systemd/man/machine-id.xml

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<refentry id="machine-id">
  <refentryinfo>
    <title>machine-id</title>
    <productname>systemd</productname>
  </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 or boot. The machine ID is a single
    newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
    hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
    zeros.</para>

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

    <para>The machine ID may be set, for example when network booting, with the
    <varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
    option <option>--machine-id=</option> to systemd. An ID specified in this manner
    has higher priority and will be used instead of the ID stored in
    <filename>/etc/machine-id</filename>.</para>

    <para>The machine ID does not change based on local or network configuration or when
    hardware is replaced. 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>This machine ID adheres to the same format and logic as the
    D-Bus machine ID.</para>

    <para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
    untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
    needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
    should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
    ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
    the original machine ID from the application-specific one. The
    <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    API provides an implementation of such an algorithm.</para>
  </refsect1>

  <refsect1>
    <title>Initialization</title>

    <para>Each machine should have a non-empty ID in normal operation. The ID of each
    machine should be unique. To achieve those objectives,
    <filename>/etc/machine-id</filename> can be initialized in a few different ways.
    </para>

    <para>For normal operating system installations, where a custom image is created for a
    specific machine, <filename>/etc/machine-id</filename> should be populated during
    installation.</para>

    <para>
    <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    may be used by installer tools to initialize the machine ID at install time, but
    <filename>/etc/machine-id</filename> may also be written using any other means.
    </para>

    <para>For operating system images which are created once and used on multiple
    machines, for example for containers or in the cloud,
    <filename>/etc/machine-id</filename> should be either missing or an empty file in the generic file
    system image (the difference between the two options is described under "First Boot Semantics" below). An
    ID will be generated during boot and saved to this file if possible. Having an empty file in place is
    useful because it allows a temporary file to be bind-mounted over the real file, in case the image is
    used read-only.</para>

    <para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    may be used to initialize <filename>/etc/machine-id</filename> on mounted (but not
    booted) system images.</para>

    <para>When a machine is booted with
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
    or <option>--machine-id=</option> options (see first section) are specified, this
    value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
    be used. If this file is empty or missing, <filename>systemd</filename> will attempt
    to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
    value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
    <filename>product_uuid</filename> or the devicetree <filename>vm,uuid</filename>
    (on KVM systems), and finally a randomly generated UUID.</para>

    <para>After the machine ID is established,
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
    will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
    It is an error if the file system is read-only and does not contain a (possibly empty)
    <filename>/etc/machine-id</filename> file.</para>

    <para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    will attempt to write the machine ID to the file system if
    <filename>/etc/machine-id</filename> or <filename>/etc/</filename> are read-only during
    early boot but become writable later on.</para>
  </refsect1>

  <refsect1>
    <title>First Boot Semantics</title>

    <para><filename>/etc/machine-id</filename> is used to decide whether a boot is the first one.  The rules
    are as follows:</para>

    <orderedlist>
      <listitem><para>If <filename>/etc/machine-id</filename> does not exist, this is a first boot.  During
      early boot, <command>systemd</command> will write <literal>uninitialized\n</literal> to this file and overmount
      a temporary file which contains the actual machine ID.  Later (after <filename>first-boot-complete.target</filename>
      has been reached), the real machine ID will be written to disk.</para></listitem>

      <listitem><para>If <filename>/etc/machine-id</filename> contains the string <literal>uninitialized</literal>,
      a boot is also considered the first boot.  The same mechanism as above applies.</para></listitem>

      <listitem><para>If <filename>/etc/machine-id</filename> exists and is empty, a boot is
      <emphasis>not</emphasis> considered the first boot.  <command>systemd</command> will still bind-mount a file
      containing the actual machine-id over it and later try to commit it to disk (if <filename>/etc/</filename> is
      writable).</para></listitem>

      <listitem><para>If <filename>/etc/machine-id</filename> already contains a valid machine-id, this is
      not a first boot.</para></listitem>
    </orderedlist>

    <para>If by any of the above rules, a first boot is detected, units with <varname>ConditionFirstBoot=yes</varname>
    will be run.</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 Variant 1 Version 4 UUIDs, as per RFC 4122.</para>

    <para>In order to maintain compatibility with existing installations, an application requiring a strictly
    RFC 4122 compliant UUID should decode the machine ID, and then (non-reversibly) apply the following
    operations to turn it into a valid RFC 4122 Variant 1 Version 4 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>