diff options
Diffstat (limited to 'man/machine-id.xml')
| -rw-r--r-- | man/machine-id.xml | 43 |
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] & 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> <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> |