Configurando NTP usando ntpd

El Protocolo de Hora de Red (NTP) habilita la propagación segura de la información de fecha y hora con el objetivo de mantener los relojes de los sistemas de ordenadores en red sincronizados a una referencia común en la red o en Internet. Muchos organismos de normalización de todo el mundo tienen relojes atómicos que pueden estar disponibles como referencia. Los relojes que conforman el Sistema de Posicionamiento Global contienen más de un reloj atómico, haciendo sus señales de hora muy seguras potencialmente. Sus señales pueden ser deliberadamente degradadas por razones militares. Una situación ideal sería que cada sitio tenga un servidor, con su propio reloj de referencia enganchado para actuar como servidor horario para todo el sitio. Existen muchos dispositivos que obtienen la fecha y la hora por medio de transmisiones de radio de baja frecuencia o del Sistema de Posicionamiento Global (GPS).Sin embargo para la mayoría de las situaciones, se puede utilizar una gama de servidores de tiempo de acceso público conectados a Internet en ubicaciones geográficamente dispersas. Estos servidores NTP suministran "Hora Universal Coordinada" (UTC). La información sobre estos servidores se puede encontrar en www.pool.ntp.org.

Mantener una hora precisa es importante por diversas razones en las Tecnologías de la Información. En la red, por ejemplo, se requieren sellos de tiempo precisos en los paquetes y en los registros. Los registros se usan para investigar cuestiones de servicio y de seguridad y por lo tanto los sellos de tiempo de diferentes sistemas deben ser puestos por relojes sincronizados para tener un valor real. Puesto que los sistemas y las redes son cada vez más rápidos, existe una necesidad de que los relojes incrementen su precisión y resolución. En algunos países existe la obligación legal de mantener relojes precisos sincronizados. Visite www.ntp.org para más información. En los sistemas Linux, NTP está implementado por un demonio que corre en el espacio de usuario. El demonio NTP de espacio de usuario predeterminado en 29 es chronyd. Debe ser deshabilitado si desea usar el demonio ntpd. Vea Configurando NTP Usando la Suite chrony para información sobre chrony.

El demonio de espacio de usuario actualiza el reloj del sistema, que es un reloj software corriendo en el kernel. Linux utiliza un reloj software como su reloj de sistema en lugar del típico reloj hardware integrado conocido como "Real Time Clock" (RTC). Vea las páginas de manual rtc(4) y hwclock(8) para información sobre relojes hardware. El reloj del sistema puede mantener la hora utilizando diversos relojes fuente. Normalmente se usa el Time Stamp Counter (TSC). El TSC es un registro de CPU que cuenta el número ciclos desde el último reinicio. Es muy rápido, tiene una alta resolución y no tiene interrupciones. Al arranque del sistema, el reloj del sistema lee la fecha y la hora desde el RTC. La hora que mantiene el RTC variará hasta cinco minutos de la hora real debido a variaciones de temperatura. Por eso la necesidad de que el reloj del sistema esté constantemente sincronizado con referencias de hora externas. Cuando el reloj del sistema está siendo sincronizado por ntpd, el kernel a su vez actualizará el RTC cada 11 minutos automáticamente.

LOs servidores NTP están clasificados de acuerdo a su distancia de sincronización de los relojes atómicos que son la fuente de las señales de hora. Los servidores están pensados para estar organizados en capas, o estratos, de 1 a 15 de arriba hacia abajo. Por eso se usa la palabra estrato para referirse a una capa específica. Los relojes atómicos se denominan Estrato 0 ya que está es la fuente, pero por la Internet no se envían paquetes de Estrato 0, todos los relojes atómicos de estrato 0 están adjuntos a un servidor al que nos referimos como estrato 1. Estos servidores envían paquetes marcados como Estrato 1. Un servidor que se sincroniza con paquetes de estrato n pertenece al siguiente, más bajo, estrato y marcará sus paquetes como estrato n+1. Los servidores del mismo estrato pueden intercambiar paquetes entre si pero están designados como pertenecientes a un solo estrato, el estrato uno por debajo de la mejor referencia a la que estén sincronizados. La designación de Estrato 16 se usa para indicar que el servidor no está actualmente sincronizado con ninguna fuente de hora fiable.

Advierta que de modo predeterminado los clientes NTP actúan como servidores de aquellos sistemas que están el estrato de debajo de ellos.

