mirror of
https://github.com/systemd/systemd
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135 lines
6.8 KiB
XML
135 lines
6.8 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-timesyncd.service" conditional='ENABLE_TIMESYNCD'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-timesyncd.service</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-timesyncd.service</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-timesyncd.service</refname>
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<refname>systemd-timesyncd</refname>
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<refpurpose>Network Time Synchronization</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-timesyncd.service</filename></para>
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<para><filename>/usr/lib/systemd/systemd-timesyncd</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-timesyncd</filename> is a system service that may be used to synchronize the
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local system clock with a remote Network Time Protocol (NTP) server. It also saves the local time to disk
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every time the clock has been synchronized and uses this to possibly advance the system realtime clock on
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subsequent reboots to ensure it (roughly) monotonically advances even if the system lacks a
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battery-buffered RTC chip.</para>
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<para>The <filename>systemd-timesyncd</filename> service implements SNTP only. This minimalistic service
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will step the system clock for large offsets or slowly adjust it for smaller deltas. Complex use cases
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that require full NTP support (and where SNTP is not sufficient) are not covered by
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<filename>systemd-timesyncd</filename>.</para>
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<para>The NTP servers contacted are determined from the global settings in
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<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, the
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per-link static settings in <filename>.network</filename> files, and the per-link dynamic settings
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received over DHCP. See
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<citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
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further details.</para>
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<para><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
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<command>set-ntp</command> command may be used to enable and start, or disable and stop this
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service.</para>
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<para><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
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<command>timesync-status</command> or <command>show-timesync</command> command can be used to show the
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current status of this service.</para>
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<para><filename>systemd-timesyncd</filename> initialization delays the start of units that are ordered
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after <filename>time-set.target</filename> (see
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
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details) until the local time has been updated from <filename>/var/lib/systemd/timesync/clock</filename>
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(see below) in order to make it roughly monotonic. It does not delay other units until synchronization
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with an accurate reference time sources has been reached. Use
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<citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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to achieve that, which will delay start of units that are ordered after
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<filename>time-sync.target</filename> until synchronization to an accurate reference clock is
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reached.</para>
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</refsect1>
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<refsect1>
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<title>Files</title>
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<variablelist>
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<varlistentry>
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<term><filename>/var/lib/systemd/timesync/clock</filename></term>
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<listitem>
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<para>The modification time ("mtime") of this file is updated on each successful NTP
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synchronization or after each <varname>SaveIntervalSec=</varname> time interval, as specified in
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<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
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<para>When initializing, the local clock is advanced to the modification time of this file (if the
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file timestamp is in the past this adjustment is not made). If the file does not exist yet, the
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clock is instead advanced to the modification time of <filename>/usr/lib/clock-epoch</filename> –
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if it exists – or to a time derived from the source tree at build time. This mechanism is used to
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ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across
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reboots, in case no battery-buffered local RTC is available.</para>
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<xi:include href="version-info.xml" xpointer="v219"/>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><filename>/usr/lib/clock-epoch</filename></term>
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<listitem><para>The modification time ("mtime") of this file is used for advancing the system clock
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in case <filename>/var/lib/systemd/timesync/clock</filename> does not exist yet, see
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above.</para>
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<xi:include href="version-info.xml" xpointer="v254"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><filename>/run/systemd/timesync/synchronized</filename></term>
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<listitem>
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<para>A file that is touched on each successful synchronization, to assist
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<filename>systemd-time-wait-sync</filename> and other applications to detecting synchronization
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with accurate reference clocks.</para>
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<xi:include href="version-info.xml" xpointer="v239"/>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>timedatectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>localtime</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry project='man-pages'><refentrytitle>hwclock</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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