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11a1589223
Files which are installed as-is (any .service and other unit files, .conf files, .policy files, etc), are left as is. My assumption is that SPDX identifiers are not yet that well known, so it's better to retain the extended header to avoid any doubt. I also kept any copyright lines. We can probably remove them, but it'd nice to obtain explicit acks from all involved authors before doing that.
186 lines
8.7 KiB
XML
186 lines
8.7 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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This file is part of systemd.
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Copyright 2010 Lennart Poettering
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-->
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<refentry id="machine-id">
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<refentryinfo>
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<title>machine-id</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>machine-id</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>machine-id</refname>
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<refpurpose>Local machine ID configuration file</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/etc/machine-id</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of
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the local system that is set during installation or boot. The machine ID is a single
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newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from
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hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all
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zeros.</para>
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<para>The machine ID is usually generated from a random source during system
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installation or first boot and stays constant for all subsequent boots. Optionally,
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for stateless systems, it is generated during runtime during early boot if necessary.
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</para>
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<para>The machine ID may be set, for example when network booting, with the
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<varname>systemd.machine_id=</varname> kernel command line parameter or by passing the
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option <option>--machine-id=</option> to systemd. An ID is specified in this manner
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has higher priority and will be used instead of the ID stored in
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<filename>/etc/machine-id</filename>.</para>
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<para>The machine ID does not change based on local or network configuration or when
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hardware is replaced. Due to this and its greater length, it is a more useful
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replacement for the
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<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call that POSIX specifies.</para>
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<para>This machine ID adheres to the same format and logic as the
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D-Bus machine ID.</para>
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<para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
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untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
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needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
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should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
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ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
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the original machine ID from the application-specific one. The
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<citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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API provides an implementation of such an algorithm.</para>
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</refsect1>
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<refsect1>
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<title>Initialization</title>
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<para>Each machine should have a non-empty ID in normal operation. The ID of each
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machine should be unique. To achive those objectives,
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<filename>/etc/machine-id</filename> can be initialized in a few different ways.
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</para>
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<para>For normal operating system installations, where a custom image is created for a
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specific machine, <filename>/etc/machine-id</filename> should be populated during
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installation.</para>
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<para>
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<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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may be used by installer tools to initialize the machine ID at install time, but
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<filename>/etc/machine-id</filename> may also be written using any other means.
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</para>
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<para>For operating system images which are created once and used on multiple
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machines, for example for containers or in the cloud,
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<filename>/etc/machine-id</filename> should be an empty file in the generic file
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system image. An ID will be generated during boot and saved to this file if
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possible. Having an empty file in place is useful because it allows a temporary file
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to be bind-mounted over the real file, in case the image is used read-only.</para>
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<para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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may be used to to initialize <filename>/etc/machine-id</filename> on mounted (but not
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booted) system images.</para>
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<para>When a machine is booted with
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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the ID of the machine will be established. If <varname>systemd.machine_id=</varname>
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or <option>--machine-id=</option> options (see first section) are specified, this
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value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will
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be used. If this file is empty or missing, <filename>systemd</filename> will attempt
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to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the
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value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI
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<filename>product_uuid</filename> (on KVM systems), and finally a randomly generated
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UUID.</para>
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<para>After the machine ID is established,
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it
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will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>.
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It is an error if the file system is read-only and does not contain a (possibly empty)
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<filename>/etc/machine-id</filename> file.</para>
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<para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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will attempt to write the machine ID to the file system if
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<filename>/etc/machine-id</filename> or <filename>/etc</filename> are read-only during
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early boot but become writable later on.</para>
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</refsect1>
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<refsect1>
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<title>Relation to OSF UUIDs</title>
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<para>Note that the machine ID historically is not an OSF UUID as
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defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
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4122</ulink>, nor a Microsoft GUID; however, starting with systemd
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v30, newly generated machine IDs do qualify as v4 UUIDs.</para>
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<para>In order to maintain compatibility with existing
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installations, an application requiring a UUID should decode the
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machine ID, and then apply the following operations to turn it
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into a valid OSF v4 UUID. With <literal>id</literal> being an
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unsigned character array:</para>
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<programlisting>/* Set UUID version to 4 --- truly random generation */
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id[6] = (id[6] & 0x0F) | 0x40;
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/* Set the UUID variant to DCE */
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id[8] = (id[8] & 0x3F) | 0x80;</programlisting>
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<para>(This code is inspired by
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<literal>generate_random_uuid()</literal> of
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<filename>drivers/char/random.c</filename> from the Linux kernel
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sources.)</para>
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</refsect1>
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<refsect1>
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<title>History</title>
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<para>The simple configuration file format of
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<filename>/etc/machine-id</filename> originates in the
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<filename>/var/lib/dbus/machine-id</filename> file introduced by
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D-Bus. In fact, this latter file might be a symlink to
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<filename>/etc/machine-id</filename>.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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