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. Use
  89                 <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  90                 to initialize it on mounted (but not booted) system
  91                 images.</para>
  92         </refsect1>
  93
  94         <refsect1>
  95                 <title>Relation to OSF UUIDs</title>
  96
  97                 <para>Note that the machine ID historically is not an
  98                 OSF UUID as defined by <ulink
  99                 url="https://tools.ietf.org/html/rfc4122">RFC
 100                 4122</ulink>, nor a Microsoft GUID; however, starting with
 101                 systemd v30, newly generated machine IDs do
 102                 qualify as v4 UUIDs.</para>
 103
 104                 <para>In order to maintain compatibility with existing
 105                 installations, an application requiring a UUID should
 106                 decode the machine ID, and then apply the following
 107                 operations to turn it into a valid OSF v4 UUID. With
 108                 <literal>id</literal> being an unsigned character
 109                 array:</para>
 110
 111                 <programlisting>/* Set UUID version to 4 --- truly random generation */
 112 id[6] = (id[6] &amp; 0x0F) | 0x40;
 113 /* Set the UUID variant to DCE */
 114 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 115
 116                 <para>(This code is inspired by
 117                 <literal>generate_random_uuid()</literal> of
 118                 <filename>drivers/char/random.c</filename> from the
 119                 Linux kernel sources.)</para>
 120
 121         </refsect1>
 122
 123         <refsect1>
 124                 <title>History</title>
 125
 126                 <para>The simple configuration file format of
 127                 <filename>/etc/machine-id</filename> originates in the
 128                 <filename>/var/lib/dbus/machine-id</filename> file
 129                 introduced by D-Bus. In fact, this latter file might be a
 130                 symlink to
 131                 <varname>/etc/machine-id</varname>.</para>
 132         </refsect1>
 133
 134         <refsect1>
 135                   <title>See Also</title>
 136                   <para>
 137                           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 138                           <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 139                           <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 140                           <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 141                           <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 142                           <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 143                           <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 144                           <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 145                           <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 146                   </para>
 147         </refsect1>
 148
 149 </refentry>