Adding Virtualization Support

Author: Peter Boy (pboy) | Creation Date: 2021-03-31 | Last update: 2022-11-15 | Fedora Version(s) 34-37

Qemu-kvm in combination with Libvirt management toolkit is the standard virtualization methodology in Fedora. This optionally includes a local virtual network that you may use for protected communication between the virtual guest systems with each other and with the host. Its default configuration enables also natted access to the public network, specifically usful for virtual machines or containers without its own direct access to the public interface.

Preparation

Hardware requirements

QEMU / KVM require hardware virtualization support. The first thing to do is to make sure that it is available.

[…]# egrep --color 'vmx|svm' /proc/cpuinfo

The command will return one line per cpu core if virtualization is enabled. If not, you should first check in the BIOS whether virtualization is disabled.

Storage set up

Libvirt stores its data including the image files of the virtual hard disk(s) for the guest systems in /var/lib/libvirt. If you adhere to the default partitioning concept, the libvirt application data is stored in its own logical volume that you have to create in advance. You need to specify the size of the storage area, a unique name, and the accommodating VG (fedora_fedora in case of default partitioning). In the new logical volume, create an xfs file system and mount at /var/lib/libvirt.

Cockpit

The easiest way is to use Cockpit. Start your favorite browser and navigate to your server, named example.com here.

https://example.com:9090

If there is no valid public certificate installed so far, a browser warning appears and you have to accept an exception for the self-signed certificate. The subsequent login can use either the root account or an unprivileged administrative user account.

In Cockpit select "Storage" in the left navigation column and then the target volume group in the device list at the top of the right column of the opening window. The center content area changes to show the selected volume group at the top and a list of existing logical volumes below it that may be empty for now.

To create a logical volume select "Create logical volume" next to the 'Logical volumes' section title. In the form that opens, fill in the name of the new logical volume at the top, e.g. in this case 'libvirt'. Leave the usage field as 'File system' and adjust the size at the bottom, e.g. 500 GiB. Then create the LV.

In the 'Logical volumes' list, a new line appears with the LV name, libvirt in this example, as part of the device part on the right side. Expand that line and select 'format' on the right side. In the form that opens, fill in the name of the new file system, e.g. in this case 'libvirt', and the mount point, /var/lib/libvirt in this case. Leave the other fields at their default values. Select 'Format' and Cockpit will handle everything else.

After completion, the file system is immediately available and is also permanently configured in the system accordingly.

Command line

Some administrators may prefer the command line for easy scripting. Adjust size and VG name accordingly.

[…]# lvcreate -L 50G -n libvirt  fedora
[…]# mkfs.xfs /dev/fedora/libvirt
[…]# mkdir -p /var/lib/libvirt
[…]# vim /etc/fstab
/dev/mapper/fedora-root     /                  xfs   defaults    0 0
/dev/mapper/fedora-libvirt  /var/lib/libvirt   xfs   defaults    0 0
[…]# mount -a

Installing libvirt virtualization software

Installing the software is quite simple.

[…]# dnf install qemu-kvm-core libvirt virt-install cockpit-machines guestfs-tools

Check to really install guestfs-tools, not libguestfs-tools (unless you need additional windows guests related software). The package guestfs-tools provides a basic set of various useful tools to maintain virtual disks. Additional packages provide support for specific use cases, e.g. various file systems or forensic support. Use dnf search guestfs to get a list of available packages.

Do never install the group @virtualization onto a Fedora Server. It includes various graphical programs and libraries that are not usable on headless servers.

Next check the SELinux labels

[…]# ls -alZ /var/lib/libvirt

Usually, the installation adjusts the SELinux labels accordingly. Otherwise, adjustment it manually.

[…]# /sbin/restorecon -R -vF /var/lib/libvirt

If everything is correct, the next step is to activate autostart after re-boot and start KVM and libvirtd.

With Fedora 35 libvirt switched to a modular archtecture (version 7.6.0-3) while used a single monolithic libvirt daemon up to Fedora 34 (version 7.0.0-x). The installation procedure is the same because the packaging system takes care of the differences. But activation and start up differs as well as installation and configuration of an internal protected virtual network between VMs und host and VMs.

Activation and startup with Fedora 34

Enable automatic startup at boot and start libvirt.

