Index: sys/net/route/nhop_ctl.c
===================================================================
--- sys/net/route/nhop_ctl.c
+++ sys/net/route/nhop_ctl.c
@@ -381,7 +381,10 @@
 	/* 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 */
+	if ((nh->nh_priv->rt_flags &
+	    (RTF_BLACKHOLE | RTF_REJECT | RTF_GATEWAY)) != 0)
+		return (EINVAL);
 
 	/* Allow some flags (FLAG1,STATIC,BLACKHOLE,REJECT) to be toggled on change. */
 	nh->nh_priv->rt_flags &= ~RTF_FMASK;