39 Time synchronization with NTP #

The NTP daemon (chronyd) coming with the chrony package is preset to use the local computer hardware clock as a time reference. The precision of the hardware clock heavily depends on its time source. For example, an atomic clock or GPS receiver is a very precise time source, while a common RTC chip is not a reliable time source. YaST simplifies the configuration of an NTP client.

In the YaST NTP client configuration (Network Services › NTP Configuration) window, you can specify when to start the NTP daemon, the type of the configuration source, and add custom time servers.

Figure 39.1: NTP configuration window #

  

39.1.3 Configure time servers #

  

Time servers for the client to query are listed in the lower part of the NTP Configuration window. Modify this list as needed with Add, Edit, and Delete.

Click Add to add a new time server:

Figure 39.2: Adding a time server #

  

1. In the Address field, type the URL of the time server or pool of time servers with which you want to synchronize the machine time. After the URL is complete, click Test to verify that it points to a valid time source.

2. Activate Quick Initial Sync to speed up the time synchronization by sending more requests at the chronyd daemon start.

3. Activate Start Offline to speed up the boot time on systems that start the chronyd daemon automatically and may not have an Internet connection at boot time. This option is useful, for example, for laptops with network connections managed by NetworkManager.

4. Confirm with OK.

chrony reads its configuration from the /etc/chrony.conf file. To keep the computer clock synchronized, you need to tell chrony what time servers to use. You can use specific server names or IP addresses, for example:

0.suse.pool.ntp.org
1.suse.pool.ntp.org
2.suse.pool.ntp.org
3.suse.pool.ntp.org

You can also specify a pool name. Pool name resolves to several IP addresses:

pool pool.ntp.org

Tip: Computers on the same network

To synchronize time on multiple computers on the same network, we do not recommend to synchronize all of them with an external server. A good practice is to make one computer the time server which is synchronized with an external time server, and the other computers act as its clients. Add a local directive to the server's /etc/chrony.conf to distinguish it from an authoritative time server:

local stratum 10

To start chrony, run:

systemctl start chronyd.service

After initializing chronyd, it takes some time before the time is stabilized and the drift file for correcting the local computer clock is created. With the drift file, the systematic error of the hardware clock can be computed when the computer is powered on. The correction is used immediately, resulting in a higher stability of the system time.

To enable the service so that chrony starts automatically at boot time, run:

systemctl enable chronyd.service

Warning: Conflicting yast-timesync.service service

In addition to the chronyd.service service, SLED includes yast-timesync.service. yast-timesync.service is triggered by a timer every 5 minutes and runs chronyd with the -q option to set the system time and exit. Because only one instance of chronyd can be running at any given time, do not enable or start both chronyd-related services at the same time.

You can use chronyc to change the behavior of chronyd at runtime. It also generates status reports about the operation of chronyd.

You can run chronyc either in interactive or non-interactive mode. To run chronyc interactively, enter chronyc on the command line. It displays a prompt and waits for your command input. For example, to check how many NTP sources are online or offline, run:

# chronyc
chronyc> activity
200 OK
4 sources online
2 sources offline
1 sources doing burst (return to online)
1 sources doing burst (return to offline)
0 sources with unknown address

To exit chronyc's prompt, enter quit or exit.

If you do not need to use the interactive prompt, enter the command directly:

# chronyc activity

Note: Temporary changes

Changes made using chronyc are not permanent. They will be lost after the next chronyd restart. For permanent changes, modify /etc/chrony.conf.

For a complete list of chronyc commands, see its manual page (man 1 chronyc).

Although chronyd starts up normally on a system that boots without a network connection, the tool cannot resolve the DNS names of the time servers specified in the configuration file.

chronyd keeps trying to resolve the time server names specified by the server, pool, and peer directives in an increasing time interval until it succeeds.

If the time server will not be reachable when chronyd is started, you can specify the offline option:

server server_address offline

chronyd will then not try to poll the server until it is enabled using the following command:

# chronyc online server_address

When the auto_offline option is set, chronyd assumes that the time server has gone offline when two requests have been sent to it without receiving a response. This option avoids the need to run the 'offline' command from chronyc when disconnecting the network link.

The software package chrony relies on other programs (such as gpsd) to access the timing data via the SHM or SOCK driver. Use the refclock directive in /etc/chrony.conf to specify a hardware reference clock to be used as a time source. It has two mandatory parameters: a driver name and a driver-specific parameter. The two parameters are followed by zero or more refclock options. chronyd includes the following drivers:

For more information on individual drivers' options, see man 8 chrony.conf.

Support for clock synchronization to an external time reference (ETR) is available. The external time reference sends an oscillator signal and a synchronization signal every 2**20 (2 to the power of 20) microseconds to keep TOD clocks of all connected servers synchronized.

For availability two ETR units can be connected to a machine. If the clock deviates for more than the sync-check tolerance all CPUs get a machine check that indicates that the clock is not synchronized. If this happens, all DASD I/O to XRC enabled devices is stopped until the clock is synchronized again.

The ETR support is activated via two sysfs attributes; run the following commands as root:

echo 1 > /sys/devices/system/etr/etr0/online
echo 1 > /sys/devices/system/etr/etr1/online