man/machine-id.xml

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  11   Copyright 2010 Lennart Poettering
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  26
  27 <refentry id="machine-id">
  28   <refentryinfo>
  29     <title>machine-id</title>
  30     <productname>systemd</productname>
  31
  32     <authorgroup>
  33       <author>
  34         <contrib>Developer</contrib>
  35         <firstname>Lennart</firstname>
  36         <surname>Poettering</surname>
  37         <email>lennart@poettering.net</email>
  38       </author>
  39     </authorgroup>
  40   </refentryinfo>
  41
  42   <refmeta>
  43     <refentrytitle>machine-id</refentrytitle>
  44     <manvolnum>5</manvolnum>
  45   </refmeta>
  46
  47   <refnamediv>
  48     <refname>machine-id</refname>
  49     <refpurpose>Local machine ID configuration file</refpurpose>
  50   </refnamediv>
  51
  52   <refsynopsisdiv>
  53     <para><filename>/etc/machine-id</filename></para>
  54   </refsynopsisdiv>
  55
  56   <refsect1>
  57     <title>Description</title>
  58
  59     <para>The <filename>/etc/machine-id</filename> file contains the
  60     unique machine ID of the local system that is set during
  61     installation. The machine ID is a single newline-terminated,
  62     hexadecimal, 32-character, lowercase machine ID string. When
  63     decoded from hexadecimal, this corresponds with a 16-byte/128-bit
  64     string.</para>
  65
  66     <para>The machine ID is usually generated from a random source
  67     during system installation and stays constant for all subsequent
  68     boots. Optionally, for stateless systems, it is generated during
  69     runtime at boot if it is found to be empty.</para>
  70
  71     <para>The machine ID does not change based on user configuration
  72     or when hardware is replaced.</para>
  73
  74     <para>This machine ID adheres to the same format and logic as the
  75     D-Bus machine ID.</para>
  76
  77     <para>Programs may use this ID to identify the host with a
  78     globally unique ID in the network, which does not change even if
  79     the local network configuration changes. Due to this and its
  80     greater length, it is a more useful replacement for the
  81     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  82     call that POSIX specifies.</para>
  83
  84     <para>The
  85     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  86     tool may be used by installer tools to initialize the machine ID
  87     at install time. Use
  88     <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  89     to initialize it on mounted (but not booted) system images.</para>
  90   </refsect1>
  91
  92   <refsect1>
  93     <title>Relation to OSF UUIDs</title>
  94
  95     <para>Note that the machine ID historically is not an OSF UUID as
  96     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  97     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
  98     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
  99
 100     <para>In order to maintain compatibility with existing
 101     installations, an application requiring a UUID should decode the
 102     machine ID, and then apply the following operations to turn it
 103     into a valid OSF v4 UUID. With <literal>id</literal> being an
 104     unsigned character array:</para>
 105
 106     <programlisting>/* Set UUID version to 4 --- truly random generation */
 107 id[6] = (id[6] &amp; 0x0F) | 0x40;
 108 /* Set the UUID variant to DCE */
 109 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 110
 111     <para>(This code is inspired by
 112     <literal>generate_random_uuid()</literal> of
 113     <filename>drivers/char/random.c</filename> from the Linux kernel
 114     sources.)</para>
 115
 116   </refsect1>
 117
 118   <refsect1>
 119     <title>History</title>
 120
 121     <para>The simple configuration file format of
 122     <filename>/etc/machine-id</filename> originates in the
 123     <filename>/var/lib/dbus/machine-id</filename> file introduced by
 124     D-Bus. In fact, this latter file might be a symlink to
 125     <varname>/etc/machine-id</varname>.</para>
 126   </refsect1>
 127
 128   <refsect1>
 129       <title>See Also</title>
 130       <para>
 131         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 132         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 133         <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 134         <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 135         <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 136         <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 137         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 138         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 139         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 140       </para>
 141   </refsect1>
 142
 143 </refentry>