systemd/man/machine-id.xml

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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.</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>
                  </para>
        </refsect1>

</refentry>