This is a nice concept but has a big problem.  The ip_input() routine
   (the function that decides to route or drop or accept as local any incoming
   IP packet) has no sure knowledge of which interface that packet actually
   came in on.  The link layer device (ethernet, slip, ppp, etc), throws the
   incoming packet on the protocol input queue and schedules a software
   interrupt.

I guess you missed this:

	m->m_pkthdr.rcvif = &sc->sc_arpcom.ac_if;