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d7ccca2e | 1 | <?xml version='1.0'?> <!--*-nxml-*--> |
d7ccca2e | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
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4 | |
5 | <!-- | |
6 | This file is part of systemd. | |
7 | ||
8 | Copyright 2010 Lennart Poettering | |
9 | ||
10 | systemd is free software; you can redistribute it and/or modify it | |
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11 | under the terms of the GNU Lesser General Public License as published by |
12 | the Free Software Foundation; either version 2.1 of the License, or | |
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13 | (at your option) any later version. |
14 | ||
15 | systemd is distributed in the hope that it will be useful, but | |
16 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
17 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
5430f7f2 | 18 | Lesser General Public License for more details. |
d7ccca2e | 19 | |
5430f7f2 | 20 | You should have received a copy of the GNU Lesser General Public License |
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21 | along with systemd; If not, see <http://www.gnu.org/licenses/>. |
22 | --> | |
23 | ||
24 | <refentry id="machine-id"> | |
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25 | <refentryinfo> |
26 | <title>machine-id</title> | |
27 | <productname>systemd</productname> | |
28 | ||
29 | <authorgroup> | |
30 | <author> | |
31 | <contrib>Developer</contrib> | |
32 | <firstname>Lennart</firstname> | |
33 | <surname>Poettering</surname> | |
34 | <email>lennart@poettering.net</email> | |
35 | </author> | |
36 | </authorgroup> | |
37 | </refentryinfo> | |
38 | ||
39 | <refmeta> | |
40 | <refentrytitle>machine-id</refentrytitle> | |
41 | <manvolnum>5</manvolnum> | |
42 | </refmeta> | |
43 | ||
44 | <refnamediv> | |
45 | <refname>machine-id</refname> | |
46 | <refpurpose>Local machine ID configuration file</refpurpose> | |
47 | </refnamediv> | |
48 | ||
49 | <refsynopsisdiv> | |
50 | <para><filename>/etc/machine-id</filename></para> | |
51 | </refsynopsisdiv> | |
52 | ||
53 | <refsect1> | |
54 | <title>Description</title> | |
55 | ||
56 | <para>The <filename>/etc/machine-id</filename> file contains the | |
57 | unique machine ID of the local system that is set during | |
58 | installation. The machine ID is a single newline-terminated, | |
59 | hexadecimal, 32-character, lowercase machine ID string. When | |
60 | decoded from hexadecimal, this corresponds with a 16-byte/128-bit | |
61 | string.</para> | |
62 | ||
63 | <para>The machine ID is usually generated from a random source | |
64 | during system installation and stays constant for all subsequent | |
65 | boots. Optionally, for stateless systems, it is generated during | |
4a9b1dd4 | 66 | runtime at early boot if it is found to be empty.</para> |
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67 | |
68 | <para>The machine ID does not change based on user configuration | |
69 | or when hardware is replaced.</para> | |
70 | ||
71 | <para>This machine ID adheres to the same format and logic as the | |
72 | D-Bus machine ID.</para> | |
73 | ||
74 | <para>Programs may use this ID to identify the host with a | |
75 | globally unique ID in the network, which does not change even if | |
76 | the local network configuration changes. Due to this and its | |
77 | greater length, it is a more useful replacement for the | |
3ba3a79d | 78 | <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry> |
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79 | call that POSIX specifies.</para> |
80 | ||
81 | <para>The | |
82 | <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
83 | tool may be used by installer tools to initialize the machine ID | |
84 | at install time. Use | |
85 | <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
86 | to initialize it on mounted (but not booted) system images.</para> | |
87 | </refsect1> | |
88 | ||
89 | <refsect1> | |
90 | <title>Relation to OSF UUIDs</title> | |
91 | ||
92 | <para>Note that the machine ID historically is not an OSF UUID as | |
93 | defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC | |
94 | 4122</ulink>, nor a Microsoft GUID; however, starting with systemd | |
95 | v30, newly generated machine IDs do qualify as v4 UUIDs.</para> | |
96 | ||
97 | <para>In order to maintain compatibility with existing | |
98 | installations, an application requiring a UUID should decode the | |
99 | machine ID, and then apply the following operations to turn it | |
100 | into a valid OSF v4 UUID. With <literal>id</literal> being an | |
101 | unsigned character array:</para> | |
102 | ||
103 | <programlisting>/* Set UUID version to 4 --- truly random generation */ | |
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104 | id[6] = (id[6] & 0x0F) | 0x40; |
105 | /* Set the UUID variant to DCE */ | |
106 | id[8] = (id[8] & 0x3F) | 0x80;</programlisting> | |
107 | ||
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108 | <para>(This code is inspired by |
109 | <literal>generate_random_uuid()</literal> of | |
110 | <filename>drivers/char/random.c</filename> from the Linux kernel | |
111 | sources.)</para> | |
112 | ||
113 | </refsect1> | |
114 | ||
115 | <refsect1> | |
116 | <title>History</title> | |
117 | ||
118 | <para>The simple configuration file format of | |
119 | <filename>/etc/machine-id</filename> originates in the | |
120 | <filename>/var/lib/dbus/machine-id</filename> file introduced by | |
121 | D-Bus. In fact, this latter file might be a symlink to | |
22065311 | 122 | <filename>/etc/machine-id</filename>.</para> |
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123 | </refsect1> |
124 | ||
125 | <refsect1> | |
126 | <title>See Also</title> | |
127 | <para> | |
128 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
129 | <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
3ba3a79d | 130 | <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
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131 | <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
132 | <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
133 | <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
134 | <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
135 | <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
136 | <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
137 | </para> | |
138 | </refsect1> | |
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139 | |
140 | </refentry> |