man/machine-id.xml

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