--- sys/netinet/in_mcast.c.orig	2007-11-30 18:01:05.000000000 +0100
+++ sys/netinet/in_mcast.c	2007-12-07 21:34:28.000000000 +0100
@@ -980,11 +980,40 @@
 		struct ip_mreq_source	 mreqs;
 
 		if (sopt->sopt_name == IP_ADD_MEMBERSHIP) {
-			error = sooptcopyin(sopt, &mreqs,
-			    sizeof(struct ip_mreq),
-			    sizeof(struct ip_mreq));
 			/*
-			 * Do argument switcharoo from ip_mreq into
+			 * Handle the case where a struct ip_mreqn
+			 * was passed to the IP_ADD_MEMBERSHIP ioctl to
+			 * specify an interface index. This is an
+			 * undocumented modification of the IPv4 ASM API
+			 * obtained from Linux.
+			 */
+			if (sopt->sopt_valsize == sizeof(struct ip_mreqn)) {
+				struct ip_mreqn *mreqn =
+				    (struct ip_mreqn *)&mreqs;
+
+				error = sooptcopyin(sopt, mreqn,
+				    sizeof(struct ip_mreqn),
+				    sizeof(struct ip_mreqn));
+				if (error)
+					return (error);
+
+				if (mreqn->imr_ifindex < 0 ||
+				    if_index < mreqn->imr_ifindex)
+					return (EINVAL);
+				if (mreqn->imr_ifindex == 0) {
+					ifp = NULL;
+				} else {
+					ifp = ifnet_byindex(mreqn->imr_ifindex);
+					if (ifp == NULL)
+						return (EADDRNOTAVAIL);
+				}
+			} else {
+				error = sooptcopyin(sopt, &mreqs,
+				    sizeof(struct ip_mreq),
+				    sizeof(struct ip_mreq));
+			}
+			/*
+			 * Do argument switcharoo from ip_mreq[n] into
 			 * ip_mreq_source to avoid using two instances.
 			 */
 			mreqs.imr_interface = mreqs.imr_sourceaddr;
@@ -1008,10 +1037,12 @@
 		}
 
 		/*
-		 * Obtain ifp. If no interface address was provided,
-		 * use the interface of the route in the unicast FIB for
-		 * the given multicast destination; usually, this is the
-		 * default route.
+		 * If ifp was not already overridden, obtain it.
+		 *
+		 * If no interface address was provided, use the interface
+		 * of the route in the unicast FIB for the given multicast
+		 * destination; usually, this is the default route.
+		 *
 		 * If this lookup fails, attempt to use the first non-loopback
 		 * interface with multicast capability in the system as a
 		 * last resort. The legacy IPv4 ASM API requires that we do
@@ -1020,9 +1051,9 @@
 		 * If all of these conditions fail, return EADDRNOTAVAIL, and
 		 * reject the IPv4 multicast join.
 		 */
-		if (mreqs.imr_interface.s_addr != INADDR_ANY) {
-			ifp = ip_multicast_if(&mreqs.imr_interface);
-		} else {
+		if (ifp == NULL && mreqs.imr_interface.s_addr != INADDR_ANY) {
+			INADDR_TO_IFP(mreqs.imr_interface, ifp);
+		} else if (ifp == NULL) {
 			struct route ro;
 
 			ro.ro_rt = NULL;
@@ -1235,12 +1266,42 @@
 	case IP_DROP_MEMBERSHIP:
 	case IP_DROP_SOURCE_MEMBERSHIP:
 		if (sopt->sopt_name == IP_DROP_MEMBERSHIP) {
-			error = sooptcopyin(sopt, &mreqs,
-			    sizeof(struct ip_mreq),
-			    sizeof(struct ip_mreq));
+			/*
+			 * Handle the case where a struct ip_mreqn
+			 * was passed to the IP_DROP_MEMBERSHIP ioctl to
+			 * specify an interface index. This is an
+			 * undocumented modification of the IPv4 ASM API
+			 * obtained from Linux.
+			 */
+			if (sopt->sopt_valsize == sizeof(struct ip_mreqn)) {
+				struct ip_mreqn *mreqn =
+				    (struct ip_mreqn *)&mreqs;
+
+				error = sooptcopyin(sopt, mreqn,
+				    sizeof(struct ip_mreqn),
+				    sizeof(struct ip_mreqn));
+				if (error)
+					return (error);
+
+				if (mreqn->imr_ifindex < 0 ||
+				    if_index < mreqn->imr_ifindex)
+					return (EINVAL);
+				if (mreqn->imr_ifindex == 0) {
+					ifp = NULL;
+				} else {
+					ifp = ifnet_byindex(mreqn->imr_ifindex);
+					if (ifp == NULL)
+						return (EADDRNOTAVAIL);
+				}
+			} else {
+				error = sooptcopyin(sopt, &mreqs,
+				    sizeof(struct ip_mreq),
+				    sizeof(struct ip_mreq));
+			}
+
 			/*
 			 * Swap interface and sourceaddr arguments,
-			 * as ip_mreq and ip_mreq_source are laid
+			 * as ip_mreq[n] and ip_mreq_source are laid
 			 * out differently.
 			 */
 			mreqs.imr_interface = mreqs.imr_sourceaddr;
@@ -1263,7 +1324,7 @@
 			ssa->sin.sin_addr = mreqs.imr_sourceaddr;
 		}
 
-		if (gsa->sin.sin_addr.s_addr != INADDR_ANY)
+		if (ifp == NULL && gsa->sin.sin_addr.s_addr != INADDR_ANY)
 			INADDR_TO_IFP(mreqs.imr_interface, ifp);
 
 #ifdef DIAGNOSTIC