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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="systemd-timesyncd.service" conditional='ENABLE_TIMESYNCD'
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>systemd-timesyncd.service</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-timesyncd.service</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-timesyncd.service</refname>
<refname>systemd-timesyncd</refname>
<refpurpose>Network Time Synchronization</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>systemd-timesyncd.service</filename></para>
<para><filename>/usr/lib/systemd/systemd-timesyncd</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><filename>systemd-timesyncd</filename> 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.</para>
<para>The <filename>systemd-timesyncd</filename> 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
<filename>systemd-timesyncd</filename>.</para>
<para>The NTP servers contacted are determined from the global settings in
<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, the
per-link static settings in <filename>.network</filename> files, and the per-link dynamic settings
received over DHCP. See
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
further details.</para>
<para><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<command>set-ntp</command> command may be used to enable and start, or disable and stop this
service.</para>
<para><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<command>timesync-status</command> or <command>show-timesync</command> command can be used to show the
current status of this service.</para>
<para><filename>systemd-timesyncd</filename> initialization delays the start of units that are ordered
after <filename>time-set.target</filename> (see
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
details) until the local time has been updated from <filename>/var/lib/systemd/timesync/clock</filename>
(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
<citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
to achieve that, which will delay start of units that are ordered after
<filename>time-sync.target</filename> until synchronization to an accurate reference clock is
reached.</para>
</refsect1>
<refsect1>
<title>Files</title>
<variablelist>
<varlistentry>
<term><filename>/var/lib/systemd/timesync/clock</filename></term>
<listitem>
<para>The modification time ("mtime") of this file is updated on each successful NTP
synchronization or after each <varname>SaveIntervalSec=</varname> time interval, as specified in
<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
<para>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 <filename>/usr/lib/clock-epoch</filename> –
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.</para>
<xi:include href="version-info.xml" xpointer="v219"/>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>/usr/lib/clock-epoch</filename></term>
<listitem><para>The modification time ("mtime") of this file is used for advancing the system clock
in case <filename>/var/lib/systemd/timesync/clock</filename> does not exist yet, see
above.</para>
<xi:include href="version-info.xml" xpointer="v254"/></listitem>
</varlistentry>
<varlistentry>
<term><filename>/run/systemd/timesync/synchronized</filename></term>
<listitem>
<para>A file that is touched on each successful synchronization, to assist
<filename>systemd-time-wait-sync</filename> and other applications to detecting synchronization
with accurate reference clocks.</para>
<xi:include href="version-info.xml" xpointer="v239"/>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>localtime</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>hwclock</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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