browser or client.
%s Represents the status code.
%t Represents the date and time of the request.
%u If the authentication is required, it represents the remote user. If not, a hyphen (that is, -) is used instead.
%{field} Represents the content of the HTTP header field. The common options include %{Referer} (the URL of the web page that referred the client to the server) and %{User-Agent} (the type of the web browser making the request).

Example 12.48. Using the LogFormat directive
LogFormat "%h %l %u %t \"%r\" %>s %b" common

LogLevel
The LogLevel directive allows you to customize the verbosity level of the error log. It takes the following form:
LogLevel option
The option has to be a valid keyword as described in Table 12.12, “Available LogLevel options”. The default option is warn.
Table 12.12. Available LogLevel options
Option Description
emerg Only the emergency situations when the server cannot perform its work are logged.
alert All situations when an immediate action is required are logged.
crit All critical conditions are logged.
error All error messages are logged.
warn All warning messages are logged.
notice Even normal, but still significant situations are logged.
info Various informational messages are logged.
debug Various debugging messages are logged.

Example 12.49. Using the LogLevel directive
LogLevel warn

MaxKeepAliveRequests
The MaxKeepAliveRequests directive allows you to specify the maximum number of requests for a persistent connection. It takes the following form:
MaxKeepAliveRequests number
A high number can improve the performance of the server. Note that using 0 allows unlimited number of requests. The default option is 100.
Example 12.50. Using the MaxKeepAliveRequests option
MaxKeepAliveRequests 100

NameVirtualHost
The NameVirtualHost directive allows you to specify the IP address and port number for a name-based virtual host. It takes the following form:
NameVirtualHost ip-address[:port]
The ip-address can be either a full IP address, or an asterisk (that is, *) representing all interfaces. Note that IPv6 addresses have to be enclosed in square brackets (that is, [ and ]). The port is optional.
Name-based virtual hosting allows one Apache HTTP Server to serve different domains without using multiple IP addresses.

Using secure HTTP connections

Name-based virtual hosts only work with non-secure HTTP connections. If using virtual hosts with a secure server, use IP address-based virtual hosts instead.
Example 12.51. Using the NameVirtualHost directive
NameVirtualHost *:80

Options
The Options directive allows you to specify which server features are available in a particular directory. It takes the following form:
Options option
The option has to be a valid keyword as described in Table 12.13, “Available server features”.
Table 12.13. Available server features
Option Description
ExecCGI Enables the execution of CGI scripts.
FollowSymLinks Enables following symbolic links in the directory.
Includes Enables server-side includes.
IncludesNOEXEC Enables server-side includes, but does not allow the execution of commands.
Indexes Enables server-generated directory listings.
MultiViews Enables content-negotiated MultiViews.
SymLinksIfOwnerMatch Enables following symbolic links in the directory when both the link and the target file have the same owner.
All Enables all of the features above with the exception of MultiViews.
None Disables all of the features above.

Example 12.52. Using the Options directive
Options Indexes FollowSymLinks

Order
The Order directive allows you to specify the order in which the Allow and Deny directives are evaluated. It takes the following form:
Order option
The option has to be a valid keyword as described in Table 12.14, “Available Order options”. The default option is allow,deny.
Table 12.14. Available Order options
Option Description
allow,deny Allow directives are evaluated first.
deny,allow Deny directives are evaluated first.

Example 12.53. Using the Order directive
Order allow,deny

PidFile
The PidFile directive allows you to specify a file to which the process ID (PID) of the server is stored. It takes the following form:
PidFile path
The path refers to a pid file, and can be either absolute, or relative to the directory that is specified by the ServerRoot directive (that is, /etc/httpd/ by default). The default option is run/httpd.pid.
Example 12.54. Using the PidFile directive
PidFile run/httpd.pid

ProxyRequests
The ProxyRequests directive allows you to enable forward proxy requests. It takes the following form:
ProxyRequests option
The option has to be a valid keyword as described in Table 12.15, “Available ProxyRequests options”. The default option is Off.
Table 12.15. Available ProxyRequests options
Option Description
On Enables forward proxy requests.
Off Disables forward proxy requests.

Example 12.55. Using the ProxyRequests directive
ProxyRequests On

ReadmeName
The ReadmeName directive allows you to specify a file to be appended to the end of the server-generated directory listing. It takes the following form:
ReadmeName filename
The filename is a name of the file to look for in the requested directory. By default, the server looks for README.html.
Example 12.56. Using the ReadmeName directive
ReadmeName README.html

Redirect
The Redirect directive allows you to redirect a client to another URL. It takes the following form:
Redirect [status] path url
The status is optional, and if provided, it has to be a valid keyword as described in Table 12.16, “Available status options”. The path refers to the old location, and must be relative to the directory specified by the DocumentRoot directive (for example, /docs). The url refers to the current location of the content (for example, http://docs.example.com).
Table 12.16. Available status options
Status Description
permanent Indicates that the requested resource has been moved permanently. The 301 (Moved Permanently) status code is returned to a client.
temp Indicates that the requested resource has been moved only temporarily. The 302 (Found) status code is returned to a client.
seeother Indicates that the requested resource has been replaced. The 303 (See Other) status code is returned to a client.
gone Indicates that the requested resource has been removed permanently. The 410 (Gone) status is returned to a client.

Note that for more advanced redirection techniques, you can use the mod_rewrite module that is part of the Apache HTTP Server installation.
Example 12.57. Using the Redirect directive
Redirect permanent /docs http://docs.example.com

ScriptAlias
The ScriptAlias directive allows you to specify the location of CGI scripts. It takes the following form:
ScriptAlias url-path real-path
The url-path must be relative to the directory specified by the DocumentRoot directive (for example, /cgi-bin/). The real-path is a full path to a file or directory in the local file system.
This directive is typically followed by the Directory tag with additional permissions to access the target directory. By default, the /cgi-bin/ alias is created so that the scripts located in the /var/www/cgi-bin/ are accessible.
The ScriptAlias directive is used for security reasons to prevent CGI scripts from being viewed as ordinary text documents.
Example 12.58. Using the ScriptAlias directive
ScriptAlias /cgi-bin/ /var/www/cgi-bin/

<Directory "/var/www/cgi-bin">
  AllowOverride None
  Options None
  Order allow,deny
  Allow from all
</Directory>

ServerAdmin
The ServerAdmin directive allows you to specify the email address of the server administrator to be displayed in server-generated web pages. It takes the following form:
ServerAdmin email
The default option is root@localhost.
This directive is commonly set to webmaster@hostname, where hostname is the address of the server. Once set, alias webmaster to the person responsible for the web server in /etc/aliases, and as superuser, run the newaliases command.
Example 12.59. Using the ServerAdmin directive
ServerAdmin webmaster@penguin.example.com

ServerName
The ServerName directive allows you to specify the hostname and the port number of a web server. It takes the following form:
ServerName hostname[:port]
The hostname has to be a fully qualified domain name (FQDN) of the server. The port is optional, but when supplied, it has to match the number specified by the Listen directive.
When using this directive, make sure that the IP address and server name pair are included in the /etc/hosts file.
Example 12.60. Using the ServerName directive
ServerName penguin.example.com:80

ServerRoot
The ServerRoot directive allows you to specify the directory in which the server operates. It takes the following form:
ServerRoot directory
The directory must be a full path to an existing directory in the local file system. The default option is /etc/httpd/.
Example 12.61. Using the ServerRoot directive
ServerRoot /etc/httpd

ServerSignature
The ServerSignature directive allows you to enable displaying information about the server on server-generated documents. It takes the following form:
ServerSignature option
The option has to be a valid keyword as described in Table 12.17, “Available ServerSignature options”. The default option is On.
Table 12.17. Available ServerSignature options
Option Description
On Enables appending the server name and version to server-generated pages.
Off Disables appending the server name and version to server-generated pages.
EMail Enables appending the server name, version, and the email address of the system administrator as specified by the ServerAdmin directive to server-generated pages.

