systemd/man/machine-id.xml
2013-07-16 17:42:56 +02:00

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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>