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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<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 project='man-pages'><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. Use
<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
to initialize it on mounted (but not booted) system images.</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] & 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 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 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>
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