Example 12.62. Using the ServerSignature directive
ServerSignature On

ServerTokens
The ServerTokens directive allows you to customize what information are included in the Server response header. It takes the following form:
ServerTokens option
The option has to be a valid keyword as described in Table 12.18, “Available ServerTokens options”. The default option is OS.
Table 12.18. Available ServerTokens options
Option Description
Prod Includes the product name only (that is, Apache).
Major Includes the product name and the major version of the server (for example, 2).
Minor Includes the product name and the minor version of the server (for example, 2.2).
Min Includes the product name and the minimal version of the server (for example, 2.2.15).
OS Includes the product name, the minimal version of the server, and the type of the operating system it is running on (for example, Red Hat).
Full Includes all the information above along with the list of loaded modules.

Note that for security reasons, it is recommended to reveal as little information about the server as possible.
Example 12.63. Using the ServerTokens directive
ServerTokens Prod

SuexecUserGroup
The SuexecUserGroup directive allows you to specify the user and group under which the CGI scripts will be run. It takes the following form:
SuexecUserGroup user group
The user has to be an existing user, and the group must be a valid UNIX group.
For security reasons, the CGI scripts should not be run with root privileges. Note that in <VirtualHost>, SuexecUserGroup replaces the User and Group directives.
Example 12.64. Using the SuexecUserGroup directive
SuexecUserGroup apache apache

Timeout
The Timeout directive allows you to specify the amount of time to wait for an event before closing a connection. It takes the following form:
Timeout time
The time is specified in seconds. The default option is 60.
Example 12.65. Using the Timeout directive
Timeout 60

TypesConfig
The TypesConfig allows you to specify the location of the MIME types configuration file. It takes the following form:
TypesConfig path
The path refers to an existing MIME types configuration file, and can be either absolute, or relative to the directory that is specified by the ServerRoot directive (that is, /etc/httpd/ by default). The default option is /etc/mime.types.
Note that instead of editing /etc/mime.types, the recommended way to add MIME type mapping to the Apache HTTP Server is to use the AddType directive.
Example 12.66. Using the TypesConfig directive
TypesConfig /etc/mime.types

UseCanonicalName
The UseCanonicalName allows you to specify the way the server refers to itself. It takes the following form:
UseCanonicalName option
The option has to be a valid keyword as described in Table 12.19, “Available UseCanonicalName options”. The default option is Off.
Table 12.19. Available UseCanonicalName options
Option Description
On Enables the use of the name that is specified by the ServerName directive.
Off Disables the use of the name that is specified by the ServerName directive. The hostname and port number provided by the requesting client are used instead.
DNS Disables the use of the name that is specified by the ServerName directive. The hostname determined by a reverse DNS lookup is used instead.

Example 12.67. Using the UseCanonicalName directive
UseCanonicalName Off

User
The User directive allows you to specify the user under which the httpd service will run. It takes the following form:
User user
The user has to be an existing UNIX user. The default option is apache.
For security reasons, the httpd service should not be run with root privileges. Note that User is no longer supported inside <VirtualHost>, and has been replaced by the SuexecUserGroup directive.
Example 12.68. Using the User directive
User apache

UserDir
The UserDir directive allows you to enable serving content from users' home directories. It takes the following form:
UserDir option
The option can be either a name of the directory to look for in user's home directory (typically public_html), or a valid keyword as described in Table 12.20, “Available UserDir options”. The default option is disabled.
Table 12.20. Available UserDir options
Option Description
enabled user Enables serving content from home directories of given users.
disabled [user] Disables serving content from home directories, either for all users, or, if a space separated list of users is supplied, for given users only.

Set the correct permissions

In order for the web server to access the content, the permissions on relevant directories and files must be set correctly. Make sure that all users are able to access the home directories, and that they can access and read the content of the directory specified by the UserDir directive. For example, to allow access to public_html/ in the home directory of user joe, type the following at a shell prompt as root:
~]# chmod a+x /home/joe/
~]# chmod a+rx /home/joe/public_html/
All files in this directory must be set accordingly.
Example 12.69. Using the UserDir directive
UserDir public_html

12.1.5.2. Common ssl.conf Directives

The Secure Sockets Layer (SSL) directives allow you to customize the behavior of the Apache HTTP Secure Server, and in most cases, they are configured appropriately during the installation. Be careful when changing these settings, as incorrect configuration can lead to security vulnerabilities.
The following directive is commonly used in /etc/httpd/conf.d/ssl.conf:
SetEnvIf
The SetEnvIf directive allows you to set environment variables based on the headers of incoming connections. It takes the following form:
SetEnvIf option pattern [!]variable[=value]…
The option can be either a HTTP header field, a previously defined environment variable name, or a valid keyword as described in Table 12.21, “Available SetEnvIf options”. The pattern is a regular expression. The variable is an environment variable that is set when the option matches the pattern. If the optional exclamation mark (that is, !) is present, the variable is removed instead of being set.
Table 12.21. Available SetEnvIf options
Option Description
Remote_Host Refers to the client's hostname.
Remote_Addr Refers to the client's IP address.
Server_Addr Refers to the server's IP address.
Request_Method Refers to the request method (for example, GET).
Request_Protocol Refers to the protocol name and version (for example, HTTP/1.1).
Request_URI Refers to the requested resource.

The SetEnvIf directive is used to disable HTTP keepalives, and to allow SSL to close the connection without a closing notification from the client browser. This is necessary for certain web browsers that do not reliably shut down the SSL connection.
Example 12.70. Using the SetEnvIf directive
SetEnvIf User-Agent ".*MSIE.*" \
         nokeepalive ssl-unclean-shutdown \
         downgrade-1.0 force-response-1.0

Note that for the /etc/httpd/conf.d/ssl.conf file to be present, the mod_ssl needs to be installed. Refer to Section 12.1.8, “Setting Up an SSL Server” for more information on how to install and configure an SSL server.