[…]# systemctl enable libvirtd  --now

By default, libvirt creates a (virtual) bridge with an interface virbr0, the IP 192.168.122.1 and the libvirt-internal name default. In addition, a separate firewall zone libvirt is set up and assigned to the internal interface. Check if everything is running as expected.

[…]# ip a
[…]# firewall-cmd --get-active-zones

Activation and startup with Fedora 35-37

The libvirt Fedora installation procedure provides systemd startup scripts that take care of enabling and starting the various unix sockets and services as needed. This includes support for qemu, xen and lxc. Configuration of vbox is disabled by default. The drivers determine during startup whether the required prerequisites are met and abort otherwise. The default services, in case of Fedora Server qemu and lxc, are started at boot time. If not used for about one minute they are deactivated, but will restart on demand as soon as a virtual machine gets started (either by command line or Cockpit service). No need for administrator intervention at all.

The network configuration is slightly different. The services don’t start at boot time, like qemu, lxc, etc., but on demand at first access. Therefore, you won’t get an interface virbr0 until any libvirt servive request it. That’s sometimes inconvenient, e.g. if you use that interface for non-libvirt services, too (e.g. lxd or nspawn container).You may prefer to enable virt-network service anyway:

  1. Optionally, if you are planning to use the virtual network independently from starting virtual machines or you need the virbr0 interface at boot time anyway, enable libvirt’s internal network. Otherwise you may skip this step.

    […]# systemctl enable virtnetworkd.service  --now

    Alternatively, you may want do completly discard libvirt’s internal network. You’ll take this path, if you set up an iternal network with NetworkManager tools,

    […]# systemctl disable virtnetworkd.socket  --now
  2. Reboot the system to activate and test the virtualization functionality

  3. Check the successful start via a status query

    […]# systemctl status virtnetworkd.service
    ● virtnetworkd.service - Virtualization network daemon
         Loaded: loaded (/usr/lib/systemd/system/virtnetworkd.service; enabled; vendor preset: disabled)
         Active: active (running) since Tue 2021-xx-xx hh:mm:ss UTC; 48s ago
    TriggeredBy: ● virtnetworkd.socket
                 ● virtnetworkd-ro.socket
                 ● virtnetworkd-admin.socket
           Docs: man:libvirtd(8)
                 https://libvirt.org
       Main PID: xyz (virtnetworkd)
          Tasks: ...
         Memory: 10.9M
            CPU: xxms
         CGroup: /system.slice/virtnetworkd.service
                 └─xyz ....
    
    mm dd hh:mm:ss host.example.com systemd[1]: Starting Virtualization network daemon...
    mm dd hh:mm:ss host.example.com systemd[1]: Started Virtualization network daemon.

    Upon completion of a boot, an interface virbr0 as well as a dnsmasq server is now available, regardless of when and if a libvirt service is started. The virtnetwork service will terminate if no libvirt service starts up within the first 1-2 minutes (and later restart automatically if needed). But the interface and the dnsmasq server remain regardless.

    The virtualization services virtqemud.service, etc will be dormant it no VM has been started as well. But the socket is active and will start the corresponding service on demand.

Adjusting libvirt internal network configuration

The default configuration of the internal network (virbr0) activates just a DHCP Server. If the virtual machines should also be able to communicate with each other and the host, then adding a DNS server is at least very advantageous. It is easier and less error-prone to address VMs and the host by name instead of IP numbers.

The first step is to decide about a domain name. A top-level ".local" is explicitly not recommended, nor taking one of the official top-level names. But for example, you can take the official domain name and replace the last, top-level part with 'lan' or 'internal'. An official domain example.com would translate to an internal domain example.lan. We use that one throughout this tutorial. The host gets the internal name host.example.lan.

Use the libvirt tool to adjust the default network. Replace names and placeholders as required. Delete the line with "forward mode = 'nat'" if you do not want to allow access to the public network via the virtual network.

[…]# virsh  net-edit  default
<network>
  <name>default</name>
  <uuid>aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee</uuid>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:xx:yy:zz'/>
  <forward mode='nat'/>
  <mtu size='8000'/>
  <domain name='example.lan' localOnly='yes'/>
  <dns forwardPlainNames='no'>
    <forwarder domain='example.lan' />
    <host ip='192.168.122.1'>
      <hostname>host</hostname>
      <hostname>host.example.lan</hostname>
    </host>
  </dns>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>

Activate the modified configuration.

