8d41a963d6
Newly generated machine IDs now qualify as randomized v4 UUIds. This is trivial to do and hopefully increases adoption of the ID for various purposes.
138 lines
5.8 KiB
XML
138 lines
5.8 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 General Public License as published by
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the Free Software Foundation; either version 2 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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General Public License for more details.
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You should have received a copy of the GNU 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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</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>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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