12.1.5.3. Common Multi-Processing Module Directives

The Multi-Processing Module (MPM) directives allow you to customize the behavior of a particular MPM specific server-pool. Since its characteristics differ depending on which MPM is used, the directives are embedded in IfModule. By default, the server-pool is defined for both the prefork and worker MPMs.
The following MPM directives are commonly used in /etc/httpd/conf/httpd.conf:
MaxClients
The MaxClients directive allows you to specify the maximum number of simultaneously connected clients to process at one time. It takes the following form:
MaxClients number
A high number can improve the performance of the server, although it is not recommended to exceed 256 when using the prefork MPM.
Example 12.71. Using the MaxClients directive
MaxClients 256

MaxRequestsPerChild
The MaxRequestsPerChild directive allows you to specify the maximum number of request a child process can serve before it dies. It takes the following form:
MaxRequestsPerChild number
Setting the number to 0 allows unlimited number of requests.
The MaxRequestsPerChild directive is used to prevent long-lived processes from causing memory leaks.
Example 12.72. Using the MaxRequestsPerChild directive
MaxRequestsPerChild 4000

MaxSpareServers
The MaxSpareServers directive allows you to specify the maximum number of spare child processes. It takes the following form:
MaxSpareServers number
This directive is used by the prefork MPM only.
Example 12.73. Using the MaxSpareServers directive
MaxSpareServers 20

MaxSpareThreads
The MaxSpareThreads directive allows you to specify the maximum number of spare server threads. It takes the following form:
MaxSpareThreads number
The number must be greater than or equal to the sum of MinSpareThreads and ThreadsPerChild. This directive is used by the worker MPM only.
Example 12.74. Using the MaxSpareThreads directive
MaxSpareThreads 75

MinSpareServers
The MinSpareServers directive allows you to specify the minimum number of spare child processes. It takes the following form:
MinSpareServers number
Note that a high number can create a heavy processing load on the server. This directive is used by the prefork MPM only.
Example 12.75. Using the MinSpareServers directive
MinSpareServers 5

MinSpareThreads
The MinSpareThreads directive allows you to specify the minimum number of spare server threads. It takes the following form:
MinSpareThreads number
This directive is used by the worker MPM only.
Example 12.76. Using the MinSpareThreads directive
MinSpareThreads 75

StartServers
The StartServers directive allows you to specify the number of child processes to create when the service is started. It takes the following form:
StartServers number
Since the child processes are dynamically created and terminated according to the current traffic load, it is usually not necessary to change this value.
Example 12.77. Using the StartServers directive
StartServers 8

ThreadsPerChild
The ThreadsPerChild directive allows you to specify the number of threads a child process can create. It takes the following form:
ThreadsPerChild number
This directive is used by the worker MPM only.
Example 12.78. Using the ThreadsPerChild directive
ThreadsPerChild 25

12.1.6. Working with Modules

Being a modular application, the httpd service is distributed along with a number of Dynamic Shared Objects (DSOs), which can be dynamically loaded or unloaded at runtime as necessary. By default, these modules are located in /usr/lib/httpd/modules/ on 32-bit and in /usr/lib64/httpd/modules/ on 64-bit systems.

12.1.6.1. Loading a Module

To load a particular DSO module, use the LoadModule directive as described in Section 12.1.5.1, “Common httpd.conf Directives”. Note that modules provided by a separate package often have their own configuration file in the /etc/httpd/conf.d/ directory.
Example 12.79. Loading the mod_ssl DSO
LoadModule ssl_module modules/mod_ssl.so

Once you are finished, restart the web server to reload the configuration. Refer to Section 12.1.4.3, “Restarting the Service” for more information on how to restart the httpd service.

12.1.6.2. Writing a Module

If you intend to create a new DSO module, make sure you have the httpd-devel package installed. To do so, type the following at a shell prompt as root:
yum install httpd-devel
This package contains the include files, the header files, and the APache eXtenSion (apxs) utility required to compile a module.
Once written, you can build the module with the following command:
apxs -i -a -c module_name.c
If the build was successful, you should be able to load the module the same way as any other module that is distributed with the Apache HTTP Server.

12.1.7. Setting Up Virtual Hosts

The Apache HTTP Server's built in virtual hosting allows the server to provide different information based on which IP address, hostname, or port is being requested.
To create a name-based virtual host, find the virtual host container provided in /etc/httpd/conf/httpd.conf as an example, remove the hash sign (that is, #) from the beginning of each line, and customize the options according to your requirements as shown in Example 12.80, “Sample virtual host configuration”.
Example 12.80. Sample virtual host configuration
NameVirtualHost penguin.example.com:80

<VirtualHost penguin.example.com:80>
    ServerAdmin webmaster@penguin.example.com
    DocumentRoot /www/docs/penguin.example.com
    ServerName penguin.example.com:80
    ErrorLog logs/penguin.example.com-error_log
    CustomLog logs/penguin.example.com-access_log common
</VirtualHost>

Note that ServerName must be a valid DNS name assigned to the machine. The <VirtualHost> container is highly customizable, and accepts most of the directives available within the main server configuration. Directives that are not supported within this container include User and Group, which were replaced by SuexecUserGroup.

Changing the port number

If you configure a virtual host to listen on a non-default port, make sure you update the Listen directive in the global settings section of the /etc/httpd/conf/httpd.conf file accordingly.
To activate a newly created virtual host, the web server has to be restarted first. Refer to Section 12.1.4.3, “Restarting the Service” for more information on how to restart the httpd service.

12.1.8. Setting Up an SSL Server

Secure Sockets Layer (SSL) is a cryptographic protocol that allows a server and a client to communicate securely. Along with its extended and improved version called Transport Layer Security (TLS), it ensures both privacy and data integrity. The Apache HTTP Server in combination with mod_ssl, a module that uses the OpenSSL toolkit to provide the SSL/TLS support, is commonly referred to as the SSL server.
Unlike a regular HTTP connection that can be read and possibly modified by anybody who is able to intercept it, the use of mod_ssl prevents any inspection or modification of the transmitted content. This section provides basic information on how to enable this module in the Apache HTTP Server configuration, and guides you through the process of generating private keys and self-signed certificates.

12.1.8.1. An Overview of Certificates and Security

Secure communication is based on the use of keys. In conventional or symmetric cryptography, both ends of the transaction have the same key they can use to decode each other's transmissions. On the other hand, in public or asymmetric cryptography, two keys co-exist: a private key that is kept a secret, and a public key that is usually shared with the public. While the data encoded with the public key can only be decoded with the private key, data encoded with the private key can in turn only be decoded with the public key.
To provide secure communications using SSL, an SSL server must use a digital certificate signed by a Certificate Authority (CA). The certificate lists various attributes of the server (that is, the server hostname, the name of the company, its location, etc.), and the signature produced using the CA's private key. This signature ensures that a particular certificate authority has issued the certificate, and that the certificate has not been modified in any way.
When a web browser establishes a new SSL connection, it checks the certificate provided by the web server. If the certificate does not have a signature from a trusted CA, or if the hostname listed in the certificate does not match the hostname used to establish the connection, it refuses to communicate with the server and usually presents a user with an appropriate error message.
By default, most web browsers are configured to trust a set of widely used certificate authorities. Because of this, an appropriate CA should be chosen when setting up a secure server, so that target users can trust the connection, otherwise they will be presented with an error message, and will have to accept the certificate manually. Since encouraging users to override certificate errors can allow an attacker to intercept the connection, you should use a trusted CA whenever possible. For more information on this, see Table 12.22, “CA lists for most common web browsers”.
Table 12.22. CA lists for most common web browsers
Web Browser Link
Mozilla Firefox Mozilla root CA list.
Opera The Opera Rootstore.
Internet Explorer Windows root certificate program members.

When setting up an SSL server, you need to generate a certificate request and a private key, and then send the certificate request, proof of the company's identity, and payment to a certificate authority. Once the CA verifies the certificate request and your identity, it will send you a signed certificate you can use with your server. Alternatively, you can create a self-signed certificate that does not contain a CA signature, and thus should be used for testing purposes only.

12.1.8.2. Enabling the mod_ssl Module

If you intend to set up an SSL server, make sure you have the mod_ssl (the mod_ssl module) and openssl (the OpenSSL toolkit) packages installed. To do so, type the following at a shell prompt as root:
yum install mod_ssl openssl
This will create the mod_ssl configuration file at /etc/httpd/conf.d/ssl.conf, which is included in the main Apache HTTP Server configuration file by default. For the module to be loaded, restart the httpd service as described in Section 12.1.4.3, “Restarting the Service”.

