Chrony

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This article describes how to set up and run Chrony, an alternative NTP client and server that is roaming friendly and designed specifically for systems that are not online all the time.

Installation

chrony is available from the official repositories.

Alternatives to the Chrony, are NTPd, the reference implementation of NTP, and OpenNTPD, part of the OpenBSD project and currently not maintained for Linux.

Configuration

The first thing you define in your /etc/chrony.conf is the servers your machine will synchronize to. NTP servers are classified in a hierarchical system with many levels called strata: the devices which are considered independent time sources are classified as stratum 0 sources; the servers directly connected to stratum 0 devices are classified as stratum 1 sources; servers connected to stratum 1 sources are then classified as stratum 2 sources and so on.

It has to be understood that a server's stratum cannot be taken as an indication of its accuracy or reliability. Typically, stratum 2 servers are used for general synchronization purposes: if you do not already know the servers you are going to connect to, you should use the pool.ntp.org servers (alternate link) and choose the server pool that is closest to your location.

The following lines are just an example:

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

If your computer is not connected to the internet on startup, it is recommended to use the offline option, to tell Chrony not to try and connect to the servers, until it has been given the go:

server 0.pool.ntp.org offline
server 1.pool.ntp.org offline
server 2.pool.ntp.org offline
server 3.pool.ntp.org offline

It may also be a good idea to either use IP addresses instead of host names, or to map the hostnames to IP addresses in your /etc/hosts file, as DNS resolving won't be available until you've made a connection.

To tell chronyd that a connection has been established, you need to be able to log in with chronyc. You will have to configure chronyd with an administrator password to be able to do this. Setting up an administrator password is as simple as creating the file /etc/chrony.keys with a single line:

/etc/chrony.keys
1 xyzzy

as well as adding the following line somewhere in /etc/chrony.conf:

commandkey 1

The smallest useful configuration file (using IP addresses instead of a hostname) would look something like:

/etc/chrony.conf
server 1.2.3.4 offline
server 5.6.7.8 offline
server 9.10.11.12 offline
rtcfile /etc/chrony.rtc
rtconutc
keyfile /etc/chrony.keys
commandkey 1
driftfile /etc/chrony.drift

Telling chronyd an internet connection has been made

For this to work, you'll need to configure the commandkey option in /etc/chrony.conf as shown above. If you've done this, start chronyc and enter the following commands if you are connected to the internet:

chronyc> password xyzzy
200 OK
chronyc> online
200 OK
chronyc> exit

Chrony should now connect to the configured time servers and update your clock if needed.

To tell chrony that you are not connected to the internet anymore, execute the following:

chronyc> password xyzzy
200 OK
chronyc> offline
200 OK
chronyc> exit

In conclusion, don't forget the user guide at /usr/share/doc/chrony/chrony.txt, which is likely to answer any doubts you could still have. It is also available online. See also the related man pages: man {chrony|chronyc|chronyd|chrony.conf}).

Usage

Starting chronyd

The package provides chrony.service, see systemd for details.

Synchronising chrony hardware clock from the system clock

During boot the initial time is read from the hardware clock (RTC) and the system time is then set, and synchronised over a period of minutes once the chrony daemon has been running for a while. If the hardware clock is out of sync then the initial system time can be some minutes away from the true time. If that is the case it may be necessary to reset the hardware clock.

You can use chronyc to force the current system time to be synced to hardware:

# chronyc> password xyzzy
Password:
200 OK
chronyc> trimrtc
200 OK
chronyc> quit

Then exit from chronyc and the RTC and system time should be within a few microseconds of each other and should then be approximately correct on boot and fully synchronise a short time later.

Using NetworkManager to let chronyd go online

chronyd can be go into online/offline mode along with a network connection through the use of NetworkManager's dispatcher scripts. You can install networkmanager-dispatcher-chronyAUR from the AUR.

See also