<?xml version='1.0'?> <!--*-nxml-*--> <?xml-stylesheet type="text/xsl" href="http://docbook.sourceforge.net/release/xsl/current/xhtml/docbook.xsl"?> <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> <!-- This file is part of systemd. Copyright 2010 Lennart Poettering systemd is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. systemd is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with systemd; If not, see <http://www.gnu.org/licenses/>. --> <refentry id="machine-id"> <refentryinfo> <title>machine-id</title> <productname>systemd</productname> <authorgroup> <author> <contrib>Developer</contrib> <firstname>Lennart</firstname> <surname>Poettering</surname> <email>lennart@poettering.net</email> </author> </authorgroup> </refentryinfo> <refmeta> <refentrytitle>machine-id</refentrytitle> <manvolnum>5</manvolnum> </refmeta> <refnamediv> <refname>machine-id</refname> <refpurpose>Local machine ID configuration file</refpurpose> </refnamediv> <refsynopsisdiv> <para><filename>/etc/machine-id</filename></para> </refsynopsisdiv> <refsect1> <title>Description</title> <para>The <filename>/etc/machine-id</filename> file contains the unique machine ID of the local system that is set during installation. The machine ID is a single newline-terminated, hexadecimal, 32-character, lowercase machine ID string. When decoded from hexadecimal, this corresponds with a 16-byte/128-bit string.</para> <para>The machine ID is usually generated from a random source during system installation and stays constant for all subsequent boots. Optionally, for stateless systems, it is generated during runtime at boot if it is found to be empty.</para> <para>The machine ID does not change based on user configuration or when hardware is replaced.</para> <para>This machine ID adheres to the same format and logic as the D-Bus machine ID.</para> <para>Programs may use this ID to identify the host with a globally unique ID in the network, which does not change even if the local network configuration changes. Due to this and its greater length, it is a more useful replacement for the <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry> call that POSIX specifies.</para> <para>The <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool may be used by installer tools to initialize the machine ID at install time.</para> </refsect1> <refsect1> <title>Relation to OSF UUIDs</title> <para>Note that the machine ID historically is not an OSF UUID as defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC 4122</ulink>, nor a Microsoft GUID; however, starting with systemd v30, newly generated machine IDs do qualify as v4 UUIDs.</para> <para>In order to maintain compatibility with existing installations, an application requiring a UUID should decode the machine ID, and then apply the following operations to turn it into a valid OSF v4 UUID. With <literal>id</literal> being an unsigned character array:</para> <programlisting>/* Set UUID version to 4 --- truly random generation */ id[6] = (id[6] & 0x0F) | 0x40; /* Set the UUID variant to DCE */ id[8] = (id[8] & 0x3F) | 0x80;</programlisting> <para>(This code is inspired by <literal>generate_random_uuid()</literal> of <filename>drivers/char/random.c</filename> from the Linux kernel sources.)</para> </refsect1> <refsect1> <title>History</title> <para>The simple configuration file format of <filename>/etc/machine-id</filename> originates in the <filename>/var/lib/dbus/machine-id</filename> file introduced by D-Bus. In fact, this latter file might be a symlink to <varname>/etc/machine-id</varname>.</para> </refsect1> <refsect1> <title>See Also</title> <para> <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>, <citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>, <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>, <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry> </para> </refsect1> </refentry>