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d7ccca2e | 1 | <?xml version='1.0'?> <!--*-nxml-*--> |
3a54a157 | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
db9ecf05 | 4 | <!-- SPDX-License-Identifier: LGPL-2.1-or-later --> |
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5 | |
6 | <refentry id="machine-id"> | |
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7 | <refentryinfo> |
8 | <title>machine-id</title> | |
9 | <productname>systemd</productname> | |
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10 | </refentryinfo> |
11 | ||
12 | <refmeta> | |
13 | <refentrytitle>machine-id</refentrytitle> | |
14 | <manvolnum>5</manvolnum> | |
15 | </refmeta> | |
16 | ||
17 | <refnamediv> | |
18 | <refname>machine-id</refname> | |
19 | <refpurpose>Local machine ID configuration file</refpurpose> | |
20 | </refnamediv> | |
21 | ||
22 | <refsynopsisdiv> | |
23 | <para><filename>/etc/machine-id</filename></para> | |
24 | </refsynopsisdiv> | |
25 | ||
26 | <refsect1> | |
27 | <title>Description</title> | |
28 | ||
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29 | <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of |
30 | the local system that is set during installation or boot. The machine ID is a single | |
31 | newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from | |
32 | hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all | |
33 | zeros.</para> | |
34 | ||
35 | <para>The machine ID is usually generated from a random source during system | |
36 | installation or first boot and stays constant for all subsequent boots. Optionally, | |
37 | for stateless systems, it is generated during runtime during early boot if necessary. | |
38 | </para> | |
39 | ||
40 | <para>The machine ID may be set, for example when network booting, with the | |
41 | <varname>systemd.machine_id=</varname> kernel command line parameter or by passing the | |
e9dd6984 | 42 | option <option>--machine-id=</option> to systemd. An ID specified in this manner |
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43 | has higher priority and will be used instead of the ID stored in |
44 | <filename>/etc/machine-id</filename>.</para> | |
45 | ||
46 | <para>The machine ID does not change based on local or network configuration or when | |
47 | hardware is replaced. Due to this and its greater length, it is a more useful | |
48 | replacement for the | |
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49 | <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry> |
50 | call that POSIX specifies.</para> | |
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51 | |
52 | <para>This machine ID adheres to the same format and logic as the | |
53 | D-Bus machine ID.</para> | |
54 | ||
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55 | <para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in |
56 | untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is | |
57 | needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID | |
58 | should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the | |
59 | ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve | |
60 | the original machine ID from the application-specific one. The | |
61 | <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
62 | API provides an implementation of such an algorithm.</para> | |
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63 | </refsect1> |
64 | ||
65 | <refsect1> | |
66 | <title>Initialization</title> | |
67 | ||
68 | <para>Each machine should have a non-empty ID in normal operation. The ID of each | |
1b2ad5d9 | 69 | machine should be unique. To achieve those objectives, |
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70 | <filename>/etc/machine-id</filename> can be initialized in a few different ways. |
71 | </para> | |
72 | ||
73 | <para>For normal operating system installations, where a custom image is created for a | |
74 | specific machine, <filename>/etc/machine-id</filename> should be populated during | |
75 | installation.</para> | |
798d3a52 | 76 | |
74a79c65 | 77 | <para> |
798d3a52 | 78 | <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry> |
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79 | may be used by installer tools to initialize the machine ID at install time, but |
80 | <filename>/etc/machine-id</filename> may also be written using any other means. | |
81 | </para> | |
82 | ||
83 | <para>For operating system images which are created once and used on multiple | |
84 | machines, for example for containers or in the cloud, | |
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85 | <filename>/etc/machine-id</filename> should be either missing or an empty file in the generic file |
86 | system image (the difference between the two options is described under "First Boot Semantics" below). An | |
87 | ID will be generated during boot and saved to this file if possible. Having an empty file in place is | |
88 | useful because it allows a temporary file to be bind-mounted over the real file, in case the image is | |
89 | used read-only.</para> | |
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90 | |
91 | <para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
1b2ad5d9 | 92 | may be used to initialize <filename>/etc/machine-id</filename> on mounted (but not |
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93 | booted) system images.</para> |
94 | ||
95 | <para>When a machine is booted with | |
96 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
97 | the ID of the machine will be established. If <varname>systemd.machine_id=</varname> | |
98 | or <option>--machine-id=</option> options (see first section) are specified, this | |
99 | value will be used. Otherwise, the value in <filename>/etc/machine-id</filename> will | |
100 | be used. If this file is empty or missing, <filename>systemd</filename> will attempt | |