12.1.8.3. Using an Existing Key and Certificate

If you have a previously created key and certificate, you can configure the SSL server to use these files instead of generating new ones. There are only two situations where this is not possible:
  1. You are changing the IP address or domain name.
    Certificates are issued for a particular IP address and domain name pair. If one of these values changes, the certificate becomes invalid.
  2. You have a certificate from VeriSign, and you are changing the server software.
    VeriSign, a widely used certificate authority, issues certificates for a particular software product, IP address, and domain name. Changing the software product renders the certificate invalid.
In either of the above cases, you will need to obtain a new certificate. For more information on this topic, refer to Section 12.1.8.4, “Generating a New Key and Certificate”.
If you wish to use an existing key and certificate, move the relevant files to the /etc/pki/tls/private/ and /etc/pki/tls/certs/ directories respectively. You can do so by running the following commands as root:
mv key_file.key /etc/pki/tls/private/hostname.key
mv certificate.crt /etc/pki/tls/certs/hostname.crt
Then add the following lines to the /etc/httpd/conf.d/ssl.conf configuration file:
SSLCertificateFile /etc/pki/tls/certs/hostname.crt
SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
To load the updated configuration, restart the httpd service as described in Section 12.1.4.3, “Restarting the Service”.
Example 12.81. Using a key and certificate from the Red Hat Secure Web Server
~]# mv /etc/httpd/conf/httpsd.key /etc/pki/tls/private/penguin.example.com.key
~]# mv /etc/httpd/conf/httpsd.crt /etc/pki/tls/certs/penguin.example.com.crt

12.1.8.4. Generating a New Key and Certificate

In order to generate a new key and certificate pair, you must to have the crypto-utils package installed in your system. As root, you can install it by typing the following at a shell prompt:
yum install crypto-utils
This package provides a set of tools to generate and manage SSL certificates and private keys, and includes genkey, the Red Hat Keypair Generation utility that will guide you through the key generation process.

Replacing an existing certificate

If the server already has a valid certificate and you are replacing it with a new one, specify a different serial number. This ensures that client browsers are notified of this change, update to this new certificate as expected, and do not fail to access the page. To create a new certificate with a custom serial number, as root, use the following command instead of genkey:
openssl req -x509 -new -set_serial number -key hostname.key -out hostname.crt

Remove a previously created key

If there already is a key file for a particular hostname in your system, genkey will refuse to start. In this case, remove the existing file using the following command as root:
rm /etc/pki/tls/private/hostname.key
To run the utility, as root, run the genkey command followed by the appropriate hostname (for example, penguin.example.com):
genkey hostname
To complete the key and certificate creation, take the following steps:
  1. Review the target locations in which the key and certificate will be stored.
    Running the genkey utility
    Running the genkey utility
    Figure 12.1. Running the genkey utility

    Use the Tab key to select the Next button, and press Enter to proceed to the next screen.
  2. Using the Up and down arrow keys, select the suitable key size. Note that while the large key increases the security, it also increases the response time of your server. Because of this, the recommended option is 1024 bits.
    Selecting the key size
    Selecting the key size
    Figure 12.2. Selecting the key size

    Once finished, use the Tab key to select the Next button, and press Enter to initiate the random bits generation process. Depending on the selected key size, this may take some time.
  3. Decide whether you wish to send a certificate request to a certificate authority.
    Generating a certificate request
    Generating a certificate request
    Figure 12.3. Generating a certificate request

    Use the Tab key to select Yes to compose a certificate request, or No to generate a self-signed certificate. Then press Enter to confirm your choice.
  4. Using the Spacebar key, enable ([*]) or disable ([ ]) the encryption of the private key.
    Encrypting the private key
    Encrypting the private key
    Figure 12.4. Encrypting the private key

    Use the Tab key to select the Next button, and press Enter to proceed to the next screen.
  5. If you have enabled the private key encryption, enter an adequate passphrase. Note that for security reasons, it is not displayed as you type, and it must be at least five characters long.
    Entering a passphrase
    Entering a passphrase
    Figure 12.5. Entering a passphrase

    Use the Tab key to select the Next button, and press Enter to proceed to the next screen.

    Do not forget the passphrase

    Entering the correct passphrase is required in order for the server to start. If you lose it, you will need to generate a new key and certificate.
  6. Customize the certificate details.
    Specifying certificate information
    Specifying certificate information
    Figure 12.6. Specifying certificate information

    Use the Tab key to select the Next button, and press Enter to finish the key generation.
  7. If you have previously enabled the certificate request generation, you will be prompted to send it to a certificate authority.
    Instructions on how to send a certificate request
    Instructions on how to send a certificate request
    Figure 12.7. Instructions on how to send a certificate request

    Press Enter to return to a shell prompt.
Once generated, add the key and certificate locations to the /etc/httpd/conf.d/ssl.conf configuration file:
SSLCertificateFile /etc/pki/tls/certs/hostname.crt
SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
Finally, restart the httpd service as described in Section 12.1.4.3, “Restarting the Service”, so that the updated configuration is loaded.

12.1.9. Additional Resources

To learn more about the Apache HTTP Server, refer to the following resources.

12.1.9.1. Installed Documentation

http://localhost/manual/
The official documentation for the Apache HTTP Server with the full description of its directives and available modules. Note that in order to access this documentation, you must have the httpd-manual package installed, and the web server must be running.
man httpd
The manual page for the httpd service containing the complete list of its command line options.
man genkey
The manual page for genkey containing the full documentation on its usage.

12.1.9.2. Useful Websites

http://httpd.apache.org/
The official website for the Apache HTTP Server with documentation on all the directives and default modules.
http://www.modssl.org/
The official website for the mod_ssl module.
http://www.openssl.org/
The OpenSSL home page containing further documentation, frequently asked questions, links to the mailing lists, and other useful resources.

Chapter 13. Mail Servers

Email was born in the 1960s. The mailbox was a file in a user's home directory that was readable only by that user. Primitive mail applications appended new text messages to the bottom of the file, making the user wade through the constantly growing file to find any particular message. This system was only capable of sending messages to users on the same system.
The first network transfer of an electronic mail message file took place in 1971 when a computer engineer named Ray Tomlinson sent a test message between two machines via ARPANET—the precursor to the Internet. Communication via email soon became very popular, comprising 75 percent of ARPANET's traffic in less than two years.
Today, email systems based on standardized network protocols have evolved into some of the most widely used services on the Internet. Fedora offers many advanced applications to serve and access email.
This chapter reviews modern email protocols in use today, and some of the programs designed to send and receive email.

13.1. Email Protocols

Today, email is delivered using a client/server architecture. An email message is created using a mail client program. This program then sends the message to a server. The server then forwards the message to the recipient's email server, where the message is then supplied to the recipient's email client.
To enable this process, a variety of standard network protocols allow different machines, often running different operating systems and using different email programs, to send and receive email.
The following protocols discussed are the most commonly used in the transfer of email.

13.1.1. Mail Transport Protocols

Mail delivery from a client application to the server, and from an originating server to the destination server, is handled by the Simple Mail Transfer Protocol (SMTP).

