multicast packets from bpf

[Bruce M. Simpson]

Seems reasonable, both patches are syntactically sane. There are 
arguments in favour of both changes.

I favour the first approach, however, it may make more sense to put the 
logic into bpf_movein() as it already builds a sockaddr based on the 
header data provided to bpf during a write.

For the first patch: I previously fixed tapwrite() to check injected 
frames in the same way, as this was causing a problem with my own use of 
if_bridge. There is no way that I see for bpf to be able to tell if a 
frame is link-layer multicast or not, and checking at that layer does 
introduce a little pollution. Ethernet is the most common case so it 
could be argued that's OK, as we have ethernet-specific fields in struct 
mbuf now. Your change is the parallel change in the bpfwrite path to 
what I have in the tapwrite path.

The second patch: Conceptually similar to the loopback check in 
ip_output() for multicast. we wind up doing this check elsewhere, in 
particular netgraph. It is a relatively cheap check although it does 
involve changing the flags on a potentially read-only mbuf chain, which 
is bending the rules a bit (the stack often needs to change stuff in 
m_pkthdr even if the clusters are read-only).

BTW this patch looks like it touches the paths which would need to be 
changed if IGMP snooping were to be implemented for if_bridge.

regards,
BMS