man/machine-id.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   Copyright 2010 Lennart Poettering
   9 -->
  10
  11 <refentry id="machine-id">
  12   <refentryinfo>
  13     <title>machine-id</title>
  14     <productname>systemd</productname>
  15
  16     <authorgroup>
  17       <author>
  18         <contrib>Developer</contrib>
  19         <firstname>Lennart</firstname>
  20         <surname>Poettering</surname>
  21         <email>lennart@poettering.net</email>
  22       </author>
  23     </authorgroup>
  24   </refentryinfo>
  25
  26   <refmeta>
  27     <refentrytitle>machine-id</refentrytitle>
  28     <manvolnum>5</manvolnum>
  29   </refmeta>
  30
  31   <refnamediv>
  32     <refname>machine-id</refname>
  33     <refpurpose>Local machine ID configuration file</refpurpose>
  34   </refnamediv>
  35
  36   <refsynopsisdiv>
  37     <para><filename>/etc/machine-id</filename></para>
  38   </refsynopsisdiv>
  39
  40   <refsect1>
  41     <title>Description</title>
  42
  43     <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of
  44     the local system that is set during installation or boot. The machine ID is a single
  45     newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
  46     hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
  47     zeros.</para>
  48
  49     <para>The machine ID is usually generated from a random source during system
  50     installation or first boot and stays constant for all subsequent boots. Optionally,
  51     for stateless systems, it is generated during runtime during early boot if necessary.
  52     </para>
  53
  54     <para>The machine ID may be set, for example when network booting, with the
  55     <varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
  56     option <option>--machine-id=</option> to systemd. An ID is specified in this manner
  57     has higher priority and will be used instead of the ID stored in
  58     <filename>/etc/machine-id</filename>.</para>
  59
  60     <para>The machine ID does not change based on local or network configuration or when
  61     hardware is replaced. Due to this and its greater length, it is a more useful
  62     replacement for the
  63     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  64     call that POSIX specifies.</para>
  65
  66     <para>This machine ID adheres to the same format and logic as the
  67     D-Bus machine ID.</para>
  68
  69     <para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
  70     untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
  71     needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
  72     should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
  73     ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
  74     the original machine ID from the application-specific one. The
  75     <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  76     API provides an implementation of such an algorithm.</para>
  77   </refsect1>
  78
  79   <refsect1>
  80     <title>Initialization</title>
  81
  82     <para>Each machine should have a non-empty ID in normal operation. The ID of each
  83     machine should be unique. To achieve those objectives,
  84     <filename>/etc/machine-id</filename> can be initialized in a few different ways.
  85     </para>
  86
  87     <para>For normal operating system installations, where a custom image is created for a
  88     specific machine, <filename>/etc/machine-id</filename> should be populated during
  89     installation.</para>
  90
  91     <para>
  92     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  93     may be used by installer tools to initialize the machine ID at install time, but
  94     <filename>/etc/machine-id</filename> may also be written using any other means.
  95     </para>
  96
  97     <para>For operating system images which are created once and used on multiple
  98     machines, for example for containers or in the cloud,
  99     <filename>/etc/machine-id</filename> should be an empty file in the generic file
 100     system image. An ID will be generated during boot and saved to this file if
 101     possible. Having an empty file in place is useful because it allows a temporary file
 102     to be bind-mounted over the real file, in case the image is used read-only.</para>
 103
 104     <para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 105     may be used to initialize <filename>/etc/machine-id</filename> on mounted (but not
 106     booted) system images.</para>
 107
 108     <para>When a machine is booted with
 109     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 110     the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
 111     or <option>--machine-id=</option> options (see first section) are specified, this
 112     value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
 113     be used. If this file is empty or missing, <filename>systemd</filename> will attempt
 114     to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
 115     value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
 116     <filename>product_uuid</filename> (on KVM systems), and finally a randomly generated
 117     UUID.</para>
 118
 119     <para>After the machine ID is established,
 120     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 121     will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
 122     will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
 123     It is an error if the file system is read-only and does not contain a (possibly empty)
 124     <filename>/etc/machine-id</filename> file.</para>
 125
 126     <para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 127     will attempt to write the machine ID to the file system if
 128     <filename>/etc/machine-id</filename> or <filename>/etc</filename> are read-only during
 129     early boot but become writable later on.</para>
 130   </refsect1>
 131
 132   <refsect1>
 133     <title>Relation to OSF UUIDs</title>
 134
 135     <para>Note that the machine ID historically is not an OSF UUID as
 136     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
 137     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
 138     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
 139
 140     <para>In order to maintain compatibility with existing
 141     installations, an application requiring a UUID should decode the
 142     machine ID, and then apply the following operations to turn it
 143     into a valid OSF v4 UUID. With <literal>id</literal> being an
 144     unsigned character array:</para>
 145
 146     <programlisting>/* Set UUID version to 4 --- truly random generation */
 147 id[6] = (id[6] &amp; 0x0F) | 0x40;
 148 /* Set the UUID variant to DCE */
 149 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 150
 151     <para>(This code is inspired by
 152     <literal>generate_random_uuid()</literal> of
 153     <filename>drivers/char/random.c</filename> from the Linux kernel
 154     sources.)</para>
 155
 156   </refsect1>
 157
 158   <refsect1>
 159     <title>History</title>
 160
 161     <para>The simple configuration file format of
 162     <filename>/etc/machine-id</filename> originates in the
 163     <filename>/var/lib/dbus/machine-id</filename> file introduced by
 164     D-Bus. In fact, this latter file might be a symlink to
 165     <filename>/etc/machine-id</filename>.</para>
 166   </refsect1>
 167
 168   <refsect1>
 169       <title>See Also</title>
 170       <para>
 171         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 172         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 173         <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 174         <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 175         <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 176         <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 177         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 178         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 179         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 180       </para>
 181   </refsect1>
 182
 183 </refentry>