13.1.1.1. SMTP

The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for email clients as well. To send email, the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery. For this reason, it is necessary to specify an SMTP server when configuring an email client.
Under Fedora, a user can configure an SMTP server on the local machine to handle mail delivery. However, it is also possible to configure remote SMTP servers for outgoing mail.
One important point to make about the SMTP protocol is that it does not require authentication. This allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this characteristic of SMTP that makes junk email or spam possible. Imposing relay restrictions limits random users on the Internet from sending email through your SMTP server, to other servers on the internet. Servers that do not impose such restrictions are called open relay servers.
Fedora provides the Postfix and Sendmail SMTP programs.

13.1.2. Mail Access Protocols

There are two primary protocols used by email client applications to retrieve email from mail servers: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP).

13.1.2.1. POP

The default POP server under Fedora is Dovecot and is provided by the dovecot package.

Installing the dovecot package

In order to use Dovecot, first ensure the dovecot package is installed on your system by running, as root:
yum install dovecot
For more information on installing packages with Yum, refer to Section 4.2.4, “Installing Packages”.
When using a POP server, email messages are downloaded by email client applications. By default, most POP email clients are automatically configured to delete the message on the email server after it has been successfully transferred, however this setting usually can be changed.
POP is fully compatible with important Internet messaging standards, such as Multipurpose Internet Mail Extensions (MIME), which allow for email attachments.
POP works best for users who have one system on which to read email. It also works well for users who do not have a persistent connection to the Internet or the network containing the mail server. Unfortunately for those with slow network connections, POP requires client programs upon authentication to download the entire content of each message. This can take a long time if any messages have large attachments.
The most current version of the standard POP protocol is POP3.
There are, however, a variety of lesser-used POP protocol variants:
  • APOPPOP3 with MDS (Monash Directory Service) authentication. An encoded hash of the user's password is sent from the email client to the server rather then sending an unencrypted password.
  • KPOPPOP3 with Kerberos authentication.
  • RPOPPOP3 with RPOP authentication. This uses a per-user ID, similar to a password, to authenticate POP requests. However, this ID is not encrypted, so RPOP is no more secure than standard POP.
For added security, it is possible to use Secure Socket Layer (SSL) encryption for client authentication and data transfer sessions. This can be enabled by using the pop3s service, or by using the /usr/sbin/stunnel application. For more information on securing email communication, refer to Section 13.5.1, “Securing Communication”.

13.1.2.2. IMAP

The default IMAP server under Fedora is Dovecot and is provided by the dovecot package. Refer to Section 13.1.2.1, “POP” for information on how to install Dovecot.
When using an IMAP mail server, email messages remain on the server where users can read or delete them. IMAP also allows client applications to create, rename, or delete mail directories on the server to organize and store email.
IMAP is particularly useful for users who access their email using multiple machines. The protocol is also convenient for users connecting to the mail server via a slow connection, because only the email header information is downloaded for messages until opened, saving bandwidth. The user also has the ability to delete messages without viewing or downloading them.
For convenience, IMAP client applications are capable of caching copies of messages locally, so the user can browse previously read messages when not directly connected to the IMAP server.
IMAP, like POP, is fully compatible with important Internet messaging standards, such as MIME, which allow for email attachments.
For added security, it is possible to use SSL encryption for client authentication and data transfer sessions. This can be enabled by using the imaps service, or by using the /usr/sbin/stunnel program. For more information on securing email communication, refer to Section 13.5.1, “Securing Communication”.
Other free, as well as commercial, IMAP clients and servers are available, many of which extend the IMAP protocol and provide additional functionality.

13.1.2.3. Dovecot

The imap-login and pop3-login processes which implement the IMAP and POP3 protocols are spawned by the master dovecot daemon included in the dovecot package. The use of IMAP and POP is configured through the /etc/dovecot/dovecot.conf configuration file; by default dovecot runs IMAP and POP3 together with their secure versions using SSL. To configure dovecot to use POP, complete the following steps:
  1. Edit the /etc/dovecot/dovecot.conf configuration file to make sure the protocols variable is uncommented (remove the hash sign (#) at the beginning of the line) and contains the pop3 argument. For example:
    protocols = imap imaps pop3 pop3s
    When the protocols variable is left commented out, dovecot will use the default values specified for this variable.
  2. Make that change operational for the current session by running the following command as root:
    systemctl restart dovecot.service
  3. Make that change operational after the next reboot by running the command:
    systemctl enable dovecot.service

    The dovecot service starts the POP3 server

    Please note that dovecot only reports that it started the IMAP server, but also starts the POP3 server.
Unlike SMTP, both IMAP and POP3 require connecting clients to authenticate using a username and password. By default, passwords for both protocols are passed over the network unencrypted.
To configure SSL on dovecot:
  • Edit the /etc/pki/dovecot/dovecot-openssl.conf configuration file as you prefer. However, in a typical installation, this file does not require modification.
  • Rename, move or delete the files /etc/pki/dovecot/certs/dovecot.pem and /etc/pki/dovecot/private/dovecot.pem.
  • Execute the /usr/libexec/dovecot/mkcert.sh script which creates the dovecot self signed certificates. These certificates are copied in the /etc/pki/dovecot/certs and /etc/pki/dovecot/private directories. To implement the changes, restart dovecot by typing the following at a shell prompt as root:
    systemctl restart dovecot.service
More details on dovecot can be found online at http://www.dovecot.org.

13.2. Email Program Classifications

In general, all email applications fall into at least one of three classifications. Each classification plays a specific role in the process of moving and managing email messages. While most users are only aware of the specific email program they use to receive and send messages, each one is important for ensuring that email arrives at the correct destination.

13.2.1. Mail Transport Agent

A Mail Transport Agent (MTA) transports email messages between hosts using SMTP. A message may involve several MTAs as it moves to its intended destination.
While the delivery of messages between machines may seem rather straightforward, the entire process of deciding if a particular MTA can or should accept a message for delivery is quite complicated. In addition, due to problems from spam, use of a particular MTA is usually restricted by the MTA's configuration or the access configuration for the network on which the MTA resides.
Many modern email client programs can act as an MTA when sending email. However, this action should not be confused with the role of a true MTA. The sole reason email client programs are capable of sending email like an MTA is because the host running the application does not have its own MTA. This is particularly true for email client programs on non-UNIX-based operating systems. However, these client programs only send outbound messages to an MTA they are authorized to use and do not directly deliver the message to the intended recipient's email server.
Since Fedora offers two MTAs—Postfix and Sendmail—email client programs are often not required to act as an MTA. Fedora also includes a special purpose MTA called Fetchmail.
For more information on Postfix, Sendmail, and Fetchmail, refer to Section 13.3, “Mail Transport Agents”.

13.2.2. Mail Delivery Agent

A Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the proper user's mailbox. In many cases, the MDA is actually a Local Delivery Agent (LDA), such as mail or Procmail.
Any program that actually handles a message for delivery to the point where it can be read by an email client application can be considered an MDA. For this reason, some MTAs (such as Sendmail and Postfix) can fill the role of an MDA when they append new email messages to a local user's mail spool file. In general, MDAs do not transport messages between systems nor do they provide a user interface; MDAs distribute and sort messages on the local machine for an email client application to access.

13.2.3. Mail User Agent

A Mail User Agent (MUA) is synonymous with an email client application. An MUA is a program that, at the very least, allows a user to read and compose email messages. Many MUAs are capable of retrieving messages via the POP or IMAP protocols, setting up mailboxes to store messages, and sending outbound messages to an MTA.
MUAs may be graphical, such as Evolution, or have simple text-based interfaces, such as pine.

