Looking through my old mail, I see:

   I've added bpf filters to the input and output side of the ip forwarding
   code and been using them for some months.  The performance is reasonable
   for slow links (like up to 56k).  The optimizer in tcpdump is actually
   pretty good.

   If you are interested, I'll send you the diffs.  There is currently no
   notification of dropped packets.

I'd be interested in these diffs.