101 | to use the D-Bus machine ID from <filename>/var/lib/dbus/machine-id</filename>, the | |
102 | value of the kernel command line option <varname>container_uuid</varname>, the KVM DMI | |
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103 | <filename>product_uuid</filename> or the devicetree <filename>vm,uuid</filename> |
104 | (on KVM systems), and finally a randomly generated UUID.</para> | |
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105 | |
106 | <para>After the machine ID is established, | |
107 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
108 | will attempt to save it to <filename>/etc/machine-id</filename>. If this fails, it | |
109 | will attempt to bind-mount a temporary file over <filename>/etc/machine-id</filename>. | |
110 | It is an error if the file system is read-only and does not contain a (possibly empty) | |
111 | <filename>/etc/machine-id</filename> file.</para> | |
112 | ||
113 | <para><citerefentry><refentrytitle>systemd-machine-id-commit.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
114 | will attempt to write the machine ID to the file system if | |
3b121157 | 115 | <filename>/etc/machine-id</filename> or <filename>/etc/</filename> are read-only during |
74a79c65 | 116 | early boot but become writable later on.</para> |
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117 | </refsect1> |
118 | ||
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119 | <refsect1> |
120 | <title>First Boot Semantics</title> | |
121 | ||
122 | <para><filename>/etc/machine-id</filename> is used to decide whether a boot is the first one. The rules | |
123 | are as follows:</para> | |
124 | ||
125 | <orderedlist> | |
126 | <listitem><para>If <filename>/etc/machine-id</filename> does not exist, this is a first boot. During | |
377a9545 | 127 | early boot, <command>systemd</command> will write <literal>uninitialized\n</literal> to this file and overmount |
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128 | a temporary file which contains the actual machine ID. Later (after <filename>first-boot-complete.target</filename> |
129 | has been reached), the real machine ID will be written to disk.</para></listitem> | |
130 | ||
131 | <listitem><para>If <filename>/etc/machine-id</filename> contains the string <literal>uninitialized</literal>, | |
132 | a boot is also considered the first boot. The same mechanism as above applies.</para></listitem> | |
133 | ||
134 | <listitem><para>If <filename>/etc/machine-id</filename> exists and is empty, a boot is | |
135 | <emphasis>not</emphasis> considered the first boot. <command>systemd</command> will still bind-mount a file | |
136 | containing the actual machine-id over it and later try to commit it to disk (if <filename>/etc/</filename> is | |
137 | writable).</para></listitem> | |
138 | ||
139 | <listitem><para>If <filename>/etc/machine-id</filename> already contains a valid machine-id, this is | |
140 | not a first boot.</para></listitem> | |
141 | </orderedlist> | |
142 | ||
143 | <para>If by any of the above rules, a first boot is detected, units with <varname>ConditionFirstBoot=yes</varname> | |
144 | will be run.</para> | |
145 | </refsect1> | |
146 | ||
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147 | <refsect1> |
148 | <title>Relation to OSF UUIDs</title> | |
149 | ||
150 | <para>Note that the machine ID historically is not an OSF UUID as | |
151 | defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC | |
152 | 4122</ulink>, nor a Microsoft GUID; however, starting with systemd | |
153 | v30, newly generated machine IDs do qualify as v4 UUIDs.</para> | |
154 | ||
155 | <para>In order to maintain compatibility with existing | |
156 | installations, an application requiring a UUID should decode the | |
157 | machine ID, and then apply the following operations to turn it | |
158 | into a valid OSF v4 UUID. With <literal>id</literal> being an | |
159 | unsigned character array:</para> | |
160 | ||
161 | <programlisting>/* Set UUID version to 4 --- truly random generation */ | |
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162 | id[6] = (id[6] & 0x0F) | 0x40; |
163 | /* Set the UUID variant to DCE */ | |
164 | id[8] = (id[8] & 0x3F) | 0x80;</programlisting> | |
165 | ||
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166 | <para>(This code is inspired by |
167 | <literal>generate_random_uuid()</literal> of | |
168 | <filename>drivers/char/random.c</filename> from the Linux kernel | |
169 | sources.)</para> | |
170 | ||
171 | </refsect1> | |
172 | ||
173 | <refsect1> | |
174 | <title>History</title> | |
175 | ||
176 | <para>The simple configuration file format of | |
177 | <filename>/etc/machine-id</filename> originates in the | |
178 | <filename>/var/lib/dbus/machine-id</filename> file introduced by | |
179 | D-Bus. In fact, this latter file might be a symlink to | |
22065311 | 180 | <filename>/etc/machine-id</filename>.</para> |
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181 | </refsect1> |
182 | ||
183 | <refsect1> | |
184 | <title>See Also</title> | |
185 | <para> | |
186 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
187 | <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
3ba3a79d | 188 | <citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
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189 | <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
190 | <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
191 | <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
192 | <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
193 | <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
194 | <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
195 | </para> | |
196 | </refsect1> | |
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197 | |
198 | </refentry> |