Index: sys/netinet6/in6.h
===================================================================
RCS file: /home/ncvs/src/sys/netinet6/in6.h,v
retrieving revision 1.36.2.9
diff -u -p -r1.36.2.9 in6.h
--- sys/netinet6/in6.h	1 Sep 2008 22:57:56 -0000	1.36.2.9
+++ sys/netinet6/in6.h	28 Sep 2008 21:27:09 -0000
@@ -575,7 +575,8 @@ struct ip6_mtuinfo {
 /* to define items, should talk with KAME guys first, for *BSD compatibility */
 #define IPV6CTL_STEALTH		45
 #define IPV6CTL_RTHDR0_ALLOWED  46
-#define IPV6CTL_MAXID		47
+#define	ICMPV6CTL_ND6_ONLINKNSRFC4861	47
+#define IPV6CTL_MAXID		48
 #endif /* __BSD_VISIBLE */
 
 /*
Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /home/ncvs/src/sys/netinet6/in6_proto.c,v
retrieving revision 1.32.2.9
diff -u -p -r1.32.2.9 in6_proto.c
--- sys/netinet6/in6_proto.c	1 Sep 2008 22:57:56 -0000	1.32.2.9
+++ sys/netinet6/in6_proto.c	28 Sep 2008 21:26:24 -0000
@@ -354,6 +354,7 @@ DOMAIN_SET(inet6);
 #ifndef	IPV6_SENDREDIRECTS
 #define	IPV6_SENDREDIRECTS	1
 #endif
+int	nd6_onlink_ns_rfc4861 = 0; /* allow 'on-link' nd6 NS (as in RFC 4861) */
 
 int	ip6_forwarding = IPV6FORWARDING;	/* act as router? */
 int	ip6_sendredirects = IPV6_SENDREDIRECTS;
@@ -553,3 +554,6 @@ SYSCTL_INT(_net_inet6_icmp6, ICMPV6CTL_N
 	nd6_maxnudhint, CTLFLAG_RW,	&nd6_maxnudhint, 0, "");
 SYSCTL_INT(_net_inet6_icmp6, ICMPV6CTL_ND6_DEBUG,
 	nd6_debug, CTLFLAG_RW,	&nd6_debug,		0, "");
+SYSCTL_INT(_net_inet6_icmp6, ICMPV6CTL_ND6_ONLINKNSRFC4861,
+	nd6_onlink_ns_rfc4861, CTLFLAG_RW, &nd6_onlink_ns_rfc4861, 0,
+	"Accept 'on-link' nd6 NS in compliance with RFC 4861.");
Index: sys/netinet6/nd6.h
===================================================================
RCS file: /home/ncvs/src/sys/netinet6/nd6.h,v
retrieving revision 1.19.2.3
diff -u -p -r1.19.2.3 nd6.h
--- sys/netinet6/nd6.h	1 Sep 2008 22:57:56 -0000	1.19.2.3
+++ sys/netinet6/nd6.h	28 Sep 2008 21:26:24 -0000
@@ -339,6 +339,7 @@ extern struct llinfo_nd6 llinfo_nd6;
 extern struct nd_drhead nd_defrouter;
 extern struct nd_prhead nd_prefix;
 extern int nd6_debug;
+extern int nd6_onlink_ns_rfc4861;
 
 #define nd6log(x)	do { if (nd6_debug) log x; } while (/*CONSTCOND*/ 0)
 
Index: sys/netinet6/nd6_nbr.c
===================================================================
RCS file: /home/ncvs/src/sys/netinet6/nd6_nbr.c,v
retrieving revision 1.29.2.10
diff -u -p -r1.29.2.10 nd6_nbr.c
--- sys/netinet6/nd6_nbr.c	1 Sep 2008 22:57:56 -0000	1.29.2.10
+++ sys/netinet6/nd6_nbr.c	28 Sep 2008 21:26:24 -0000
@@ -148,6 +148,24 @@ nd6_ns_input(m, off, icmp6len)
 			    "(wrong ip6 dst)\n"));
 			goto bad;
 		}
+	} else if (!nd6_onlink_ns_rfc4861) {
+		struct sockaddr_in6 src_sa6;
+
+		/*
+		 * According to recent IETF discussions, it is not a good idea
+		 * to accept a NS from an address which would not be deemed
+		 * to be a neighbor otherwise.  This point is expected to be
+		 * clarified in future revisions of the specification.
+		 */
+		bzero(&src_sa6, sizeof(src_sa6));
+		src_sa6.sin6_family = AF_INET6;
+		src_sa6.sin6_len = sizeof(src_sa6);
+		src_sa6.sin6_addr = saddr6;
+		if (!nd6_is_addr_neighbor(&src_sa6, ifp)) {
+			nd6log((LOG_INFO, "nd6_ns_input: "
+				"NS packet from non-neighbor\n"));
+			goto bad;
+		}
 	}
 
 	if (IN6_IS_ADDR_MULTICAST(&taddr6)) {