In case no real/physical IEEE 802 address is available, both the expired
"draft-leach-uuids-guids-01" (section "4. Node IDs when no IEEE 802
network card is available") and RFC 2518 (section "6.4.1 Node Field
Generation Without the IEEE 802 Address") recommend (quoted from RFC
2518):
"The ideal solution is to obtain a 47 bit cryptographic quality random
number, and use it as the low 47 bits of the node ID, with the _most_
significant bit of the first octet of the node ID set to 1. This bit
is the unicast/multicast bit, which will never be set in IEEE 802
addresses obtained from network cards; hence, there can never be a
conflict between UUIDs generated by machines with and without network
cards."
Unfortunately, this incorrectly explains how to implement this and
the FreeBSD UUID generator code inherited this generation bug from
the broken reference code in the standards draft. They should instead
specify the "_least_ significant bit of the first octet of the node ID"
as the multicast bit in a memory and hexadecimal string representation
of a 48-bit IEEE 802 MAC address.
This standards bug arised from a false interpretation, as the multicast
bit is actually the _most_ significant bit in IEEE 802.3 (Ethernet)
_transmission order_ of an IEEE 802 MAC address. The standards authors
forgot that the bitwise order of an _octet_ from a MAC address _memory_
and hexadecimal string representation is still always from left (MSB,
bit 7) to right (LSB, bit 0).
Fortunately, this UUID generation bug could have occurred on systems
without any Ethernet NICs only.
The uuidgen command, by means of the uuidgen syscall, generates one
or more Universally Unique Identifiers compatible with OSF/DCE 1.1
version 1 UUIDs.
From the Perforce logs (change 11995):
Round of cleanups:
o Give uuidgen() the correct prototype in syscalls.master
o Define struct uuid according to DCE 1.1 in sys/uuid.h
o Use struct uuid instead of uuid_t. The latter is defined
in sys/uuid.h but should not be used in kernel land.
o Add snprintf_uuid(), printf_uuid() and sbuf_printf_uuid()
to kern_uuid.c for use in the kernel (currently geom_gpt.c).
o Rename the non-standard struct uuid in kern/kern_uuid.c
to struct uuid_private and give it a slightly better definition
for better byte-order handling. See below.
o In sys/gpt.h, fix the broken uuid definitions to match the now
compliant struct uuid definition. See below.
o In usr.bin/uuidgen/uuidgen.c catch up with struct uuid change.
A note about byte-order:
The standard failed to provide a non-conflicting and
unambiguous definition for the binary representation. My initial
implementation always wrote the timestamp as a 64-bit little-endian
(2s-complement) integral. The clock sequence was always written
as a 16-bit big-endian (2s-complement) integral. After a good
nights sleep and couple of Pan Galactic Gargle Blasters (not
necessarily in that order :-) I reread the spec and came to the
conclusion that the time fields are always written in the native
by order, provided the the low, mid and hi chopping still occurs.
The spec mentions that you "might need to swap bytes if you talk
to a machine that has a different byte-order". The clock sequence
is always written in big-endian order (as is the IEEE 802 address)
because its division is resulting in bytes, making the ordering
unambiguous.
