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