chiark / gitweb /
udevadm-settle: add missing brace
[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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9   Copyright 2010 Lennart Poettering
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24
25 <refentry id="machine-id">
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
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, 32-character,
61                 lowercase 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, which 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 that POSIX specifies.</para>
84
85                 <para>The
86                 <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
87                 tool may be used by installer tools to initialize the
88                 machine ID at install time.</para>
89         </refsect1>
90
91         <refsect1>
92                 <title>Relation to OSF UUIDs</title>
93
94                 <para>Note that the machine ID historically is not an
95                 OSF UUID as defined by <ulink
96                 url="https://tools.ietf.org/html/rfc4122">RFC
97                 4122</ulink>, nor a Microsoft GUID; however, starting with
98                 systemd v30, newly generated machine IDs do
99                 qualify as v4 UUIDs.</para>
100
101                 <para>In order to maintain compatibility with existing
102                 installations, an application requiring a UUID should
103                 decode the machine ID, and then apply the following
104                 operations to turn it into a valid OSF v4 UUID. With
105                 <literal>id</literal> being an unsigned character
106                 array:</para>
107
108                 <programlisting>/* Set UUID version to 4 --- truly random generation */
109 id[6] = (id[6] &amp; 0x0F) | 0x40;
110 /* Set the UUID variant to DCE */
111 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
112
113                 <para>(This code is inspired by
114                 <literal>generate_random_uuid()</literal> of
115                 <filename>drivers/char/random.c</filename> from the
116                 Linux kernel sources.)</para>
117
118         </refsect1>
119
120         <refsect1>
121                 <title>History</title>
122
123                 <para>The simple configuration file format of
124                 <filename>/etc/machine-id</filename> originates in the
125                 <filename>/var/lib/dbus/machine-id</filename> file
126                 introduced by D-Bus. In fact, this latter file might be a
127                 symlink to
128                 <varname>/etc/machine-id</varname>.</para>
129         </refsect1>
130
131         <refsect1>
132                   <title>See Also</title>
133                   <para>
134                           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
135                           <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
136                           <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
137                           <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
138                           <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
139                           <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
140                           <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
141                           <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
142                   </para>
143         </refsect1>
144
145 </refentry>