Aquí hay un resumen de los Estratos NTP:

This process continues down to Stratum 15 which is the lowest valid stratum. The label Stratum 16 is used to indicated an unsynchronized state.

The version of NTP used by Fedora is as described in RFC 1305 Network Time Protocol (Version 3) Specification, Implementation and Analysis and RFC 5905 Network Time Protocol Version 4: Protocol and Algorithms Specification

This implementation of NTP enables sub-second accuracy to be achieved. Over the Internet, accuracy to 10s of milliseconds is normal. On a Local Area Network (LAN), 1 ms accuracy is possible under ideal conditions. This is because clock drift is now accounted and corrected for, which was not done in earlier, simpler, time protocol systems. A resolution of 233 picoseconds is provided by using 64-bit time stamps. The first 32-bits of the time stamp is used for seconds, the last 32-bits are used for fractions of seconds.

NTP represents the time as a count of the number of seconds since 00:00 (midnight) 1 January, 1900 GMT. As 32-bits is used to count the seconds, this means the time will "roll over" in 2036. However NTP works on the difference between time stamps so this does not present the same level of problem as other implementations of time protocols have done. If a hardware clock that is within 68 years of the correct time is available at boot time then NTP will correctly interpret the current date. The NTP4 specification provides for an "Era Number" and an "Era Offset" which can be used to make software more robust when dealing with time lengths of more than 68 years. Note, please do not confuse this with the Unix Year 2038 problem.

The NTP protocol provides additional information to improve accuracy. Four time stamps are used to allow the calculation of round-trip time and server response time. In order for a system in its role as NTP client to synchronize with a reference time server, a packet is sent with an "originate time stamp". When the packet arrives, the time server adds a "receive time stamp". After processing the request for time and date information and just before returning the packet, it adds a "transmit time stamp". When the returning packet arrives at the NTP client, a "receive time stamp" is generated. The client can now calculate the total round trip time and by subtracting the processing time derive the actual traveling time. By assuming the outgoing and return trips take equal time, the single-trip delay in receiving the NTP data is calculated. The full NTP algorithm is much more complex than presented here.

When a packet containing time information is received it is not immediately responded to, but is first subject to validation checks and then processed together with several other time samples to arrive at an estimate of the time. This is then compared to the system clock to determine the time offset, the difference between the system clock’s time and what ntpd has determined the time should be. The system clock is adjusted slowly, at most at a rate of 0.5ms per second, to reduce this offset by changing the frequency of the counter being used. It will take at least 2000 seconds to adjust the clock by 1 second using this method. This slow change is referred to as slewing and cannot go backwards. If the time offset of the clock is more than 128ms (the default setting), ntpd can "step" the clock forwards or backwards. If the time offset at system start is greater than 1000 seconds then the user, or an installation script, should make a manual adjustment. See Configuring the Date and Time. With the -g option to the ntpd command (used by default), any offset at system start will be corrected, but during normal operation only offsets of up to 1000 seconds will be corrected.

Some software may fail or produce an error if the time is changed backwards. For systems that are sensitive to step changes in the time, the threshold can be changed to 600s instead of 128ms using the -x option (unrelated to the -g option). Using the -x option to increase the stepping limit from 0.128s to 600s has a drawback because a different method of controlling the clock has to be used. It disables the kernel clock discipline and may have a negative impact on the clock accuracy. The -x option can be added to the /etc/sysconfig/ntpd configuration file.

The drift file is used to store the frequency offset between the system clock running at its nominal frequency and the frequency required to remain in synchronization with UTC. If present, the value contained in the drift file is read at system start and used to correct the clock source. Use of the drift file reduces the time required to achieve a stable and accurate time. The value is calculated, and the drift file replaced, once per hour by ntpd. The drift file is replaced, rather than just updated, and for this reason the drift file must be in a directory for which the ntpd has write permissions.

As NTP is entirely in UTC (Universal Time, Coordinated), Timezones and DST (Daylight Saving Time) are applied locally by the system. The file /etc/localtime is a copy of, or symlink to, a zone information file from /usr/share/zoneinfo. The RTC may be in localtime or in UTC, as specified by the 3rd line of /etc/adjtime, which will be one of LOCAL or UTC to indicate how the RTC clock has been set. Users can easily change this setting using the checkbox System Clock Uses UTC in the Date and Time graphical configuration tool. See Configuring the Date and Time for information on how to use that tool. Running the RTC in UTC is recommended to avoid various problems when daylight saving time is changed.

