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headers: fix git URLs for source files
[elogind.git] / man / machine-id.xml
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"
4         "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
5
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7   This file is part of systemd.
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9   Copyright 2010 Lennart Poettering
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24
25 <refentry id="machine-id">
26         <refentryinfo>
27                 <title>/etc/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
58                 contains the unique machine id of the local system
59                 that is set during installation. The machine ID is a
60                 single newline-terminated, hexadecimal, lowercase 32
61                 character machine ID string. (When decoded from
62                 hexadecimal this corresponds with a 16 byte/128 bit
63                 string.)</para>
64
65                 <para>The machine ID is usually generated from a
66                 random source during system installation and stays
67                 constant for all subsequent boots. Optionally, for
68                 stateless systems it is generated during runtime at
69                 boot if it is found to be empty.</para>
70
71                 <para>The machine ID does not change based on user
72                 configuration, or when hardware is replaced.</para>
73
74                 <para>This machine ID adheres to the same format and
75                 logic as the D-Bus machine ID.</para>
76
77                 <para>Programs may use this ID to identify the host
78                 with a globally unique ID in the network, that does
79                 not change even if the local network configuration
80                 changes. Due to this and its greater length it is
81                 a more useful replacement for the
82                 <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
83                 call POSIX specifies.</para>
84         </refsect1>
85
86         <refsect1>
87                 <title>Relation to OSF UUIDs</title>
88
89                 <para>Note that the machine ID historically is not an
90                 OSF UUID as defined by <ulink
91                 url="http://tools.ietf.org/html/rfc4122">RFC
92                 4122</ulink>, nor a Microsoft GUID. Starting with
93                 systemd v30 newly generated machine IDs however do
94                 qualify as v4 UUIDs.</para>
95
96                 <para>In order to maintain compatibility with existing
97                 installations, an application requiring a UUID should
98                 decode the machine ID, and then apply the following
99                 operations to turn it into a valid OSF v4 UUID. With
100                 <literal>id</literal> being an unsigned character
101                 array:</para>
102
103                 <programlisting>/* Set UUID version to 4 --- truly random generation */
104 id[6] = (id[6] &amp; 0x0F) | 0x40;
105 /* Set the UUID variant to DCE */
106 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
107
108                 <para>(This code is inspired by
109                 <literal>generate_random_uuid()</literal> of
110                 <filename>drivers/char/random.c</filename> from the
111                 kernel 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
121                 introduced by D-Bus. In fact this latter file might be a
122                 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>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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                   </para>
135         </refsect1>
136
137 </refentry>