systemd-timesyncd.service, systemd-timesyncd — Network Time Synchronization
systemd-timesyncd.service
/usr/lib/systemd/systemd-timesyncd
systemd-timesyncd is a system service that may be used to synchronize the
local system clock with a remote Network Time Protocol (NTP) server. It also saves the local time to disk
every time the clock has been synchronized and uses this to possibly advance the system realtime clock on
subsequent reboots to ensure it (roughly) monotonically advances even if the system lacks a
battery-buffered RTC chip.
The systemd-timesyncd service implements SNTP only. This minimalistic service
will step the system clock for large offsets or slowly adjust it for smaller deltas. Complex use cases
that require full NTP support (and where SNTP is not sufficient) are not covered by
systemd-timesyncd.
The NTP servers contacted are determined from the global settings in
timesyncd.conf(5), the
per-link static settings in .network files, and the per-link dynamic settings
received over DHCP. See
systemd.network(5) for
further details.
timedatectl(1)'s set-ntp command may be used to enable and start, or disable and stop this service.
timedatectl(1)'s timesync-status or show-timesync command can be used to show the current status of this service.
systemd-timesyncd initialization delays the start of units that are ordered
after time-set.target (see
systemd.special(7) for
details) until the local time has been updated from /var/lib/systemd/timesync/clock
(see below) in order to make it roughly monotonic. It does not delay other units until synchronization
with an accurate reference time sources has been reached. Use
systemd-time-wait-sync.service(8)
to achieve that, which will delay start of units that are ordered after
time-sync.target until synchronization to an accurate reference clock is
reached.
/var/lib/systemd/timesync/clock¶The modification time ("mtime") of this file is updated on each successful NTP
synchronization or after each SaveIntervalSec= time interval, as specified in
timesyncd.conf(5).
When initializing, the local clock is advanced to the modification time of this file (if the
file timestamp is in the past this adjustment is not made). If the file does not exist yet, the
clock is instead advanced to the modification time of /usr/lib/clock-epoch –
if it exists – or to a time derived from the source tree at build time. This mechanism is used to
ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across
reboots, in case no battery-buffered local RTC is available.
/usr/lib/clock-epoch¶The modification time ("mtime") of this file is used for advancing the system clock
in case /var/lib/systemd/timesync/clock does not exist yet, see
above.
/run/systemd/timesync/synchronized¶A file that is touched on each successful synchronization, to assist
systemd-time-wait-sync and other applications to detecting synchronization
with accurate reference clocks.