Install the package.
The smallest useful configuration file (using IP addresses instead of a hostname) would look something like:
server 220.127.116.11 offline server 18.104.22.168 offline server 22.214.171.124 offline driftfile /var/lib/chrony/drift rtconutc # This sets the rtc, but throws out RTC drift tracking. rtcfile is the recommended alternative: see "RTC options" below. rtcsync
/usr/share/doc/chrony/README, which will point you to the right answer to any doubts you could still have. Documentation is also available online. See also the related man pages: , , .
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 tells chrony to pick 4 sources from the NTP pool (chrony has special handling of pools, so as to not confuse its tracking of server-side drift), and use a burst behavior on startup:
pool pool.ntp.org iburst maxsources 4
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:
pool pool.ntp.org iburst maxsources 4 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 will not be available until you have made a connection.
Using NTS servers
server time.cloudflare.com iburst nts
You can find a list of all known NTPSec-supporting servers here.
The hardware clock (RTC)
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. Chrony.conf has three different mechanisms for handling the RTC:
- The first mechanism is
rtcsync, which simply writes the current time to the RTC periodically. This is the classical method used by ntpd, but turns off RTC drift tracking: this is bad for intermittently running desktops, which does a lot of time-keeping on the RTC.
- The second mechanism is
rtcautotrim, which overwrites the RTC time only if it goes above a difference threshold. This method can be used with
rtcfile, which allows for keeping track of RTC error.
- The final mechanism is to do nothing about the RTC, but record its error and drift in
rtcfile. The RTC time will stay wrong, but the system time will become correct as chrony has an idea of how wrong it is. The
rtctrimcommand in chronyc can still sync the RTC as needed:
# chronyc chronyc> trimrtc 200 OK chronyc> quit
rtcfilecannot be used at the same time. Using
rtcfilealso prevents tools like
timedatectlfrom accessing the RTC. See for details.
rtconutc describes whether RTC runs on UTC.
Example: intermittently running desktops
An intermittently running desktop would require the use of
rtcfile to keep track of RTC error. A machine running Arch Linux for five years, accumulated a 300 s error within the RTC. After a reboot it took chrony a long time to adjust this difference using the above configuration. If we go for the below instead:
dumponexit dumpdir /var/lib/chrony rtcfile /var/lib/chrony/rtc
This keeps, interestingly, the RTC still out-of-date, but after each re-start, chrony adjusts the accumulated error of the RTC and the system time is quite synchronous to NTP even shortly after a start.
RTC remains out-of-date because we forgot to add the
rtcautotrim line telling chrony to adjust the RTC. If we do add it, both the RTC and the system time will become correct.
Other interesting options
makestep: allow chrony to change the time by abrupt sets instead of frequency adjustments. Doing so may surprise running programs, but helps to fix large errors.
makestep 0.1 3may be desirable for computers that are often offline: only the first three changes will be stepped, so surprises are limited to computer startup.
presendmay help increase accuracy without any compatibility cost.
hwtimestamp: some network interface cards can timestamp its packages to account for delays in the network stack. Use
hwtimestamp *to turn it on: this will not do anything on adapters without such support.
tempcomp: keep track of the relationship between software clock errors (usually due to motherboard crystal temperature changes) and a temperature sensor. For those desiring ultimate precision.
The package provides
chronyd.service, see systemd for details.
systemd-timesyncd.serviceis in conflict with
chronyd, so you need to disable it first if you want to enable
- Whilst not conflicting, the similarly named
systemd-timedated.servicecan work with chrony by setting the environment variable
SYSTEMD_TIMEDATED_NTP_SERVICES=chronyd.service:systemd-timesyncd.service. See for details.
Telling chronyd an internet connection has been made
If you are connected to the internet, run:
# chronyc chronyc> online 200 OK chronyc> exit
You may also be interested in the
activity option to display status:
# chronyc activity 200 OK 3 sources online 0 sources offline 0 sources doing burst (return to online) 0 sources doing burst (return to offline) 0 sources with unknown address
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 offline 200 OK # chronyc activity 200 OK 0 sources online 3 sources offline 0 sources doing burst (return to online) 0 sources doing burst (return to offline) 0 sources with unknown address
The online/offline status can be automatically handled by dispatcher services forand , see below.
Checking configured NTP servers
To check which NTP servers chrony is actually using, and how precise they are, you can use
chronyc -N 'sources -a -v':
$ chronyc -N 'sources -a -v' .-- Source mode '^' = server, '=' = peer, '#' = local clock. / .- Source state '*' = current best, '+' = combined, '-' = not combined, | / 'x' = may be in error, '~' = too variable, '?' = unusable. || .- xxxx [ yyyy ] +/- zzzz || Reachability register (octal) -. | xxxx = adjusted offset, || Log2(Polling interval) --. | | yyyy = measured offset, || \ | | zzzz = estimated error. || | | \ MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== ^+ ptbnts1.ptb.de 1 6 377 50 -38us[ -13us] +/- 8723us ^* ptbnts2.ptb.de 1 6 377 49 +2061ns[ +27us] +/- 7538us ^+ nts.ntp.se 2 6 377 51 +594us[ +619us] +/- 15ms ^+ nts.sth1.ntp.se 2 6 377 51 +655us[ +680us] +/- 15ms ^+ nts.sth2.ntp.se 2 6 377 53 +991us[+1016us] +/- 15ms ^+ time.cloudflare.com 3 6 377 49 -1250us[-1250us] +/- 10ms
Notifying network state
If you have specified your pools as offline in
chrony.conf, you need to tell chrony that the network status has changed.
You can either use chronyc to notify chrony that your network configuration has changed, or you can use a dispatcher for your relevant network configuration manager.
chronyd can go into online/offline mode along with a network connection through the use of NetworkManager's dispatcher scripts. You can install AUR from the AUR.
InstallAUR from the AUR. This adds a hook to netctl which is run automatically for any connection.
Create the following hook:
if $if_up; then chronyc online elif $if_down; then chronyc offline fi