man/machine-id.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2010 Lennart Poettering
  11 -->
  12
  13 <refentry id="machine-id">
  14   <refentryinfo>
  15     <title>machine-id</title>
  16     <productname>systemd</productname>
  17
  18     <authorgroup>
  19       <author>
  20         <contrib>Developer</contrib>
  21         <firstname>Lennart</firstname>
  22         <surname>Poettering</surname>
  23         <email>lennart@poettering.net</email>
  24       </author>
  25     </authorgroup>
  26   </refentryinfo>
  27
  28   <refmeta>
  29     <refentrytitle>machine-id</refentrytitle>
  30     <manvolnum>5</manvolnum>
  31   </refmeta>
  32
  33   <refnamediv>
  34     <refname>machine-id</refname>
  35     <refpurpose>Local machine ID configuration file</refpurpose>
  36   </refnamediv>
  37
  38   <refsynopsisdiv>
  39     <para><filename>/etc/machine-id</filename></para>
  40   </refsynopsisdiv>
  41
  42   <refsect1>
  43     <title>Description</title>
  44
  45     <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of
  46     the local system that is set during installation or boot. The machine ID is a single
  47     newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
  48     hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
  49     zeros.</para>
  50
  51     <para>The machine ID is usually generated from a random source during system
  52     installation or first boot and stays constant for all subsequent boots. Optionally,
  53     for stateless systems, it is generated during runtime during early boot if necessary.
  54     </para>
  55
  56     <para>The machine ID may be set, for example when network booting, with the
  57     <varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
  58     option <option>--machine-id=</option> to systemd. An ID is specified in this manner
  59     has higher priority and will be used instead of the ID stored in
  60     <filename>/etc/machine-id</filename>.</para>
  61
  62     <para>The machine ID does not change based on local or network configuration or when
  63     hardware is replaced. Due to this and its greater length, it is a more useful
  64     replacement for the
  65     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  66     call that POSIX specifies.</para>
  67
  68     <para>This machine ID adheres to the same format and logic as the
  69     D-Bus machine ID.</para>
  70
  71     <para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
  72     untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
  73     needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
  74     should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
  75     ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
  76     the original machine ID from the application-specific one. The
  77     <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  78     API provides an implementation of such an algorithm.</para>
  79   </refsect1>
  80
  81   <refsect1>
  82     <title>Initialization</title>
  83
  84     <para>Each machine should have a non-empty ID in normal operation. The ID of each
  85     machine should be unique. To achive those objectives,
  86     <filename>/etc/machine-id</filename> can be initialized in a few different ways.
  87     </para>
  88
  89     <para>For normal operating system installations, where a custom image is created for a
  90     specific machine, <filename>/etc/machine-id</filename> should be populated during
  91     installation.</para>
  92
  93     <para>
  94     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  95     may be used by installer tools to initialize the machine ID at install time, but
  96     <filename>/etc/machine-id</filename> may also be written using any other means.
  97     </para>
  98
  99     <para>For operating system images which are created once and used on multiple
 100     machines, for example for containers or in the cloud,
 101     <filename>/etc/machine-id</filename> should be an empty file in the generic file
 102     system image. An ID will be generated during boot and saved to this file if
 103     possible. Having an empty file in place is useful because it allows a temporary file
 104     to be bind-mounted over the real file, in case the image is used read-only.</para>
 105
 106     <para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 107     may be used to to initialize <filename>/etc/machine-id</filename> on mounted (but not
 108     booted) system images.</para>
 109
 110     <para>When a machine is booted with
 111     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 112     the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
 113     or <option>--machine-id=</option> options (see first section) are specified, this
 114     value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
 115     be used. If this file is empty or missing, <filename>systemd</filename> will attempt
 116     to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
 117     value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
 118     <filename>product_uuid</filename> (on KVM systems), and finally a randomly generated
 119     UUID.</para>
 120
 121     <para>After the machine ID is established,
 122     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 123     will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
 124     will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
 125     It is an error if the file system is read-only and does not contain a (possibly empty)
 126     <filename>/etc/machine-id</filename> file.</para>
 127
 128     <para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 129     will attempt to write the machine ID to the file system if
 130     <filename>/etc/machine-id</filename> or <filename>/etc</filename> are read-only during
 131     early boot but become writable later on.</para>
 132   </refsect1>
 133
 134   <refsect1>
 135     <title>Relation to OSF UUIDs</title>
 136
 137     <para>Note that the machine ID historically is not an OSF UUID as
 138     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
 139     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
 140     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
 141
 142     <para>In order to maintain compatibility with existing
 143     installations, an application requiring a UUID should decode the
 144     machine ID, and then apply the following operations to turn it
 145     into a valid OSF v4 UUID. With <literal>id</literal> being an
 146     unsigned character array:</para>
 147
 148     <programlisting>/* Set UUID version to 4 --- truly random generation */
 149 id[6] = (id[6] &amp; 0x0F) | 0x40;
 150 /* Set the UUID variant to DCE */
 151 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 152
 153     <para>(This code is inspired by
 154     <literal>generate_random_uuid()</literal> of
 155     <filename>drivers/char/random.c</filename> from the Linux kernel
 156     sources.)</para>
 157
 158   </refsect1>
 159
 160   <refsect1>
 161     <title>History</title>
 162
 163     <para>The simple configuration file format of
 164     <filename>/etc/machine-id</filename> originates in the
 165     <filename>/var/lib/dbus/machine-id</filename> file introduced by
 166     D-Bus. In fact, this latter file might be a symlink to
 167     <filename>/etc/machine-id</filename>.</para>
 168   </refsect1>
 169
 170   <refsect1>
 171       <title>See Also</title>
 172       <para>
 173         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 174         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 175         <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 176         <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 177         <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 178         <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 179         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 180         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 181         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 182       </para>
 183   </refsect1>
 184
 185 </refentry>