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>/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>