13.3. Mail Transport Agents

Fedora offers two primary MTAs: Postfix and Sendmail. Postfix is configured as the default MTA, although it is easy to switch the default MTA to Sendmail. To switch the default MTA to Sendmail, as root, you can either uninstall Postfix or use the following command to switch to Sendmail:
alternatives --config mta
You can also use the following command to enable/disable the desired service:
systemctl enable|disable service.service

13.3.1. Postfix

Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-compatible MTA that is designed to be secure, fast, and easy to configure.
To improve security, Postfix uses a modular design, where small processes with limited privileges are launched by a master daemon. The smaller, less privileged processes perform very specific tasks related to the various stages of mail delivery and run in a change rooted environment to limit the effects of attacks.
Configuring Postfix to accept network connections from hosts other than the local computer takes only a few minor changes in its configuration file. Yet for those with more complex needs, Postfix provides a variety of configuration options, as well as third party add-ons that make it a very versatile and full-featured MTA.
The configuration files for Postfix are human readable and support upward of 250 directives. Unlike Sendmail, no macro processing is required for changes to take effect and the majority of the most commonly used options are described in the heavily commented files.

13.3.1.1. The Default Postfix Installation

The Postfix executable is /usr/sbin/postfix. This daemon launches all related processes needed to handle mail delivery.
Postfix stores its configuration files in the /etc/postfix/ directory. The following is a list of the more commonly used files:
  • access — Used for access control, this file specifies which hosts are allowed to connect to Postfix.
  • main.cf — The global Postfix configuration file. The majority of configuration options are specified in this file.
  • master.cf — Specifies how Postfix interacts with various processes to accomplish mail delivery.
  • transport — Maps email addresses to relay hosts.
The aliases file can be found in the /etc/ directory. This file is shared between Postfix and Sendmail. It is a configurable list required by the mail protocol that describes user ID aliases.

Configuring Postfix as a server for other clients

The default /etc/postfix/main.cf file does not allow Postfix to accept network connections from a host other than the local computer. For instructions on configuring Postfix as a server for other clients, refer to Section 13.3.1.2, “Basic Postfix Configuration”.
Restart the postfix service after changing any options in the configuration files under the /etc/postfix directory in order for those changes to take effect. To do so, run the following command as root:
systemctl restart postfix.service

13.3.1.2. Basic Postfix Configuration

By default, Postfix does not accept network connections from any host other than the local host. Perform the following steps as root to enable mail delivery for other hosts on the network:
  • Edit the /etc/postfix/main.cf file with a text editor, such as vi.
  • Uncomment the mydomain line by removing the hash sign (#), and replace domain.tld with the domain the mail server is servicing, such as example.com.
  • Uncomment the myorigin = $mydomain line.
  • Uncomment the myhostname line, and replace host.domain.tld with the hostname for the machine.
  • Uncomment the mydestination = $myhostname, localhost.$mydomain line.
  • Uncomment the mynetworks line, and replace 168.100.189.0/28 with a valid network setting for hosts that can connect to the server.
  • Uncomment the inet_interfaces = all line.
  • Comment the inet_interfaces = localhost line.
  • Restart the postfix service.
Once these steps are complete, the host accepts outside emails for delivery.
Postfix has a large assortment of configuration options. One of the best ways to learn how to configure Postfix is to read the comments within the /etc/postfix/main.cf configuration file. Additional resources including information about Postfix configuration, SpamAssassin integration, or detailed descriptions of the /etc/postfix/main.cf parameters are available online at http://www.postfix.org/.

13.3.1.3. Using Postfix with LDAP

Postfix can use an LDAP directory as a source for various lookup tables (e.g.: aliases, virtual, canonical, etc.). This allows LDAP to store hierarchical user information and Postfix to only be given the result of LDAP queries when needed. By not storing this information locally, administrators can easily maintain it.
13.3.1.3.1. The /etc/aliases lookup example
The following is a basic example for using LDAP to look up the /etc/aliases file. Make sure your /etc/postfix/main.cf contains the following:
alias_maps = hash:/etc/aliases, ldap:/etc/postfix/ldap-aliases.cf
Create a /etc/postfix/ldap-aliases.cf file if you do not have one created already and make sure it contains the following:
server_host = ldap.example.com
search_base = dc=example, dc=com
where ldap.example.com, example, and com are parameters that need to be replaced with specification of an existing available LDAP server.

The /etc/postfix/ldap-aliases.cf file

The /etc/postfix/ldap-aliases.cf file can specify various parameters, including parameters that enable LDAP SSL and STARTTLS. For more information, refer to the ldap_table(5) man page.
For more information on LDAP, refer to Section 14.1, “OpenLDAP”.

13.3.2. Sendmail

Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost every aspect of how email is handled, including the protocol used. Many system administrators elect to use Sendmail as their MTA due to its power and scalability.

13.3.2.1. Purpose and Limitations

It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not. In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the only application needed to run an email server within an organization. Technically, this is true, as Sendmail can spool mail to each users' directory and deliver outbound mail for users. However, most users actually require much more than simple email delivery. Users usually want to interact with their email using an MUA, that uses POP or IMAP, to download their messages to their local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other applications can work in conjunction with Sendmail, but they actually exist for different reasons and can operate separately from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured to do. With literally hundreds of different options and rule sets, entire volumes have been dedicated to helping explain everything that can be done and how to fix things that go wrong. Refer to the Section 13.6, “Additional Resources” for a list of Sendmail resources.
This section reviews the files installed with Sendmail by default and reviews basic configuration changes, including how to stop unwanted email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).

13.3.2.2. The Default Sendmail Installation

In order to use Sendmail, first ensure the sendmail package is installed on your system by running, as root:
yum install sendmail
In order to configure Sendmail, ensure the sendmail-cf package is installed on your system by running, as root:
yum install sendmail-cf
For more information on installing packages with Yum, refer to Section 4.2.4, “Installing Packages”.
Before using Sendmail, the default MTA has to be switched from Postfix. For more information how to switch the default MTA refer to Section 13.3, “Mail Transport Agents”.
The Sendmail executable is /usr/sbin/sendmail.
Sendmail's lengthy and detailed configuration file is /etc/mail/sendmail.cf. Avoid editing the sendmail.cf file directly. To make configuration changes to Sendmail, edit the /etc/mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf, and use the following alternatives to generate a new configuration file:
  • Use the included makefile in /etc/mail/ (~]# make all -C /etc/mail/) to create a new /etc/mail/sendmail.cf configuration file. All other generated files in /etc/mail (db files) will be regenerated if needed. The old makemap commands are still usable. The make command will automatically be used by systemctl start|restart|reload sendmail.service.
  • Alternatively you may use the m4 macro processor to create a new /etc/mail/sendmail.cf. The m4 macro processor is not installed by default. Before using it to create /etc/mail/sendmail.cf, install the m4 package as root:
    yum install m4
More information on configuring Sendmail can be found in Section 13.3.2.3, “Common Sendmail Configuration Changes”.
Various Sendmail configuration files are installed in the /etc/mail/ directory including:
  • access — Specifies which systems can use Sendmail for outbound email.
  • domaintable — Specifies domain name mapping.
  • local-host-names — Specifies aliases for the host.
  • mailertable — Specifies instructions that override routing for particular domains.
  • virtusertable — Specifies a domain-specific form of aliasing, allowing multiple virtual domains to be hosted on one machine.
Several of the configuration files in /etc/mail/, such as access, domaintable, mailertable and virtusertable, must actually store their information in database files before Sendmail can use any configuration changes. To include any changes made to these configurations in their database files, run the following command, as root:
makemap hash /etc/mail/name < /etc/mail/name
where name represents the name of the configuration file to be updated. You may also restart the sendmail service for the changes to take effect by running:
systemctl restart sendmail.service
For example, to have all emails addressed to the example.com domain delivered to , add the following line to the virtusertable file:
@example.com bob@other-example.com
To finalize the change, the virtusertable.db file must be updated:
makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable
Sendmail will create an updated virtusertable.db file containing the new configuration.

