144 lines
6.3 KiB
XML
144 lines
6.3 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<?xml-stylesheet type="text/xsl" href="http://docbook.sourceforge.net/release/xsl/current/xhtml/docbook.xsl"?>
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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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This file is part of systemd.
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Copyright 2010 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="machine-id">
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<refentryinfo>
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<title>/etc/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
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contains the unique machine id of the local system
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that is set during installation. The machine ID is a
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single newline-terminated, hexadecimal, lowercase 32
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character machine ID string. (When decoded from
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hexadecimal this corresponds with a 16 byte/128 bit
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string.)</para>
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<para>The machine ID is usually generated from a
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random source during system installation and stays
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constant for all subsequent boots. Optionally, for
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stateless systems it is generated during runtime at
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boot if it is found to be empty.</para>
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<para>The machine ID does not change based on user
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configuration, or when hardware is replaced.</para>
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<para>This machine ID adheres to the same format and
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logic as the D-Bus machine ID.</para>
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<para>Programs may use this ID to identify the host
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with a globally unique ID in the network, that does
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not change even if the local network configuration
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changes. Due to this and its greater length it is
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a more useful replacement for the
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<citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call POSIX specifies.</para>
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<para>The
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<citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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tool may be used by installer tools to initialize the
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machine ID at install time.</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
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OSF UUID as defined by <ulink
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url="http://tools.ietf.org/html/rfc4122">RFC
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4122</ulink>, nor a Microsoft GUID. Starting with
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systemd v30 newly generated machine IDs however do
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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
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decode the machine ID, and then apply the following
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operations to turn it into a valid OSF v4 UUID. With
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<literal>id</literal> being an unsigned character
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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
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kernel 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
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introduced by D-Bus. In fact this latter file might be a
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symlink to
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<varname>/etc/machine-id</varname>.</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><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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</para>
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</refsect1>
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</refentry>
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