The operation of ntpd is explained in more detail in the man page ntpd(8). The resources section lists useful sources of information. See Additional Resources.

NTPv4 added support for the Autokey Security Architecture, which is based on public asymmetric cryptography while retaining support for symmetric key cryptography. The Autokey Security Architecture is described in RFC 5906 Network Time Protocol Version 4: Autokey Specification. The man page ntp_auth(5) describes the authentication options and commands for ntpd.

An attacker on the network can attempt to disrupt a service by sending NTP packets with incorrect time information. On systems using the public pool of NTP servers, this risk is mitigated by having more than three NTP servers in the list of public NTP servers in /etc/ntp.conf. If only one time source is compromised or spoofed, ntpd will ignore that source. You should conduct a risk assessment and consider the impact of incorrect time on your applications and organization. If you have internal time sources you should consider steps to protect the network over which the NTP packets are distributed. If you conduct a risk assessment and conclude that the risk is acceptable, and the impact to your applications minimal, then you can choose not to use authentication.

The broadcast and multicast modes require authentication by default. If you have decided to trust the network then you can disable authentication by using disable auth directive in the ntp.conf file. Alternatively, authentication needs to be configured by using SHA1 or MD5 symmetric keys, or by public (asymmetric) key cryptography using the Autokey scheme. The Autokey scheme for asymmetric cryptography is explained in the ntp_auth(8) man page and the generation of keys is explained in ntp-keygen(8). To implement symmetric key cryptography, see Configuring Symmetric Authentication Using a Key for an explanation of the key option.

Virtual machines cannot access a real hardware clock and a virtual clock is not stable enough as the stability is dependent on the host systems work load. For this reason, para-virtualized clocks should be provided by the virtualization application in use (for more information see Libvirt Managed Timers in the Virtualization Administration Guide). On Fedora with KVM the default clock source is kvm-clock. See the KVM guest timing management chapter of the Virtualization Host Configuration and Guest Installation Guide.

Greenwich Mean Time (GMT) was derived by measuring the solar day, which is dependent on the Earth’s rotation. When atomic clocks were first made, the potential for more accurate definitions of time became possible. In 1958, International Atomic Time (TAI) was introduced based on the more accurate and very stable atomic clocks. A more accurate astronomical time, Universal Time 1 (UT1), was also introduced to replace GMT. The atomic clocks are in fact far more stable than the rotation of the Earth and so the two times began to drift apart. For this reason UTC was introduced as a practical measure. It is kept within one second of UT1 but to avoid making many small trivial adjustments it was decided to introduce the concept of a leap second in order to reconcile the difference in a manageable way. The difference between UT1 and UTC is monitored until they drift apart by more than half a second. Then only is it deemed necessary to introduce a one second adjustment, forward or backward. Due to the erratic nature of the Earth’s rotational speed, the need for an adjustment cannot be predicted far into the future. The decision as to when to make an adjustment is made by the International Earth Rotation and Reference Systems Service (IERS). However, these announcements are important only to administrators of Stratum 1 servers because NTP transmits information about pending leap seconds and applies them automatically.

The daemon, ntpd, reads the configuration file at system start or when the service is restarted. The default location for the file is /etc/ntp.conf and you can view the file by entering the following command:

The configuration commands are explained briefly later in this chapter, see Configure NTP, and more verbosely in the ntp.conf(5) man page.

Here follows a brief explanation of the contents of the default configuration file:

If you change this be certain that the directory is writable by ntpd. The file contains one value used to adjust the system clock frequency after every system or service start. See Understanding the Drift File for more information.

Addresses within the range 127.0.0.0/8 are sometimes required by various processes or applications. As the "restrict default" line above prevents access to everything not explicitly allowed, access to the standard loopback address for IPv4 and IPv6 is permitted by means of the following lines:

Addresses can be added underneath if specifically required by another application.

Hosts on the local network are not permitted because of the "restrict default" line above. To change this, for example to allow hosts from the 192.0.2.0/24 network to query the time and statistics but nothing more, a line in the following format is required:

To allow unrestricted access from a specific host, for example 192.0.2.250/32, a line in the following format is required:

A mask of 255.255.255.255 is applied if none is specified.

The restrict commands are explained in the ntp_acc(5) man page.

