diff -ur old/sys/netinet/ip_mroute.c new/sys/netinet/ip_mroute.c
--- old/sys/netinet/ip_mroute.c	Sat Oct  9 17:12:04 2004
+++ new/sys/netinet/ip_mroute.c	Wed May 31 14:57:04 2006
@@ -393,7 +393,7 @@
 {
     int	error, optval;
     vifi_t	vifi;
-    struct	vifctl vifc;
+    struct	vifctln vifc;
     struct	mfcctl2 mfc;
     struct	bw_upcall bw_upcall;
     uint32_t	i;
@@ -415,10 +415,10 @@
 	break;
 
     case MRT_ADD_VIF:
-	error = sooptcopyin(sopt, &vifc, sizeof vifc, sizeof vifc);
+	error = sooptcopyin(sopt, &vifc, sizeof vifc, sizeof struct vifctl);
 	if (error)
 	    break;
-	error = add_vif(&vifc);
+	error = add_vif(&vifc, sopt->sopt_valsize);
 	break;
 
     case MRT_DEL_VIF:
@@ -899,7 +899,7 @@
  * Add a vif to the vif table
  */
 static int
-add_vif(struct vifctl *vifcp)
+add_vif(struct vifctln *vifcp, size_t vifcpsize)
 {
     struct vif *vifp = viftable + vifcp->vifc_vifi;
     struct sockaddr_in sin = {sizeof sin, AF_INET};
@@ -934,13 +934,25 @@
     } else
 #endif
     {
-	sin.sin_addr = vifcp->vifc_lcl_addr;
-	ifa = ifa_ifwithaddr((struct sockaddr *)&sin);
-	if (ifa == NULL) {
-	    VIF_UNLOCK();
-	    return EADDRNOTAVAIL;
+	if (vifcpsize < sizeof struct vifctln || vifcp->vifc_ifindex <= 0) {
+	    sin.sin_addr = vifcp->vifc_lcl_addr;
+	    ifa = ifa_ifwithaddr((struct sockaddr *)&sin);
+	    if (ifa == NULL) {
+		VIF_UNLOCK();
+		return EADDRNOTAVAIL;
+	    }
+	    ifp = ifa->ifa_ifp;
+	} else {
+	    if (vifcp->vifc_ifindex > if_index) {
+		VIF_UNLOCK();
+		return ENXIO;
+	    }
+	    ifp = ifnet_byindex(vifcp->vifc_ifindex);
+	    if (ifp == NULL) {
+		VIF_UNLOCK();
+		return ENXIO;
+	    }
 	}
-	ifp = ifa->ifa_ifp;
     }
 
     if (vifcp->vifc_flags & VIFF_TUNNEL) {
diff -ur old/sys/netinet/ip_mroute.h new/sys/netinet/ip_mroute.h
--- old/sys/netinet/ip_mroute.h	Tue Feb  1 00:26:36 2005
+++ new/sys/netinet/ip_mroute.h	Wed May 31 15:02:56 2006
@@ -102,6 +102,14 @@
 	struct	in_addr vifc_rmt_addr;	/* remote address (tunnels only) */
 };
 
+/*
+ * extended vifctl with interface index.
+ */
+struct vifctln {
+	struct vifctl;			/* inherit vifctl fields */
+	int vifc_ifindex;		/* interface index */
+};
+
 #define	VIFF_TUNNEL	0x1		/* vif represents a tunnel end-point */
 #define VIFF_SRCRT	0x2		/* tunnel uses IP source routing */
 #define VIFF_REGISTER	0x4		/* used for PIM Register encap/decap */