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1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <?xml-stylesheet type="text/xsl" href="http://docbook.sourceforge.net/release/xsl/current/xhtml/docbook.xsl"?> | |
3 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
798d3a52 | 4 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
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5 | |
6 | <!-- | |
7 | This file is part of systemd. | |
8 | ||
9 | Copyright 2010 Lennart Poettering | |
10 | ||
11 | systemd is free software; you can redistribute it and/or modify it | |
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12 | under the terms of the GNU Lesser General Public License as published by |
13 | the Free Software Foundation; either version 2.1 of the License, or | |
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14 | (at your option) any later version. |
15 | ||
16 | systemd is distributed in the hope that it will be useful, but | |
17 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
18 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
5430f7f2 | 19 | Lesser General Public License for more details. |
d7ccca2e | 20 | |
5430f7f2 | 21 | You should have received a copy of the GNU Lesser General Public License |
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22 | along with systemd; If not, see <http://www.gnu.org/licenses/>. |
23 | --> | |
24 | ||
25 | <refentry id="machine-id"> | |
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26 | <refentryinfo> |
27 | <title>machine-id</title> | |
28 | <productname>systemd</productname> | |
29 | ||
30 | <authorgroup> | |
31 | <author> | |
32 | <contrib>Developer</contrib> | |
33 | <firstname>Lennart</firstname> | |
34 | <surname>Poettering</surname> | |
35 | <email>lennart@poettering.net</email> | |
36 | </author> | |
37 | </authorgroup> | |
38 | </refentryinfo> | |
39 | ||
40 | <refmeta> | |
41 | <refentrytitle>machine-id</refentrytitle> | |
42 | <manvolnum>5</manvolnum> | |
43 | </refmeta> | |
44 | ||
45 | <refnamediv> | |
46 | <refname>machine-id</refname> | |
47 | <refpurpose>Local machine ID configuration file</refpurpose> | |
48 | </refnamediv> | |
49 | ||
50 | <refsynopsisdiv> | |
51 | <para><filename>/etc/machine-id</filename></para> | |
52 | </refsynopsisdiv> | |
53 | ||
54 | <refsect1> | |
55 | <title>Description</title> | |
56 | ||
57 | <para>The <filename>/etc/machine-id</filename> file contains the | |
58 | unique machine ID of the local system that is set during | |
59 | installation. The machine ID is a single newline-terminated, | |
60 | hexadecimal, 32-character, lowercase machine ID string. When | |
61 | decoded from hexadecimal, this corresponds with a 16-byte/128-bit | |
62 | string.</para> | |
63 | ||
64 | <para>The machine ID is usually generated from a random source | |
65 | during system installation and stays constant for all subsequent | |
66 | boots. Optionally, for stateless systems, it is generated during | |
67 | runtime at boot if it is found to be empty.</para> | |
68 | ||
69 | <para>The machine ID does not change based on user configuration | |
70 | or when hardware is replaced.</para> | |
71 | ||
72 | <para>This machine ID adheres to the same format and logic as the | |
73 | D-Bus machine ID.</para> | |
74 | ||
75 | <para>Programs may use this ID to identify the host with a | |
76 | globally unique ID in the network, which does not change even if | |
77 | the local network configuration changes. Due to this and its | |
78 | greater length, it is a more useful replacement for the | |
79 | <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
80 | call that POSIX specifies.</para> | |
81 | ||
82 | <para>The | |
83 | <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
84 | tool may be used by installer tools to initialize the machine ID | |
85 | at install time. Use | |
86 | <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
87 | to initialize it on mounted (but not booted) system images.</para> | |
88 | </refsect1> | |
89 | ||
90 | <refsect1> | |
91 | <title>Relation to OSF UUIDs</title> | |
92 | ||
93 | <para>Note that the machine ID historically is not an OSF UUID as | |
94 | defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC | |
95 | 4122</ulink>, nor a Microsoft GUID; however, starting with systemd | |
96 | v30, newly generated machine IDs do qualify as v4 UUIDs.</para> | |
97 | ||
98 | <para>In order to maintain compatibility with existing | |
99 | installations, an application requiring a UUID should decode the | |
100 | machine ID, and then apply the following operations to turn it | |
101 | into a valid OSF v4 UUID. With <literal>id</literal> being an | |
102 | unsigned character array:</para> | |
103 | ||
104 | <programlisting>/* Set UUID version to 4 --- truly random generation */ | |
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105 | id[6] = (id[6] & 0x0F) | 0x40; |
106 | /* Set the UUID variant to DCE */ | |
107 | id[8] = (id[8] & 0x3F) | 0x80;</programlisting> | |
108 | ||
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109 | <para>(This code is inspired by |
110 | <literal>generate_random_uuid()</literal> of | |
111 | <filename>drivers/char/random.c</filename> from the Linux kernel | |
112 | sources.)</para> | |
113 | ||
114 | </refsect1> | |
115 | ||
116 | <refsect1> | |
117 | <title>History</title> | |
118 | ||
119 | <para>The simple configuration file format of | |
120 | <filename>/etc/machine-id</filename> originates in the | |
121 | <filename>/var/lib/dbus/machine-id</filename> file introduced by | |
122 | D-Bus. In fact, this latter file might be a symlink to | |
123 | <varname>/etc/machine-id</varname>.</para> | |
124 | </refsect1> | |
125 | ||
126 | <refsect1> | |
127 | <title>See Also</title> | |
128 | <para> | |
129 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
130 | <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
131 | <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
132 | <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
133 | <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
134 | <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
135 | <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
136 | <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
137 | <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
138 | </para> | |
139 | </refsect1> | |
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140 | |
141 | </refentry> |