man/machine-id.xml

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  10   Copyright 2010 Lennart Poettering
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  25
  26 <refentry id="machine-id">
  27   <refentryinfo>
  28     <title>machine-id</title>
  29     <productname>systemd</productname>
  30
  31     <authorgroup>
  32       <author>
  33         <contrib>Developer</contrib>
  34         <firstname>Lennart</firstname>
  35         <surname>Poettering</surname>
  36         <email>lennart@poettering.net</email>
  37       </author>
  38     </authorgroup>
  39   </refentryinfo>
  40
  41   <refmeta>
  42     <refentrytitle>machine-id</refentrytitle>
  43     <manvolnum>5</manvolnum>
  44   </refmeta>
  45
  46   <refnamediv>
  47     <refname>machine-id</refname>
  48     <refpurpose>Local machine ID configuration file</refpurpose>
  49   </refnamediv>
  50
  51   <refsynopsisdiv>
  52     <para><filename>/etc/machine-id</filename></para>
  53   </refsynopsisdiv>
  54
  55   <refsect1>
  56     <title>Description</title>
  57
  58     <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of
  59     the local system that is set during installation or boot. The machine ID is a single
  60     newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
  61     hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
  62     zeros.</para>
  63
  64     <para>The machine ID is usually generated from a random source during system
  65     installation or first boot and stays constant for all subsequent boots. Optionally,
  66     for stateless systems, it is generated during runtime during early boot if necessary.
  67     </para>
  68
  69     <para>The machine ID may be set, for example when network booting, with the
  70     <varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
  71     option <option>--machine-id=</option> to systemd. An ID is specified in this manner
  72     has higher priority and will be used instead of the ID stored in
  73     <filename>/etc/machine-id</filename>.</para>
  74
  75     <para>The machine ID does not change based on local or network configuration or when
  76     hardware is replaced. Due to this and its greater length, it is a more useful
  77     replacement for the
  78     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  79     call that POSIX specifies.</para>
  80
  81     <para>This machine ID adheres to the same format and logic as the
  82     D-Bus machine ID.</para>
  83
  84     <para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
  85     untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
  86     needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
  87     should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
  88     ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
  89     the original machine ID from the application-specific one. The
  90     <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  91     API provides an implementation of such an algorithm.</para>
  92   </refsect1>
  93
  94   <refsect1>
  95     <title>Initialization</title>
  96
  97     <para>Each machine should have a non-empty ID in normal operation. The ID of each
  98     machine should be unique. To achive those objectives,
  99     <filename>/etc/machine-id</filename> can be initialized in a few different ways.
 100     </para>
 101
 102     <para>For normal operating system installations, where a custom image is created for a
 103     specific machine, <filename>/etc/machine-id</filename> should be populated during
 104     installation.</para>
 105
 106     <para>
 107     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 108     may be used by installer tools to initialize the machine ID at install time, but
 109     <filename>/etc/machine-id</filename> may also be written using any other means.
 110     </para>
 111
 112     <para>For operating system images which are created once and used on multiple
 113     machines, for example for containers or in the cloud,
 114     <filename>/etc/machine-id</filename> should be an empty file in the generic file
 115     system image. An ID will be generated during boot and saved to this file if
 116     possible. Having an empty file in place is useful because it allows a temporary file
 117     to be bind-mounted over the real file, in case the image is used read-only.</para>
 118
 119     <para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 120     may be used to to initialize <filename>/etc/machine-id</filename> on mounted (but not
 121     booted) system images.</para>
 122
 123     <para>When a machine is booted with
 124     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 125     the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
 126     or <option>--machine-id=</option> options (see first section) are specified, this
 127     value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
 128     be used. If this file is empty or missing, <filename>systemd</filename> will attempt
 129     to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
 130     value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
 131     <filename>product_uuid</filename> (on KVM systems), and finally a randomly generated
 132     UUID.</para>
 133
 134     <para>After the machine ID is established,
 135     <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 136     will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
 137     will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
 138     It is an error if the file system is read-only and does not contain a (possibly empty)
 139     <filename>/etc/machine-id</filename> file.</para>
 140
 141     <para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 142     will attempt to write the machine ID to the file system if
 143     <filename>/etc/machine-id</filename> or <filename>/etc</filename> are read-only during
 144     early boot but become writable later on.</para>
 145   </refsect1>
 146
 147   <refsect1>
 148     <title>Relation to OSF UUIDs</title>
 149
 150     <para>Note that the machine ID historically is not an OSF UUID as
 151     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
 152     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
 153     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
 154
 155     <para>In order to maintain compatibility with existing
 156     installations, an application requiring a UUID should decode the
 157     machine ID, and then apply the following operations to turn it
 158     into a valid OSF v4 UUID. With <literal>id</literal> being an
 159     unsigned character array:</para>
 160
 161     <programlisting>/* Set UUID version to 4 --- truly random generation */
 162 id[6] = (id[6] &amp; 0x0F) | 0x40;
 163 /* Set the UUID variant to DCE */
 164 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 165
 166     <para>(This code is inspired by
 167     <literal>generate_random_uuid()</literal> of
 168     <filename>drivers/char/random.c</filename> from the Linux kernel
 169     sources.)</para>
 170
 171   </refsect1>
 172
 173   <refsect1>
 174     <title>History</title>
 175
 176     <para>The simple configuration file format of
 177     <filename>/etc/machine-id</filename> originates in the
 178     <filename>/var/lib/dbus/machine-id</filename> file introduced by
 179     D-Bus. In fact, this latter file might be a symlink to
 180     <filename>/etc/machine-id</filename>.</para>
 181   </refsect1>
 182
 183   <refsect1>
 184       <title>See Also</title>
 185       <para>
 186         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 187         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 188         <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 189         <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 190         <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 191         <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 192         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 193         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 194         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 195       </para>
 196   </refsect1>
 197
 198 </refentry>