allow /31s on broadcast interfaces (eg ethernet) to work as per rfc3021.

the issue in our kernel was the broadcast address calculated on the /31
caused a ton of checks for use of broadcast addresses to kick in and
prevent one of the two addresses on the /31 from being used.

this diff basically detects if a /31 has been configured and doesnt
configure a broadcast address for it, which makes the ips usable
for normal traffic.

i wrote this so i could interoperate with "carrier" network gear
better, and sthen wants it so he can conserve address space use.

the further special casing of broadcast address handling was from claudio@

ok claudio@ markus@ sthen@ henning@

1 files changed, 7 insertions, 3 deletions

diff --git a/sys/netinet/in.c b/sys/netinet/in.c
index 5ac859edea9..9167486dd95 100644
--- a/sys/netinet/in.c
+++ b/sys/netinet/in.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: in.c,v 1.67 2011/07/03 06:24:13 dlg Exp $	*/
+/*	$OpenBSD: in.c,v 1.68 2011/07/06 01:57:37 dlg Exp $	*/
 /*	$NetBSD: in.c,v 1.26 1996/02/13 23:41:39 christos Exp $	*/
 
 /*
@@ -700,8 +700,12 @@ in_ifinit(struct ifnet *ifp, struct in_ifaddr *ia, struct sockaddr_in *sin,
 	 */
 	ia->ia_ifa.ifa_metric = ifp->if_metric;
 	if (ifp->if_flags & IFF_BROADCAST) {
-		ia->ia_broadaddr.sin_addr.s_addr =
-			ia->ia_net | ~ia->ia_netmask;
+		if (IN_RFC3021_SUBNET(ia->ia_netmask))
+			ia->ia_broadaddr.sin_addr.s_addr = 0;
+		else {
+			ia->ia_broadaddr.sin_addr.s_addr =
+			    ia->ia_net | ~ia->ia_netmask;
+		}
 	} else if (ifp->if_flags & IFF_LOOPBACK) {
 		ia->ia_dstaddr = ia->ia_addr;
 		flags |= RTF_HOST;