commit 2b5481fcdafd0b870d445a9712edf3611689bffc
Author: Ryota Ozaki <ozaki-r@iij.ad.jp>
Date:   Wed Mar 22 12:24:47 2017 +0900

    Check reachable gateway only if a new gateway is specified and different from the original

diff --git a/sys/net/rtsock.c b/sys/net/rtsock.c
index 1f591bd..4ffe374 100644
--- a/sys/net/rtsock.c
+++ b/sys/net/rtsock.c
@@ -599,22 +599,29 @@ route_output_change(struct rtentry *rt, struct rt_addrinfo *info,
     struct rt_xmsghdr *rtm)
 {
 	int error = 0;
-	struct ifnet *ifp, *new_ifp;
-	struct ifaddr *ifa, *new_ifa;
+	struct ifnet *ifp = NULL, *new_ifp;
+	struct ifaddr *ifa = NULL, *new_ifa;
 	struct psref psref_ifa, psref_new_ifa, psref_ifp;
+	bool newgw;
 
 	/*
-	 * new gateway could require new ifaddr, ifp;
+	 * New gateway could require new ifaddr, ifp;
 	 * flags may also be different; ifp may be specified
 	 * by ll sockaddr when protocol address is ambiguous
 	 */
-	ifp = rt_getifp(info, &psref_ifp);
-	ifa = rt_getifa(info, &psref_ifa);
-	if (ifa == NULL) {
-		error = ENETUNREACH;
-		goto out;
+	newgw = info->rti_info[RTAX_GATEWAY] != NULL &&
+	    sockaddr_cmp(info->rti_info[RTAX_GATEWAY], rt->rt_gateway) != 0;
+
+	if (newgw || info->rti_info[RTAX_IFP] != NULL ||
+	    info->rti_info[RTAX_IFA] != NULL) {
+		ifp = rt_getifp(info, &psref_ifp);
+		ifa = rt_getifa(info, &psref_ifa);
+		if (ifa == NULL) {
+			error = ENETUNREACH;
+			goto out;
+		}
 	}
-	if (info->rti_info[RTAX_GATEWAY]) {
+	if (newgw) {
 		error = rt_setgate(rt, info->rti_info[RTAX_GATEWAY]);
 		if (error != 0)
 			goto out;