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authorLuke Shumaker <lukeshu@sbcglobal.net>2016-09-13 21:24:07 -0400
committerLuke Shumaker <lukeshu@sbcglobal.net>2016-09-13 21:24:07 -0400
commitce31ec116f9bf4ad45952b6a200f4181fac311a3 (patch)
tree95bc9042a11e7f4773f10d72745396c9f46ed0ef /src/manpages/machine-id.xml
parentc73c7c774cbd1f0e778254d51da819490a333ab4 (diff)
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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 early 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>
+
+ <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] &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
+ <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>