13.3.2.3. Common Sendmail Configuration Changes

When altering the Sendmail configuration file, it is best not to edit an existing file, but to generate an entirely new /etc/mail/sendmail.cf file.

Backup the sendmail.cf file before changing its content

Before changing the sendmail.cf file, it is a good idea to create a backup copy.
To add the desired functionality to Sendmail, edit the /etc/mail/sendmail.mc file as root. Once you are finished, restart the sendmail service and, if the m4 package is installed, the m4 macro processor will automatically generate a new sendmail.cf configuration file:
systemctl restart sendmail.service

Configuring Sendmail as a server for other clients

The default sendmail.cf file does not allow Sendmail to accept network connections from any host other than the local computer. To configure Sendmail as a server for other clients, edit the /etc/mail/sendmail.mc file, and either change the address specified in the Addr= option of the DAEMON_OPTIONS directive from 127.0.0.1 to the IP address of an active network device or comment out the DAEMON_OPTIONS directive all together by placing dnl at the beginning of the line. When finished, regenerate /etc/mail/sendmail.cf by restarting the service:
systemctl restart sendmail.service
The default configuration which ships with Fedora works for most SMTP-only sites. However, it does not work for UUCP (UNIX-to-UNIX Copy Protocol) sites. If using UUCP mail transfers, the /etc/mail/sendmail.mc file must be reconfigured and a new /etc/mail/sendmail.cf file must be generated.
Consult the /usr/share/sendmail-cf/README file before editing any files in the directories under the /usr/share/sendmail-cf directory, as they can affect the future configuration of the /etc/mail/sendmail.cf file.

13.3.2.4. Masquerading

One common Sendmail configuration is to have a single machine act as a mail gateway for all machines on the network. For instance, a company may want to have a machine called mail.example.com that handles all of their email and assigns a consistent return address to all outgoing mail.
In this situation, the Sendmail server must masquerade the machine names on the company network so that their return address is user@example.com instead of user@host.example.com.
To do this, add the following lines to /etc/mail/sendmail.mc:
FEATURE(always_add_domain)dnl
FEATURE(`masquerade_entire_domain')dnl
FEATURE(`masquerade_envelope')dnl
FEATURE(`allmasquerade')dnl
MASQUERADE_AS(`bigcorp.com.')dnl
MASQUERADE_DOMAIN(`bigcorp.com.')dnl
MASQUERADE_AS(bigcorp.com)dnl
After generating a new sendmail.cf using the m4 macro processor, this configuration makes all mail from inside the network appear as if it were sent from bigcorp.com.

13.3.2.5. Stopping Spam

Email spam can be defined as unnecessary and unwanted email received by a user who never requested the communication. It is a disruptive, costly, and widespread abuse of Internet communication standards.
Sendmail makes it relatively easy to block new spamming techniques being employed to send junk email. It even blocks many of the more usual spamming methods by default. Main anti-spam features available in sendmail are header checks, relaying denial (default from version 8.9), access database and sender information checks.
For example, forwarding of SMTP messages, also called relaying, has been disabled by default since Sendmail version 8.9. Before this change occurred, Sendmail directed the mail host (x.edu) to accept messages from one party (y.com) and sent them to a different party (z.net). Now, however, Sendmail must be configured to permit any domain to relay mail through the server. To configure relay domains, edit the /etc/mail/relay-domains file and restart Sendmail:
systemctl restart sendmail.service
However, many times users are bombarded with spam from other servers throughout the Internet. In these instances, Sendmail's access control features available through the /etc/mail/access file can be used to prevent connections from unwanted hosts. The following example illustrates how this file can be used to both block and specifically allow access to the Sendmail server:
badspammer.com ERROR:550 "Go away and do not spam us" tux.badspammer.com OK 10.0 RELAY
This example shows that any email sent from badspammer.com is blocked with a 550 RFC-821 compliant error code, with a message sent back to the spammer. Email sent from the tux.badspammer.com sub-domain, is accepted. The last line shows that any email sent from the 10.0.*.* network can be relayed through the mail server.
Because the /etc/mail/access.db file is a database, use the makemap command to update any changes. Do this using the following command as root:
makemap hash /etc/mail/access < /etc/mail/access
Message header analysis allows you to reject mail based on header contents. SMTP servers store information about an email's journey in the message header. As the message travels from one MTA to another, each puts in a Received header above all the other Received headers. It is important to note that this information may be altered by spammers.
The above examples only represent a small part of what Sendmail can do in terms of allowing or blocking access. Refer to the /usr/share/sendmail-cf/README for more information and examples.
Since Sendmail calls the Procmail MDA when delivering mail, it is also possible to use a spam filtering program, such as SpamAssassin, to identify and file spam for users. Refer to Section 13.4.2.6, “Spam Filters” for more information about using SpamAssassin.

13.3.2.6. Using Sendmail with LDAP

Using LDAP is a very quick and powerful way to find specific information about a particular user from a much larger group. For example, an LDAP server can be used to look up a particular email address from a common corporate directory by the user's last name. In this kind of implementation, LDAP is largely separate from Sendmail, with LDAP storing the hierarchical user information and Sendmail only being given the result of LDAP queries in pre-addressed email messages.
However, Sendmail supports a much greater integration with LDAP, where it uses LDAP to replace separately maintained files, such as /etc/aliases and /etc/mail/virtusertables, on different mail servers that work together to support a medium- to enterprise-level organization. In short, LDAP abstracts the mail routing level from Sendmail and its separate configuration files to a powerful LDAP cluster that can be leveraged by many different applications.
The current version of Sendmail contains support for LDAP. To extend the Sendmail server using LDAP, first get an LDAP server, such as OpenLDAP, running and properly configured. Then edit the /etc/mail/sendmail.mc to include the following:
LDAPROUTE_DOMAIN('yourdomain.com')dnl
FEATURE('ldap_routing')dnl

Advanced configuration

This is only for a very basic configuration of Sendmail with LDAP. The configuration can differ greatly from this depending on the implementation of LDAP, especially when configuring several Sendmail machines to use a common LDAP server.
Consult /usr/share/sendmail-cf/README for detailed LDAP routing configuration instructions and examples.
Next, recreate the /etc/mail/sendmail.cf file by running the m4 macro processor and again restarting Sendmail. Refer to Section 13.3.2.3, “Common Sendmail Configuration Changes” for instructions.
For more information on LDAP, refer to Section 14.1, “OpenLDAP”.

