Index: sys/net/route/nhop_ctl.c
===================================================================
--- sys/net/route/nhop_ctl.c
+++ sys/net/route/nhop_ctl.c
@@ -376,12 +376,16 @@
 alter_nhop_from_info(struct nhop_object *nh, struct rt_addrinfo *info)
 {
 	struct sockaddr *info_gw;
+	int a;
 	int error;
 
 	/* Update MTU if set in the request*/
 	set_nhop_mtu_from_info(nh, info);
 
-	/* XXX: allow only one of BLACKHOLE,REJECT,GATEWAY */
+	/* allow only one of BLACKHOLE,REJECT,GATEWAY */
+	a = nh->nh_priv->rt_flags & (RTF_BLACKHOLE | RTF_REJECT | RTF_GATEWAY);
+	if (a && (a & (a - 1)))
+		return (EINVAL);
 
 	/* Allow some flags (FLAG1,STATIC,BLACKHOLE,REJECT) to be toggled on change. */
 	nh->nh_priv->rt_flags &= ~RTF_FMASK;