The file will be read by the ntpd init script on service start. The default contents is as follows:

The -g option enables ntpd to ignore the offset limit of 1000s and attempt to synchronize the time even if the offset is larger than 1000s, but only on system start. Without that option ntpd will exit if the time offset is greater than 1000s. It will also exit after system start if the service is restarted and the offset is greater than 1000s even with the -g option.

In order to use ntpd the default user space daemon, chronyd, must be stopped and disabled. Issue the following command as root:

To prevent it restarting at system start, issue the following command as root:

To check the status of chronyd, issue the following command:

To check if ntpd is installed, enter the following command as root:

NTP is implemented by means of the daemon or service ntpd, which is contained within the ntp package.

To install ntpd, enter the following command as root:

To enable ntpd at system start, enter the following command as root:

To check if ntpd is running and configured to run at system start, issue the following command:

To obtain a brief status report from ntpd, issue the following command:

The NTP traffic consists of UDP packets on port 123 and needs to be permitted through network and host-based firewalls in order for NTP to function.

Check if the firewall is configured to allow incoming NTP traffic for clients using the graphical Firewall Configuration tool.

To start the graphical firewall-config tool, press the Super key to enter the Activities Overview, type firewall and then press Enter. The Firewall Configuration window opens. You will be prompted for your user password.

To start the graphical firewall configuration tool using the command line, enter the following command as root user:

The Firewall Configuration window opens. Note, this command can be run as normal user but you will then be prompted for the root password from time to time.

Look for the word "Connected" in the lower left corner. This indicates that the firewall-config tool is connected to the user space daemon, firewalld.

The purpose of the ntpdate service is to set the clock during system boot. This was used previously to ensure that the services started after ntpdate would have the correct time and not observe a jump in the clock. The use of ntpdate and the list of step-tickers is considered deprecated and so Fedora uses the -g option to the ntpd command and not ntpdate by default.

The ntpdate service in Fedora is mostly useful only when used alone without ntpd. With systemd, which starts services in parallel, enabling the ntpdate service will not ensure that other services started after it will have correct time unless they specify an ordering dependency on time-sync.target, which is provided by the ntpdate service. The ntp-wait service (in the ntp-perl subpackage) provides the time-sync target for the ntpd service. In order to ensure a service starts with correct time, add After=time-sync.target to the service and enable one of the services which provide the target (ntpdate or sntp, or ntp-wait if ntpd is enabled). Some services on Fedora have the dependency included by default ( for example, dhcpd, dhcpd6, and crond).

To check if the ntpdate service is enabled to run at system start, issue the following command:

To enable the service to run at system start, issue the following command as root:

In Fedora the default /etc/ntp/step-tickers file contains 0.fedora.pool.ntp.org. To configure additional ntpdate servers, using a text editor running as root, edit /etc/ntp/step-tickers. The number of servers listed is not very important as ntpdate will only use this to obtain the date information once when the system is starting. If you have an internal time server then use that host name for the first line. An additional host on the second line as a backup is sensible. The selection of backup servers and whether the second host is internal or external depends on your risk assessment. For example, what is the chance of any problem affecting the first server also affecting the second server? Would connectivity to an external server be more likely to be available than connectivity to internal servers in the event of a network failure disrupting access to the first server?

To change the default configuration of the NTP service, use a text editor running as root user to edit the /etc/ntp.conf file. This file is installed together with ntpd and is configured to use time servers from the Fedora pool by default. The man page ntp.conf(5) describes the command options that can be used in the configuration file apart from the access and rate limiting commands which are explained in the ntp_acc(5) man page.

To configure the system clock to update the hardware clock, also known as the real-time clock (RTC), once after executing ntpdate, add the following line to /etc/sysconfig/ntpdate:

To update the hardware clock from the system clock, issue the following command as root:

When the system clock is being synchronized by ntpd or chronyd, the kernel will in turn update the RTC every 11 minutes automatically.

To list the available clock sources on your system, issue the following commands:

In the above example, the kernel is using kvm-clock. This was selected at boot time as this is a virtual machine.

To override the default clock source, append the clocksource directive to the GRUB_CMDLINE_LINUX line in the /etc/default/grub file and rebuild the grub.cfg file. For example:

The available clock source is architecture dependent.

Rebuild the grub.cfg file as follows:

The following sources of information provide additional resources regarding NTP and ntpd.