When performing IP fast forwarding, immediately drop traffic which is

destined for a blackhole route.

This also means that blackhole routes do not need to be bound to lo(4)
or disc(4) interfaces for the net.inet.ip.fastforwarding=1 case.

Submitted by:	james at towardex dot com
Sponsored by:	eXtensible Open Router Project <URL:http://www.xorp.org/>
MFC after:	3 weeks

sys/netinet/ip_fastfwd.c

@@ -432,6 +432,12 @@ ip_fastforward(struct mbuf *m)
 		return 1;	/* icmp unreach already sent */
 	ifp = ro.ro_rt->rt_ifp;
 
+	/*
+	 * Immediately drop blackholed traffic.
+	 */
+	if (ro.ro_rt->rt_flags & RTF_BLACKHOLE)
+		goto drop;
+
 	/*
 	 * Step 5: outgoing firewall packet processing
 	 */