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 ID. When decoded from hexadecimal, this corresponds to a
    16-byte/128-bit value.</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 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>

    <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>

    <para>The machine-id may also be set, for example when network
    booting, by setting the <varname>systemd.machine_id=</varname>
    kernel command line parameter or passing the option
    <option>--machine-id=</option> to systemd. A machine-id may not
    be set to all zeros.</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>