Add support for IPv4 /31 prefixes, as described in RFC3021.

To run a /31 network, participating hosts MUST drop support
for directed broadcasts, and treat the first and last addresses
on subnet as unicast. The broadcast address for the prefix
should be the link local broadcast address, INADDR_BROADCAST.

2 files changed, 11 insertions, 4 deletions

diff --git a/sys/netinet/in.c b/sys/netinet/in.c
index d0be11f..bba364d 100644
--- a/sys/netinet/in.c
+++ b/sys/netinet/in.c
@@ -897,8 +897,11 @@ 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 =
-			htonl(ia->ia_subnet | ~ia->ia_subnetmask);
+		if (ia->ia_subnetmask == IN_RFC3021_MASK)
+			ia->ia_broadaddr.sin_addr.s_addr = INADDR_BROADCAST;
+		else
+			ia->ia_broadaddr.sin_addr.s_addr =
+			    htonl(ia->ia_subnet | ~ia->ia_subnetmask);
 	} else if (ifp->if_flags & IFF_LOOPBACK) {
 		ia->ia_dstaddr = ia->ia_addr;
 		flags |= RTF_HOST;
@@ -1230,9 +1233,11 @@ in_broadcast(struct in_addr in, struct ifnet *ifp)
 		if (ifa->ifa_addr->sa_family == AF_INET &&
 		    (in.s_addr == ia->ia_broadaddr.sin_addr.s_addr ||
 		     /*
-		      * Check for old-style (host 0) broadcast.
+		      * Check for old-style (host 0) broadcast, but
+		      * taking into account that RFC 3021 obsoletes it.
 		      */
-		     t == ia->ia_subnet) &&
+		     (ia->ia_subnetmask != IN_RFC3021_MASK &&
+		     t == ia->ia_subnet)) &&
 		     /*
 		      * Check for an all one subnetmask. These
 		      * only exist when an interface gets a secondary
diff --git a/sys/netinet/in.h b/sys/netinet/in.h
index d5e4290..52844f0 100644
--- a/sys/netinet/in.h
+++ b/sys/netinet/in.h
@@ -392,6 +392,8 @@ __END_DECLS
 
 #define	IN_LOOPBACKNET		127			/* official! */
 
+#define	IN_RFC3021_MASK		(u_int32_t)0xfffffffe
+
 /*
  * Options for use with [gs]etsockopt at the IP level.
  * First word of comment is data type; bool is stored in int.