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diff --git a/man/machine-id.xml b/man/machine-id.xml
index 98c09436b..6ca999098 100644
--- a/man/machine-id.xml
+++ b/man/machine-id.xml
@@ -55,21 +55,23 @@
Description
The /etc/machine-id 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.
+ 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.)
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.
+ boot if it is found to be empty.
The machine ID does not change based on user
configuration, or when hardware is replaced.
- This machine id follows the same format and
+ This machine ID adheres to the same format and
logic as the D-Bus machine ID.
Programs may use this ID to identify the host
@@ -81,6 +83,35 @@
call POSIX specifies.
+
+ Relation to OSF UUIDs
+
+ Note that the machine ID historically is not an
+ OSF UUID as defined by RFC
+ 4122, nor a Microsoft GUID. Starting with
+ systemd v30 newly generated machine IDs however do
+ qualify as v4 UUIDs.
+
+ 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
+ id being an unsigned character
+ array:
+
+ /* 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;
+
+ (This code is inspired by
+ generate_random_uuid() of
+ drivers/char/random.c from the
+ kernel sources.)
+
+
+
History
@@ -88,7 +119,7 @@
/etc/machine-id originates in the
/var/lib/dbus/machine-id file
introduced by D-Bus. In fact this latter file might be a
- symlink to the
+ symlink to
/etc/machine-id.