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 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><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
    <varname>/etc/machine-id</varname>.</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><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>