Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU General Public License as published by
- the Free Software Foundation; either version 2 of the License, or
+ 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
- General Public License for more details.
+ Lesser General Public License for more details.
- You should have received a copy of the GNU General Public License
+ 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>/etc/machine-id</title>
+ <title>machine-id</title>
<productname>systemd</productname>
<authorgroup>
<refnamediv>
<refname>machine-id</refname>
- <refpurpose>local machine ID configuration file</refpurpose>
+ <refpurpose>Local machine ID configuration file</refpurpose>
</refnamediv>
<refsynopsisdiv>
<title>Description</title>
<para>The <filename>/etc/machine-id</filename> file
- configures the unique machine id of the local system
- that is set during installation. It should contain a
- single newline-terminated, hexadecimal, lowercase 16
- character machine ID string.</para>
+ contains the unique machine id of the local system
+ that is set during installation. The machine ID is a
+ single newline-terminated, hexadecimal, lowercase 32
+ character 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.</para>
+ 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 follows the same format and
+ <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, that does
+ 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 than the
+ a more useful replacement for the
<citerefentry><refentrytitle>gethostid</refentrytitle><manvolnum>3</manvolnum></citerefentry>
call 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="http://tools.ietf.org/html/rfc4122">RFC
+ 4122</ulink>, nor a Microsoft GUID. Starting with
+ systemd v30 newly generated machine IDs however 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
+ kernel sources.)</para>
+
</refsect1>
<refsect1>
<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 the
+ symlink to
<varname>/etc/machine-id</varname>.</para>
</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>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>