[…]# virsh net-destroy default
[…]# virsh net-start default

Check if the DNS resolution works.

[…]# nslookup host  192.168.122.1
[…]# dig  @192.168.122.1  host.example.com

Adjusting the hosts DNS resolution configuration

In order for the host to initiate communication with its virtual machines, it must query the name server set up in the previous step for internal domains. So we need Split DNS.

Since Fedora 33 system-resolved is the default DNS client in Fedora. It provides the required split DNS capabilities needed to include an internal private network in the name resolution. Unfortunately systemd-resolved currently doesn’t cooperate well with libvirt and its usage is quite unstable. Therefore we recommend to switch to dnsmasq for the time being to provide stable split DNS.

The following procedure doesn’t work for Fedora 35-37 due to a bug. Don’t use it with the defective releases.

  1. The name resolver service systemd-resolved introduced with Fedora 33 can do this automatically. But libvirt handles its interfaces on its own and must therefore inform systemd-resolved about it. A script in a hook provided by libvirt can take care of this. You have to adjust the local domain name (as of ${example.lan} in the script below) accordingly!

    […]# mkdir -p /etc/libvirt/hooks/network.d/
    […]# vim /etc/libvirt/hooks/network.d/40-config-resolved.sh
    #>--INSERT--<#
    #!/bin/bash
    # Add the internal libvirt interface virbr0 to the
    # systemd-resolved configuration.
    # Its automatic configuration of systemd-resolved cannot
    # (yet) detect a libvirt DNS server.
    
    network="$1"
    operation="$2"
    suboperation="$3"
    
    # $1 : network name, eg. default
    # $2 : one of "start" | "started" | "port-created"
    # $3 : always "begin"
    # see  https://libvirt.org/hooks.html
    
    ctstartlog="/var/log/resolve-fix.log"
    
    echo " P1: $1 - P2: $2 - P3: $3   @  $(date)  "
    echo "                                        "  >  $ctstartlog
    echo "======================================= "  >>  $ctstartlog
    echo " P1: $1 - P2: $2 - P3: $3   @  $(date)  "  >> $ctstartlog
    
    if [ "$network" == "default" ]; then
        if [ "$operation" == "started" ] && [ "$suboperation" == "begin" ]; then
    
            echo " Start fixing ....      "  >> $ctstartlog
            resolvectl dns virbr0 192.168.122.1
            resolvectl default-route virbr0 false
            resolvectl domain virbr0 example.lan
            resolvectl domain virbr0 example.lan
            echo "              .... done "  >> $ctstartlog
            echo " Checking ....          "  >> $ctstartlog
            echo " Executing resolvectl   "  >> $ctstartlog
            resolvectl status                >> $ctstartlog
            echo "                        "  >> $ctstartlog
            echo " Executing cat resolve  "  >> $ctstartlog
            cat  /etc/resolv.conf            >> $ctstartlog
            echo "              .... done "  >> $ctstartlog
    
        fi
    fi
    #>--SAVE&QUIT--<#
    
    […]# chmod +x /etc/libvirt/hooks/network.d/40-config-resolved.sh
  2. Check, if /etc/resolv.conf is a link and not a file. Activate modified local DNS resolving

    […]# ls -al /etc/resolv.conf

    In case it is a file, fix it:

    […]# cd /etc
    […]# rm -f resolv.con
    […]# systemctl  restart  systemd-resolved
    […]# ln -s ../run/systemd/resolve/stub-resolv.conf  resolv.conf
  3. Test the hook file

    […]# /etc/libvirt/hooks/network.d/40-config-resolved.sh  default started begin
     P1: default - P2: started - P3: begin   @  Mon Mar ...
  4. It is useful to modify the host’s search path resolve a short single hostname to the internal network.

    […]# /etc/libvirt/hooks/network.d/40-config-resolved.sh  default started begin
     P1: default - P2: started - P3: begin   @  Mon Mar ...
  5. Check the functionality of name resolution with internal and external addresses.

[…]# ping host
[…]# ping host.example.lan
[…]# ping host.example.com
[…]# ping guardian.co.uk

+ Everything should work fine now.

