machine-id: be nice and generate compliant v4 UUIDs

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.

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>