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   </refsect1>
  88
  89   <refsect1>
  90     <title>Relation to OSF UUIDs</title>
  91
  92     <para>Note that the machine ID historically is not an OSF UUID as
  93     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  94     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
  95     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
  96
  97     <para>In order to maintain compatibility with existing
  98     installations, an application requiring a UUID should decode the
  99     machine ID, and then apply the following operations to turn it
 100     into a valid OSF v4 UUID. With <literal>id</literal> being an
 101     unsigned character 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 Linux kernel
 111     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 introduced by
 121     D-Bus. In fact, this latter file might be a symlink to
 122     <filename>/etc/machine-id</filename>.</para>
 123   </refsect1>
 124
 125   <refsect1>
 126       <title>See Also</title>
 127       <para>
 128         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 129         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 130         <citerefentry project='man-pages'><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         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 135         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 136         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 137       </para>
 138   </refsect1>
 139
 140 </refentry>