From: Vladimír Čunát <vladimir.cunat@nic.cz>
Date: Fri, 3 Sep 2021 16:41:11 +0000 (+0200)
Subject: lib/selection: improve the NO6 behavior
X-Git-Tag: v5.4.2~4^2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=00f6768cc96c123d4c965a1ad73e3f643bf6c101;p=thirdparty%2Fknot-resolver.git

lib/selection: improve the NO6 behavior

With broken IPv6 and no knowledge of IP addresses, we were quite often
chosing to resolve a NS's AAAA and then using it... which wasn't good.
Let's give preference to A here as well.
---

diff --git a/NEWS b/NEWS
index 24813d2b4..d1a080ea5 100644
--- a/NEWS
+++ b/NEWS
@@ -6,6 +6,7 @@ Improvements
 - dns64 module: also map the reverse (PTR) subtree (#478, !1201)
 - dns64 module: allow disabling based on client address (#368, !1201)
 - dns64 module: allow configuring AAAA subnets not allowed in answer (!1201)
+- nameserver selection algorithm: improve IPv6 avoidance if broken (!1207)
 
 Bugfixes
 --------
diff --git a/lib/selection.c b/lib/selection.c
index ea6d6b073..fa5581f1d 100644
--- a/lib/selection.c
+++ b/lib/selection.c
@@ -377,6 +377,37 @@ static void shuffle_choices(struct choice choices[], int choices_len)
 	}
 }
 
+/* Adjust choice from `unresolved` in case of NO6 (broken IPv6). */
+static struct kr_transport unresolved_adjust(struct to_resolve unresolved[],
+					     int unresolved_len, int index)
+{
+	if (unresolved[index].type != KR_TRANSPORT_RESOLVE_AAAA || !no6_is_bad())
+		goto finish;
+	/* AAAA is detected as bad; let's choose randomly from others, if there are any. */
+	int aaaa_count = 0;
+	for (int i = 0; i < unresolved_len; ++i)
+		aaaa_count += (unresolved[i].type == KR_TRANSPORT_RESOLVE_AAAA);
+	if (aaaa_count == unresolved_len)
+		goto finish;
+	/* Chosen index within non-AAAA items. */
+	int i_no6 = kr_rand_bytes(1) % (unresolved_len - aaaa_count);
+	for (int i = 0; i < unresolved_len; ++i) {
+		if (unresolved[i].type == KR_TRANSPORT_RESOLVE_AAAA) {
+			//continue
+		} else if (i_no6 == 0) {
+			index = i;
+			break;
+		} else {
+			--i_no6;
+		}
+	}
+finish:
+	return (struct kr_transport){
+		.protocol = unresolved[index].type,
+		.ns_name = unresolved[index].name
+	};
+}
+
 /* Performs the actual selection (currently variation on epsilon-greedy). */
 struct kr_transport *select_transport(struct choice choices[], int choices_len,
 				      struct to_resolve unresolved[],
@@ -409,10 +440,7 @@ struct kr_transport *select_transport(struct choice choices[], int choices_len,
 		int index = kr_rand_bytes(1) % (choices_len + unresolved_len);
 		if (index < unresolved_len) {
 			// We will resolve a new NS name
-			*transport = (struct kr_transport){
-				.protocol = unresolved[index].type,
-				.ns_name = unresolved[index].name
-			};
+			*transport = unresolved_adjust(unresolved, unresolved_len, index);
 			return transport;
 		} else {
 			chosen = &choices[index - unresolved_len];
@@ -428,10 +456,7 @@ struct kr_transport *select_transport(struct choice choices[], int choices_len,
 	/* Don't try the same server again when there are other choices to be explored */
 	if (chosen->address_state->error_count && unresolved_len) {
 		int index = kr_rand_bytes(1) % unresolved_len;
-		*transport = (struct kr_transport){
-			.ns_name = unresolved[index].name,
-			.protocol = unresolved[index].type,
-		};
+		*transport = unresolved_adjust(unresolved, unresolved_len, index);
 		return transport;
 	}