IPv4 Multicast MAC Address issues

[John-Mark Gurney]

Peter Jeremy wrote this message on Wed, Jan 01, 2014 at 19:57 +1100:
> I'm trying to use multicast on my home network for the first time and
> have found an apparent anomoly in the destination MAC address.
> 
> My reading of RFC1112 section 6.4 is that the the destination MAC
> address uses the low 23 bits of the destination (multicast) IP
> address.  This is what Linux and Windows do and ifmcstat(8) on
> FreeBSD shows that as the multicast MAC filter.  Unfortunately, it
> seems that (at least on FreeBSD-10), the destination MAC address
> uses the low 23 bits of the IP address of my default route.
> 
> I am not doing any special multicast-related configuration on any
> of the hosts and have been using ping(8) to generate multicast
> packets.
> 
> Does FreeBSD need special configuration to support multicast or is this
> a bug?

This is probably a bug, and I have confirmed this on:
FreeBSD carbon.funkthat.com 11.0-CURRENT FreeBSD 11.0-CURRENT #4 r256870:258399M: Wed Nov 20 12:33:22 PST 2013     jmg at carbon.funkthat.com:/usr/src/sys/amd64/compile/lockprof  amd64


[jmg at carbon ~]$ echo foobar | nc -u 239.100.100.100 3845
sldkfjsdlfkj
# tcpdump -i re0 -n -e port 3845
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on re0, link-type EN10MB (Ethernet), capture size 65535 bytes
capability mode sandbox enabled
11:59:03.998008 xx:xx:xx:xx:xx:xx > 01:00:5e:28:00:0e, ethertype IPv4 (0x0800), length 49: 192.168.0.21.15921 > 239.100.100.100.3845: UDP, length 7

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."