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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
-->
<refentry id="machine-id">
<refentryinfo>
<title>machine-id</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>machine-id</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>machine-id</refname>
<refpurpose>Local machine ID configuration file</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>/etc/machine-id</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of
the local system that is set during installation or boot. The machine ID is a single
newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
zeros.</para>
<para>The machine ID is usually generated from a random source during system
installation or first boot and stays constant for all subsequent boots. Optionally,
for stateless systems, it is generated during runtime during early boot if necessary.
</para>
<para>The machine ID may be set, for example when network booting, with the
<varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
option <option>--machine-id=</option> to systemd. An ID is specified in this manner
has higher priority and will be used instead of the ID stored in
<filename>/etc/machine-id</filename>.</para>
<para>The machine ID does not change based on local or network configuration or when
hardware is replaced. Due to this and its greater length, it is a more useful
replacement for the
<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
call that POSIX specifies.</para>
<para>This machine ID adheres to the same format and logic as the
D-Bus machine ID.</para>
<para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
the original machine ID from the application-specific one. The
<citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
API provides an implementation of such an algorithm.</para>
</refsect1>
<refsect1>
<title>Initialization</title>
<para>Each machine should have a non-empty ID in normal operation. The ID of each
machine should be unique. To achieve those objectives,
<filename>/etc/machine-id</filename> can be initialized in a few different ways.
</para>
<para>For normal operating system installations, where a custom image is created for a
specific machine, <filename>/etc/machine-id</filename> should be populated during
installation.</para>
<para>
<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
may be used by installer tools to initialize the machine ID at install time, but
<filename>/etc/machine-id</filename> may also be written using any other means.
</para>
<para>For operating system images which are created once and used on multiple
machines, for example for containers or in the cloud,
<filename>/etc/machine-id</filename> should be an empty file in the generic file
system image. An ID will be generated during boot and saved to this file if
possible. Having an empty file in place is useful because it allows a temporary file
to be bind-mounted over the real file, in case the image is used read-only.</para>
<para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
may be used to initialize <filename>/etc/machine-id</filename> on mounted (but not
booted) system images.</para>
<para>When a machine is booted with
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
or <option>--machine-id=</option> options (see first section) are specified, this
value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
be used. If this file is empty or missing, <filename>systemd</filename> will attempt
to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
<filename>product_uuid</filename> (on KVM systems), and finally a randomly generated
UUID.</para>
<para>After the machine ID is established,
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
It is an error if the file system is read-only and does not contain a (possibly empty)
<filename>/etc/machine-id</filename> file.</para>
<para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
will attempt to write the machine ID to the file system if
<filename>/etc/machine-id</filename> or <filename>/etc</filename> are read-only during
early boot but become writable later on.</para>
</refsect1>
<refsect1>
<title>Relation to OSF UUIDs</title>
<para>Note that the machine ID historically is not an OSF UUID as
defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
4122</ulink>, nor a Microsoft GUID; however, starting with systemd
v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
<para>In order to maintain compatibility with existing
installations, an application requiring a UUID should decode the
machine ID, and then apply the following operations to turn it
into a valid OSF v4 UUID. With <literal>id</literal> being an
unsigned character array:</para>
<programlisting>/* Set UUID version to 4 --- truly random generation */
id[6] = (id[6] & 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id[8] = (id[8] & 0x3F) | 0x80;</programlisting>
<para>(This code is inspired by
<literal>generate_random_uuid()</literal> of
<filename>drivers/char/random.c</filename> from the Linux kernel
sources.)</para>
</refsect1>
<refsect1>
<title>History</title>
<para>The simple configuration file format of
<filename>/etc/machine-id</filename> originates in the
<filename>/var/lib/dbus/machine-id</filename> file introduced by
D-Bus. In fact, this latter file might be a symlink to
<filename>/etc/machine-id</filename>.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
