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206 lines
11 KiB
XML
206 lines
11 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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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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</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 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 achieve 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 machines, for example for
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containers or in the cloud, <filename>/etc/machine-id</filename> should be either missing or an empty
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file in the generic file system image (the difference between the two options is described under "First
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Boot Semantics" below). An ID will be generated during boot and saved to this file if possible. Having an
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empty file in place is useful because it allows a temporary file to be bind-mounted over the real file,
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in case the image is used read-only. Also see <ulink url="https://systemd.io/BUILDING_IMAGES">Safely
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Building Images</ulink>.</para>
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<para><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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may be used 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> or the devicetree <filename>vm,uuid</filename>
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(on KVM systems), the Xen hypervisor <filename>uuid</filename>, and finally a randomly
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generated UUID. <varname>systemd.machine_id=firmware</varname> can be set to generate the machine ID
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from the firmware.</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>First Boot Semantics</title>
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<para><filename>/etc/machine-id</filename> is used to decide whether a boot is the first one. The rules
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are as follows:</para>
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<orderedlist>
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<listitem><para>The kernel command argument <varname>systemd.condition_first_boot=</varname> may be
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used to override the autodetection logic, see
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<citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
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</para></listitem>
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<listitem><para>Otherwise, if <filename>/etc/machine-id</filename> does not exist, this is a first
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boot. During early boot, <command>systemd</command> will write <literal>uninitialized\n</literal> to
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this file and overmount a temporary file which contains the actual machine ID. Later (after
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<filename>first-boot-complete.target</filename> has been reached), the real machine ID will be written
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to disk.</para></listitem>
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<listitem><para>If <filename>/etc/machine-id</filename> contains the string <literal>uninitialized</literal>,
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a boot is also considered the first boot. The same mechanism as above applies.</para></listitem>
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<listitem><para>If <filename>/etc/machine-id</filename> exists and is empty, a boot is
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<emphasis>not</emphasis> considered the first boot. <command>systemd</command> will still bind-mount a file
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containing the actual machine-id over it and later try to commit it to disk (if <filename>/etc/</filename> is
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writable).</para></listitem>
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<listitem><para>If <filename>/etc/machine-id</filename> already contains a valid machine-id, this is
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not a first boot.</para></listitem>
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</orderedlist>
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<para>If according to the above rules a first boot is detected, units with
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<varname>ConditionFirstBoot=yes</varname> will be run and <command>systemd</command> will perform
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additional initialization steps, in particular presetting units.</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 defined by <ulink
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url="https://tools.ietf.org/html/rfc4122">RFC 4122</ulink>, nor a Microsoft GUID; however, starting with
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systemd v30, newly generated machine IDs do qualify as Variant 1 Version 4 UUIDs, as per RFC 4122.</para>
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<para>In order to maintain compatibility with existing installations, an application requiring a strictly
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RFC 4122 compliant UUID should decode the machine ID, and then (non-reversibly) apply the following
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operations to turn it into a valid RFC 4122 Variant 1 Version 4 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><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry project='man-pages'><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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