Chrony
This article describes how to set up and run Chrony, an alternative NTP client and server that is dial-up friendly and designed specifically for systems that are not online all the time.
Contents
Installation
chrony is available from the [community] repository.
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 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
chronyd runs as a daemon in the background, keeping track of the clock, and waiting for it to be told to go online and synchronize the time with the servers.
Stop the hwclock daemon (if it is running):
# rc.d stop hwclock
Start the chrony daemon:
# rc.d start chrony
Add chrony to your DAEMONS array so it starts automatically on boot and make sure hwclock is disabled:
/etc/rc.conf
DAEMONS=(... !hwclock chrony ...)
If you are using systemd instead of initscripts then you can enable the daemon using:
# systemctl enable chronyd
Then chronyd should start after the next boot. You can start or stop the daemon at any time using:
# systemctl start/stop chronyd
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. There are two methods. You can set the clock using the hwclock command but in order to do so you must temporarily stop the chrony daemon:
# systemctl stop chronyd
Then set the clock eg:
# hwclock --set --date="12/9/12 18:48:01"
or to set the system clock time into the RTC use:
# hwclock --systohc
Then restart chronyd:
# systemctl start chronyd
It is also possible to set the RTC whilst chronyd is running by using the command line chronyc - once at the chronyc prompt use:
# chronyc> password zyxxy 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.
Alternatives
Alternatives to the Chrony, are NTPd, the standard NTP client/daemon for Linux, and OpenNTPD, part of the OpenBSD project and currently not maintained for Linux.
See also
- Time (for more information on computer timekeeping)