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 the local
  57     system that is set during installation. The machine ID is a single newline-terminated,
  58     hexadecimal, 32-character, lowercase ID. When decoded from hexadecimal, this corresponds to a
  59     16-byte/128-bit value.</para>
  60
  61     <para>The machine ID is usually generated from a random source
  62     during system installation and stays constant for all subsequent
  63     boots. Optionally, for stateless systems, it is generated during
  64     runtime at early boot if it is found to be empty.</para>
  65
  66     <para>The machine ID does not change based on local or network configuration or when hardware is
  67     replaced. Due to this and its greater length, it is a more useful replacement for the
  68     <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  69     call that POSIX specifies.</para>
  70
  71     <para>This machine ID adheres to the same format and logic as the
  72     D-Bus machine ID.</para>
  73
  74     <para>This ID uniquely identifies the host. It should be considered "confidential", and must not
  75     be exposed in untrusted environments, in particular on the network. If a stable unique
  76     identifier that is tied to the machine is needed for some application, the machine ID or any
  77     part of it must not be used directly. Instead the machine ID should be hashed with a
  78     cryptographic, keyed hash function, using a fixed, application-specific key. That way the ID
  79     will be properly unique, and derived in a constant way from the machine ID but there will be no
  80     way to retrieve the original machine ID from the application-specific one.</para>
  81
  82     <para>The
  83     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  84     tool may be used by installer tools to initialize the machine ID
  85     at install time. Use
  86     <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
  87     to initialize it on mounted (but not booted) system images.</para>
  88
  89     <para>The machine-id may also be set, for example when network
  90     booting, by setting the <varname>systemd.machine_id=</varname>
  91     kernel command line parameter or passing the option
  92     <option>--machine-id=</option> to systemd. A machine-id may not
  93     be set to all zeros.</para>
  94   </refsect1>
  95
  96   <refsect1>
  97     <title>Relation to OSF UUIDs</title>
  98
  99     <para>Note that the machine ID historically is not an OSF UUID as
 100     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
 101     4122</ulink>, nor a Microsoft GUID; however, starting with systemd
 102     v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
 103
 104     <para>In order to maintain compatibility with existing
 105     installations, an application requiring a UUID should decode the
 106     machine ID, and then apply the following operations to turn it
 107     into a valid OSF v4 UUID. With <literal>id</literal> being an
 108     unsigned character array:</para>
 109
 110     <programlisting>/* Set UUID version to 4 --- truly random generation */
 111 id[6] = (id[6] &amp; 0x0F) | 0x40;
 112 /* Set the UUID variant to DCE */
 113 id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
 114
 115     <para>(This code is inspired by
 116     <literal>generate_random_uuid()</literal> of
 117     <filename>drivers/char/random.c</filename> from the Linux kernel
 118     sources.)</para>
 119
 120   </refsect1>
 121
 122   <refsect1>
 123     <title>History</title>
 124
 125     <para>The simple configuration file format of
 126     <filename>/etc/machine-id</filename> originates in the
 127     <filename>/var/lib/dbus/machine-id</filename> file introduced by
 128     D-Bus. In fact, this latter file might be a symlink to
 129     <filename>/etc/machine-id</filename>.</para>
 130   </refsect1>
 131
 132   <refsect1>
 133       <title>See Also</title>
 134       <para>
 135         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 136         <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 137         <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 138         <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 139         <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 140         <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 141         <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 142         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 143         <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 144       </para>
 145   </refsect1>
 146
 147 </refentry>