systemd/man/machine-id.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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<refentry id="machine-id">
  <refentryinfo>
    <title>machine-id</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>machine-id</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>machine-id</refname>
    <refpurpose>Local machine ID configuration file</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>/etc/machine-id</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>The <filename>/etc/machine-id</filename> file contains the
    unique machine ID of the local system that is set during
    installation. The machine ID is a single newline-terminated,
    hexadecimal, 32-character, lowercase machine ID string. When
    decoded from hexadecimal, this corresponds with a 16-byte/128-bit
    string.</para>

    <para>The machine ID is usually generated from a random source
    during system installation and stays constant for all subsequent
    boots. Optionally, for stateless systems, it is generated during
    runtime at early boot if it is found to be empty.</para>

    <para>The machine ID does not change based on user configuration
    or when hardware is replaced.</para>

    <para>This machine ID adheres to the same format and logic as the
    D-Bus machine ID.</para>

    <para>Programs may use this ID to identify the host with a
    globally unique ID in the network, which does not change even if
    the local network configuration changes. Due to this and its
    greater length, it is a more useful replacement for the
    <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    call that POSIX specifies.</para>

    <para>The
    <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    tool may be used by installer tools to initialize the machine ID
    at install time. Use
    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    to initialize it on mounted (but not booted) system images.</para>
  </refsect1>

  <refsect1>
    <title>Relation to OSF UUIDs</title>

    <para>Note that the machine ID historically is not an OSF UUID as
    defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
    4122</ulink>, nor a Microsoft GUID; however, starting with systemd
    v30, newly generated machine IDs do qualify as v4 UUIDs.</para>

    <para>In order to maintain compatibility with existing
    installations, an application requiring a UUID should decode the
    machine ID, and then apply the following operations to turn it
    into a valid OSF v4 UUID. With <literal>id</literal> being an
    unsigned character array:</para>

    <programlisting>/* Set UUID version to 4 --- truly random generation */
id[6] = (id[6] &amp; 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>

    <para>(This code is inspired by
    <literal>generate_random_uuid()</literal> of
    <filename>drivers/char/random.c</filename> from the Linux kernel
    sources.)</para>

  </refsect1>

  <refsect1>
    <title>History</title>

    <para>The simple configuration file format of
    <filename>/etc/machine-id</filename> originates in the
    <filename>/var/lib/dbus/machine-id</filename> file introduced by
    D-Bus. In fact, this latter file might be a symlink to
    <filename>/etc/machine-id</filename>.</para>
  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para>
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>