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-rw-r--r--man/machine-id.xml43
1 files changed, 37 insertions, 6 deletions
diff --git a/man/machine-id.xml b/man/machine-id.xml
index d0bfbd240d..6ca9990988 100644
--- a/man/machine-id.xml
+++ b/man/machine-id.xml
@@ -55,10 +55,12 @@
<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
@@ -69,7 +71,7 @@
<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
@@ -82,13 +84,42 @@
</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] &amp; 0x0F) | 0x40;
+/* Set the UUID variant to DCE */
+id[8] = (id[8] &amp; 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>
<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 the
+ symlink to
<varname>/etc/machine-id</varname>.</para>
</refsect1>