lib/nsrep: be more aggresive with TCP for broken delegations

libuv doesn't do connected UDP sockets, so we can't get ICMP unreachable
otherwise

lib/nsrep.c

@@ -123,9 +123,12 @@ static int eval_nsrep(const char *k, void *v, void *baton)
 		update_nsrep(ns, (const knot_dname_t *)k, addr, score);
 		ns->reputation = reputation;
 	} else {
-		/* With 5% chance, probe server with a probability given by its RTT / MAX_RTT */
-		unsigned roll = kr_rand_uint(KR_NS_MAX_SCORE);
-		if ((roll % 100 < 5) && (roll >= score)) {
+		/* With 10% chance, probe server with a probability given by its RTT / MAX_RTT */
+		if ((kr_rand_uint(100) < 10) && (kr_rand_uint(KR_NS_MAX_SCORE) >= score)) {
+			/* If this is a low-reliability probe, go with TCP to get ICMP reachability check. */
+			if (score >= KR_NS_LONG) {
+				qry->flags |= QUERY_TCP;
+			}
 			update_nsrep(ns, (const knot_dname_t *)k, addr, score);
 			ns->reputation = reputation;
 			return 1; /* Stop evaluation */

lib/nsrep.h

@@ -33,6 +33,7 @@ struct kr_query;
 enum kr_ns_score {
 	KR_NS_MAX_SCORE = KR_CONN_RTT_MAX,
 	KR_NS_TIMEOUT   = (95 * KR_NS_MAX_SCORE) / 100,
+	KR_NS_LONG      = (3 * KR_NS_TIMEOUT) / 4,
 	KR_NS_UNKNOWN   = KR_NS_TIMEOUT / 2,
 	KR_NS_GLUED     = 10
 };

lib/resolve.c

@@ -380,12 +380,10 @@ int kr_resolve_consume(struct kr_request *request, knot_pkt_t *packet)
 		state = knot_overlay_consume(&request->overlay, packet);
 	}
 
-	/* Resolution failed, invalidate current NS and reset to UDP. */
+	/* Resolution failed, invalidate current NS. */
 	if (state == KNOT_STATE_FAIL) {
 		kr_nsrep_update_rtt(&qry->ns, KR_NS_TIMEOUT, ctx->cache_rtt);
-		if (invalidate_ns(rplan, qry) == 0) {
-			qry->flags &= ~QUERY_TCP;
-		}
+		invalidate_ns(rplan, qry); /* @note Stay with TCP as it's likely this is broken delegation. */
 	/* Track RTT for iterative answers */
 	} else if (!(qry->flags & QUERY_CACHED)) {
 		struct timeval now;