man/machine-id.xml

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  23
  24 <refentry id="machine-id">
  25   <refentryinfo>
  26     <title>machine-id</title>
  27     <productname>systemd</productname>
  28
  29     <authorgroup>
  30       <author>
  31         <contrib>Developer</contrib>
  32         <firstname>Lennart</firstname>
  33         <surname>Poettering</surname>
  34         <email>lennart@poettering.net</email>
  35       </author>
  36     </authorgroup>
  37   </refentryinfo>
  38
  39   <refmeta>
  40     <refentrytitle>machine-id</refentrytitle>
  41     <manvolnum>5</manvolnum>
  42   </refmeta>
  43
  44   <refnamediv>
  45     <refname>machine-id</refname>
  46     <refpurpose>Local machine ID configuration file</refpurpose>
  47   </refnamediv>
  48
  49   <refsynopsisdiv>
  50     <para><filename>/etc/machine-id</filename></para>
  51   </refsynopsisdiv>
  52
  53   <refsect1>
  54     <title>Description</title>
  55
  56     <para>The <filename>/etc/machine-id</filename> file contains the
  57     unique machine ID of the local system that is set during
  58     installation. The machine ID is a single newline-terminated,
  59     hexadecimal, 32-character, lowercase machine ID string. When
  60     decoded from hexadecimal, this corresponds with a 16-byte/128-bit
  61     string.</para>
  62
  63     <para>The machine ID is usually generated from a random source
  64     during system installation and stays constant for all subsequent
  65     boots. Optionally, for stateless systems, it is generated during
  66     runtime at early boot if it is found to be empty.</para>
  67
  68     <para>The machine ID does not change based on user configuration
  69     or when hardware is replaced.</para>
  70
  71     <para>This machine ID adheres to the same format and logic as the
  72     D-Bus machine ID.</para>
  73
  74     <para>Programs may use this ID to identify the host with a
  75     globally unique ID in the network, which does not change even if
  76     the local network configuration changes. Due to this and its
  77     greater length, it is a more useful replacement for the
  78     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  79     call that POSIX specifies.</para>
  80
  81     <para>The
  82     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  83     tool may be used by installer tools to initialize the machine ID
  84     at install time. Use
  85     <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  86     to initialize it on mounted (but not booted) system images.</para>
  87
  88     <para>The machine-id may also be set, for example when network
  89     booting, by setting the <varname>systemd.machine_id=</varname>
  90     kernel command line parameter or passing the option
  91     <option>--machine-id=</option> to systemd. A machine-id may not
  92     be set to all zeros.</para>
  93   </refsect1>
  94
  95   <refsect1>
  96     <title>Relation to OSF UUIDs</title>
  97
  98     <para>Note that the machine ID historically is not an OSF UUID as
  99     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
 100     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
 101     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
 102
 103     <para>In order to maintain compatibility with existing
 104     installations, an application requiring a UUID should decode the
 105     machine ID, and then apply the following operations to turn it
 106     into a valid OSF v4 UUID. With <literal>id</literal> being an
 107     unsigned character array:</para>
 108
 109     <programlisting>/* Set UUID version to 4 --- truly random generation */
 110 id[6] = (id[6] &amp; 0x0F) | 0x40;
 111 /* Set the UUID variant to DCE */
 112 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 113
 114     <para>(This code is inspired by
 115     <literal>generate_random_uuid()</literal> of
 116     <filename>drivers/char/random.c</filename> from the Linux kernel
 117     sources.)</para>
 118
 119   </refsect1>
 120
 121   <refsect1>
 122     <title>History</title>
 123
 124     <para>The simple configuration file format of
 125     <filename>/etc/machine-id</filename> originates in the
 126     <filename>/var/lib/dbus/machine-id</filename> file introduced by
 127     D-Bus. In fact, this latter file might be a symlink to
 128     <filename>/etc/machine-id</filename>.</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 project='man-pages'><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         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 143       </para>
 144   </refsect1>
 145
 146 </refentry>