"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
+ SPDX-License-Identifier: LGPL-2.1+
+
This file is part of systemd.
Copyright 2010 Lennart Poettering
<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. The machine ID is a single newline-terminated,
- hexadecimal, 32-character, lowercase machine ID string. When
- decoded from hexadecimal, this corresponds with a 16-byte/128-bit
- string.</para>
-
- <para>The machine ID is usually generated from a random source
- during system installation and stays constant for all subsequent
- boots. Optionally, for stateless systems, it is generated during
- runtime at early boot if it is found to be empty.</para>
+ <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 user configuration
- or when hardware is replaced.</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>Programs may use this ID to identify the host with a
- globally unique ID in the network, which does not change even if
- the local network configuration changes. 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 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 achive 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>The
+ <para>
<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
- tool may be used by installer tools to initialize the machine ID
- at install time. Use
- <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
- to initialize it on mounted (but not booted) system images.</para>
-
- <para>The machine-id may also be set, for example when network
- booting, by setting the <varname>systemd.machine_id=</varname>
- kernel command line parameter or passing the option
- <option>--machine-id=</option> to systemd. A machine-id may not
- be set to all zeros.</para>
+ 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 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>
projects / thirdparty / systemd.git / blobdiff