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 unique machine ID of the local
  57     system that is set during installation. The machine ID is a single newline-terminated,
  58     hexadecimal, 32-character, lowercase ID. When decoded from hexadecimal, this corresponds to a
  59     16-byte/128-bit value.</para>
  60
  61     <para>The machine ID is usually generated from a random source
  62     during system installation and stays constant for all subsequent
  63     boots. Optionally, for stateless systems, it is generated during
  64     runtime at early boot if it is found to be empty.</para>
  65
  66     <para>The machine ID does not change based on local or network configuration or when hardware is
  67     replaced. Due to this and its greater length, it is a more useful replacement for the
  68     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  69     call that POSIX specifies.</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>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
  75     untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
  76     needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
  77     should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
  78     ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
  79     the original machine ID from the application-specific one. The
  80     <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  81     API provides an implementation of such an algorithm.</para>
  82
  83     <para>The
  84     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  85     tool may be used by installer tools to initialize the machine ID
  86     at install time. Use
  87     <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  88     to initialize it on mounted (but not booted) system images.</para>
  89
  90     <para>The machine-id may also be set, for example when network
  91     booting, by setting the <varname>systemd.machine_id=</varname>
  92     kernel command line parameter or passing the option
  93     <option>--machine-id=</option> to systemd. A machine-id may not
  94     be set to all zeros.</para>
  95   </refsect1>
  96
  97   <refsect1>
  98     <title>Relation to OSF UUIDs</title>
  99
 100     <para>Note that the machine ID historically is not an OSF UUID as
 101     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
 102     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
 103     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
 104
 105     <para>In order to maintain compatibility with existing
 106     installations, an application requiring a UUID should decode the
 107     machine ID, and then apply the following operations to turn it
 108     into a valid OSF v4 UUID. With <literal>id</literal> being an
 109     unsigned character 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 Linux kernel
 119     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 introduced by
 129     D-Bus. In fact, this latter file might be a symlink to
 130     <filename>/etc/machine-id</filename>.</para>
 131   </refsect1>
 132
 133   <refsect1>
 134       <title>See Also</title>
 135       <para>
 136         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 137         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 138         <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 139         <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 140         <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 141         <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 142         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 143         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 144         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 145       </para>
 146   </refsect1>
 147
 148 </refentry>