I think this is a good idea, and done just about right.  The only bit of
implementation that bugs me a little is:

> + 	if (inp->inp_flags & INP_RECVIF) {
> + 		struct sockaddr_dl sdl;
> + 
> + 		sdl.sdl_len = sizeof(sdl);
> + 		sdl.sdl_family = AF_LINK;
> + 		sdl.sdl_index = m->m_pkthdr.rcvif ?
> + 			m->m_pkthdr.rcvif->if_index : 0;
> + 		sdl.sdl_nlen = sdl.sdl_alen = sdl.sdl_slen = 0;
> + 		*mp = sbcreatecontrol((caddr_t) &sdl, sdl.sdl_len,
> + 			IP_RECVIF, IPPROTO_IP);
> + 		if (*mp)
> + 			mp = &(*mp)->m_next;
> + 	}

It seems unhappy to be copying in 20 bytes of a mostly-stack-grot-filled
sockaddr_dl to carry a two byte interface index.  Maybe the sockaddr
could be pruned at four or eight bytes?

Dennis Ferguson