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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 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] & 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
<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>
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