The loopback device doesn't have IFF_MULTICAST set, so technically
it is not a multicast-capable device, and you shouldn't be able to join a
group on it.
I think the way it ought to work is that you join the group on any device,
with
IPV6_MULTICAST_LOOP set and local guys should hear the node-local
multicasts,
but it shouldn't be sent on the wire. Multicasting could be supported on
loopback, too,
but it doesn't matter all that much unless there are no multicast-capable
real devices.
However, it appears that node-local multicasts are being sent out
the device,
at least on an early 2.6 kernel I did a quick test with. There probably
isn't anything
enforcing the node-locality in the send path, which I would consider a
bug. :-)
+-DLS
|