Switch to NetworkManager’s dnsmasq plugin

The NetworkManager Dnsmasq plugin is an easy way to add a local caching DNS server which is split DNS enabled. It provides the host with a lightweight, capable DNS server as an alternative to use system-resolved. By default it forwards all queries to an external, 'official' DNS server. We just have to configure the local libvirt domain.

Activate dnsmasq plugin

[…]# vim /etc/NetworkManager/conf.d/00-use-dnsmasq.conf
# /etc/NetworkManager/conf.d/00-use-dnsmasq.conf
#
# This enabled the dnsmasq plugin.
[main]
dns=dnsmasq

Define the local domain and DNS service

[…]# vim /etc/NetworkManager/dnsmasq.d/00-example-lan.conf
# /etc/NetworkManager/dnsmasq.d/00-example-lan.conf
#
# This file directs dnsmasq to forward any request to resolve
# names under the .example.lan domain to 192.168.122.1, the
# local libvirt DNS server.
server=/example.lan/192.168.122.1

Activate modified local DNS resolving

[…]# systemctl  stop  systemd-resolved
[…]# systemctl  disable  systemd-resolved
[…]# rm /etc/resolv.conf
[…]# nmcli con mod enp3s0 ipv4.dns-search 'example.lan'
[…]# nmcli con mod enp3s0 ipv6.dns-search 'example.lan'
[…]# systemctl  restart  NetworkManager

Check the functionality of name resolution with internal and external addresses.

[…]# ping host
[…]# ping host.example.lan
[…]# ping host.example.com
[…]# ping guardian.co.uk

Finishing Cockpit-machines configuration

Open your browser and connect to the Cockpit instance of your host server. Consult the post-installation guide to find out the possible connection paths. Log in as root or with your administrative account. In the overview (start) page select Virtual Machines in the left navigation column.

If there is no entry Virtual Machines in the navigation column, the cockpit-machines module was left off in the installation step above. Select Applications further down and then Machines for installation.

Currently, it sometimes takes a long time to display additional installation options. You may install the module with […​]# dnf install cockpit-machines instead.

Cockpit Virtual Machines Page first used

Storage pools

When first used, the list of virtual machines displayed in the center of the page is empty, of course. A box left above that list displays 0 Storage pools. Libvirt uses pools to determine the location of typical files. The installer has already created the directories. Only the pools need to be defined here.

Typically you use one Pool for installation media, stored at /var/lib/libvirt/boot. "Installation media" would be a suitable descriptive pool name. Select 0 Storage pools in the box and then Create storage pool. A new form opens.

If you are logged in as an adminstrative user (even if having used sudo su - ), you are asked to select a connection type, "system" or "session". This selection is presented in various configuration forms, so some explanation here. Use "system" for a production deployments, what you do in most cases. Select "sesssion" in the special case of testing, development, and experimentation. The "session" option does not support any custom or advanced networking, but works pretty much everwhere (including containers) and without any privileges. The libvirt project provides additional information for developers. If you are logged in directly as root, this line doesn’t show up. Instead, everything is treated as system. i.e. production deployment (never do development or experimentation as root).

Next enter "Installation media" as the name, "Filesystem directory" as the type, and /var/lib/libvirt/boot as the target path.

In version 264 there is a bug that blocks the creation of storage pools (see Bug 2068883. Proceed without creating pools for the time being. The bug is fixed in release 37

In most cases, the (virtual) hard disk used for a virtual machine is a disk image file stored in /var/lib/libvirt/images. Define another pool named "Disk images" accordingly.

Activate both pools in the drop down menu of each pool.

You may create additional pools as needed, e.g. disk images via iSCSI in a SAN or as a logical volume in a volume group (LVM) on the host’s local disk. The latter offers a better performance in theory, but the practical gain is usually rather small, if any. We will not go into further detail here for the time being.

Networks

A box on the right above the virtual machines list shows 1 Network and lists the networks managed by libvirt. By default, it contains the internal network default with the interface virbr0. The list does not contain the external interface. It is managed by the server. Nevertheless, it is as well available for virtual machines.

Completed

Virtualization is now ready to use on the server and you can start setting up guest VMs.

These nice people helped write this page:

Fredrik Arneving, Peter Boy, Jan Kuparinen

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