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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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Copyright 2010 Lennart Poettering
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<refentry id= "machine-id" >
<refentryinfo >
<title > /etc/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>
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<refpurpose > Local machine ID configuration file</refpurpose>
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</refnamediv>
<refsynopsisdiv >
<para > <filename > /etc/machine-id</filename> </para>
</refsynopsisdiv>
<refsect1 >
<title > Description</title>
<para > The <filename > /etc/machine-id</filename> file
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contains the unique machine id of the local system
that is set during installation. The machine ID is a
single newline-terminated, hexadecimal, lowercase 32
character machine ID string. (When decoded from
hexadecimal this corresponds with a 16 byte/128 bit
string.)</para>
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<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
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boot if it is found to be empty.</para>
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<para > The machine ID does not change based on user
configuration, or when hardware is replaced.</para>
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<para > This machine ID adheres to the same format and
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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, that does
not change even if the local network configuration
changes. Due to this and its greater length it is
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a more useful replacement for the
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<citerefentry > <refentrytitle > gethostid</refentrytitle> <manvolnum > 3</manvolnum> </citerefentry>
call POSIX specifies.</para>
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<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>
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</refsect1>
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<refsect1 >
<title > Relation to OSF UUIDs</title>
<para > Note that the machine ID historically is not an
OSF UUID as defined by <ulink
url="http://tools.ietf.org/html/rfc4122">RFC
4122</ulink> , nor a Microsoft GUID. Starting with
systemd v30 newly generated machine IDs however 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] & 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id[8] = (id[8] & 0x3F) | 0x80;</programlisting>
<para > (This code is inspired by
<literal > generate_random_uuid()</literal> of
<filename > drivers/char/random.c</filename> from the
kernel sources.)</para>
</refsect1>
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<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
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symlink to
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<varname > /etc/machine-id</varname> .</para>
</refsect1>
<refsect1 >
<title > See Also</title>
<para >
<citerefentry > <refentrytitle > systemd</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > systemd-machine-id-setup</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > gethostid</refentrytitle> <manvolnum > 3</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > hostname</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> ,
<citerefentry > <refentrytitle > machine-info</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > os-release</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > sd-id128</refentrytitle> <manvolnum > 3</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > sd_id128_get_machine</refentrytitle> <manvolnum > 3</manvolnum> </citerefentry>
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</para>
</refsect1>
</refentry>