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[thirdparty/systemd.git] / man / systemd-timesyncd.service.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<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>
        </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></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>
        </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>