man/machine-id.xml

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   3 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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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>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>