13.3.3. Fetchmail

Fetchmail is an MTA which retrieves email from remote servers and delivers it to the local MTA. Many users appreciate the ability to separate the process of downloading their messages located on a remote server from the process of reading and organizing their email in an MUA. Designed with the needs of dial-up users in mind, Fetchmail connects and quickly downloads all of the email messages to the mail spool file using any number of protocols, including POP3 and IMAP. It can even forward email messages to an SMTP server, if necessary.

Installing the fetchmail package

In order to use Fetchmail, first ensure the fetchmail package is installed on your system by running, as root:
yum install fetchmail
For more information on installing packages with Yum, refer to Section 4.2.4, “Installing Packages”.
Fetchmail is configured for each user through the use of a .fetchmailrc file in the user's home directory. If it does not already exist, create the .fetchmailrc file in your home directory
Using preferences in the .fetchmailrc file, Fetchmail checks for email on a remote server and downloads it. It then delivers it to port 25 on the local machine, using the local MTA to place the email in the correct user's spool file. If Procmail is available, it is launched to filter the email and place it in a mailbox so that it can be read by an MUA.

13.3.3.1. Fetchmail Configuration Options

Although it is possible to pass all necessary options on the command line to check for email on a remote server when executing Fetchmail, using a .fetchmailrc file is much easier. Place any desired configuration options in the .fetchmailrc file for those options to be used each time the fetchmail command is issued. It is possible to override these at the time Fetchmail is run by specifying that option on the command line.
A user's .fetchmailrc file contains three classes of configuration options:
  • global options — Gives Fetchmail instructions that control the operation of the program or provide settings for every connection that checks for email.
  • server options — Specifies necessary information about the server being polled, such as the hostname, as well as preferences for specific email servers, such as the port to check or number of seconds to wait before timing out. These options affect every user using that server.
  • user options — Contains information, such as username and password, necessary to authenticate and check for email using a specified email server.
Global options appear at the top of the .fetchmailrc file, followed by one or more server options, each of which designate a different email server that Fetchmail should check. User options follow server options for each user account checking that email server. Like server options, multiple user options may be specified for use with a particular server as well as to check multiple email accounts on the same server.
Server options are called into service in the .fetchmailrc file by the use of a special option verb, poll or skip, that precedes any of the server information. The poll action tells Fetchmail to use this server option when it is run, which checks for email using the specified user options. Any server options after a skip action, however, are not checked unless this server's hostname is specified when Fetchmail is invoked. The skip option is useful when testing configurations in the .fetchmailrc file because it only checks skipped servers when specifically invoked, and does not affect any currently working configurations.
The following is a sample example of a .fetchmailrc file:
set postmaster "user1"
set bouncemail

poll pop.domain.com proto pop3
    user 'user1' there with password 'secret' is user1 here

poll mail.domain2.com
    user 'user5' there with password 'secret2' is user1 here
    user 'user7' there with password 'secret3' is user1 here
In this example, the global options specify that the user is sent email as a last resort (postmaster option) and all email errors are sent to the postmaster instead of the sender (bouncemail option). The set action tells Fetchmail that this line contains a global option. Then, two email servers are specified, one set to check using POP3, the other for trying various protocols to find one that works. Two users are checked using the second server option, but all email found for any user is sent to user1's mail spool. This allows multiple mailboxes to be checked on multiple servers, while appearing in a single MUA inbox. Each user's specific information begins with the user action.

Omitting the password from the configuration

Users are not required to place their password in the .fetchmailrc file. Omitting the with password 'password' section causes Fetchmail to ask for a password when it is launched.
Fetchmail has numerous global, server, and local options. Many of these options are rarely used or only apply to very specific situations. The fetchmail man page explains each option in detail, but the most common ones are listed in the following three sections.

13.3.3.2. Global Options

Each global option should be placed on a single line after a set action.
  • daemon seconds — Specifies daemon-mode, where Fetchmail stays in the background. Replace seconds with the number of seconds Fetchmail is to wait before polling the server.
  • postmaster — Specifies a local user to send mail to in case of delivery problems.
  • syslog — Specifies the log file for errors and status messages. By default, this is /var/log/maillog.

13.3.3.3. Server Options

Server options must be placed on their own line in .fetchmailrc after a poll or skip action.
  • auth auth-type — Replace auth-type with the type of authentication to be used. By default, password authentication is used, but some protocols support other types of authentication, including kerberos_v5, kerberos_v4, and ssh. If the any authentication type is used, Fetchmail first tries methods that do not require a password, then methods that mask the password, and finally attempts to send the password unencrypted to authenticate to the server.
  • interval number — Polls the specified server every number of times that it checks for email on all configured servers. This option is generally used for email servers where the user rarely receives messages.
  • port port-number — Replace port-number with the port number. This value overrides the default port number for the specified protocol.
  • proto protocol — Replace protocol with the protocol, such as pop3 or imap, to use when checking for messages on the server.
  • timeout seconds — Replace seconds with the number of seconds of server inactivity after which Fetchmail gives up on a connection attempt. If this value is not set, a default of 300 seconds is assumed.

13.3.3.4. User Options

User options may be placed on their own lines beneath a server option or on the same line as the server option. In either case, the defined options must follow the user option (defined below).
  • fetchall — Orders Fetchmail to download all messages in the queue, including messages that have already been viewed. By default, Fetchmail only pulls down new messages.
  • fetchlimit number — Replace number with the number of messages to be retrieved before stopping.
  • flush — Deletes all previously viewed messages in the queue before retrieving new messages.
  • limit max-number-bytes — Replace max-number-bytes with the maximum size in bytes that messages are allowed to be when retrieved by Fetchmail. This option is useful with slow network links, when a large message takes too long to download.
  • password 'password' — Replace password with the user's password.
  • preconnect "command" — Replace command with a command to be executed before retrieving messages for the user.
  • postconnect "command" — Replace command with a command to be executed after retrieving messages for the user.
  • ssl — Activates SSL encryption.
  • user "username" — Replace username with the username used by Fetchmail to retrieve messages. This option must precede all other user options.

13.3.3.5. Fetchmail Command Options

Most Fetchmail options used on the command line when executing the fetchmail command mirror the .fetchmailrc configuration options. In this way, Fetchmail may be used with or without a configuration file. These options are not used on the command line by most users because it is easier to leave them in the .fetchmailrc file.
There may be times when it is desirable to run the fetchmail command with other options for a particular purpose. It is possible to issue command options to temporarily override a .fetchmailrc setting that is causing an error, as any options specified at the command line override configuration file options.

13.3.3.6. Informational or Debugging Options

Certain options used after the fetchmail command can supply important information.
  • --configdump — Displays every possible option based on information from .fetchmailrc and Fetchmail defaults. No email is retrieved for any users when using this option.
  • -s — Executes Fetchmail in silent mode, preventing any messages, other than errors, from appearing after the fetchmail command.
  • -v — Executes Fetchmail in verbose mode, displaying every communication between Fetchmail and remote email servers.
  • -V — Displays detailed version information, lists its global options, and shows settings to be used with each user, including the email protocol and authentication method. No email is retrieved for any users when using this option.

13.3.3.7. Special Options

These options are occasionally useful for overriding defaults often found in the .fetchmailrc file.
  • -a — Fetchmail downloads all messages from the remote email server, whether new or previously viewed. By default, Fetchmail only downloads new messages.
  • -k — Fetchmail leaves the messages on the remote email server after downloading them. This option overrides the default behavior of deleting messages after downloading them.
  • -l max-number-bytes — Fetchmail does not download any messages over a particular size and leaves them on the remote email server.
  • --qui