[9.18] rem: usr: Remove ineffective TCP fallback after repeated UDP timeouts

When an authoritative server failed to respond to two consecutive
UDP queries, named marked the next retry as TCP but still sent it
over UDP, producing misleading dnstap records. The ineffective
retry path has been removed; a corrected TCP fallback will be
restored in future BIND 9 versions.

Closes #5529

Backport of MR !12022

Merge branch 'backport-5529-fix-tcp-fallback-after-udp-timeouts-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!12050

bin/tests/system/dispatch/ans4/ans.py

@@ -0,0 +1,42 @@
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0.  If a copy of the MPL was not distributed with this
+# file, you can obtain one at https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+from collections.abc import AsyncGenerator
+
+from isctest.asyncserver import (
+    AsyncDnsServer,
+    DnsProtocol,
+    DnsResponseSend,
+    QueryContext,
+    ResponseAction,
+    ResponseDrop,
+    ResponseHandler,
+)
+
+
+class TcpOnlyHandler(ResponseHandler):
+    async def get_responses(
+        self, qctx: QueryContext
+    ) -> AsyncGenerator[ResponseAction, None]:
+        if qctx.protocol == DnsProtocol.TCP:
+            yield DnsResponseSend(qctx.response)
+        else:
+            yield ResponseDrop()
+
+
+def main() -> None:
+    server = AsyncDnsServer()
+    server.install_response_handler(TcpOnlyHandler())
+    server.run()
+
+
+if __name__ == "__main__":
+    main()

bin/tests/system/dispatch/ans4/tcp-only.db

@@ -0,0 +1,15 @@
+; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+;
+; SPDX-License-Identifier: MPL-2.0
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0.  If a copy of the MPL was not distributed with this
+; file, you can obtain one at https://mozilla.org/MPL/2.0/.
+;
+; See the COPYRIGHT file distributed with this work for additional
+; information regarding copyright ownership.
+
+@			3600	SOA	. . 1 1 1 1 1
+@			3600	NS	ns
+ns			3600	A	10.53.0.4
+foo			3600	A	127.0.0.1

bin/tests/system/dispatch/ns1/root.db

@@ -14,3 +14,5 @@
 ns.nil.		300	A	10.53.0.1
 example.	300	NS	ns.example.
 ns.example.	300	A	10.53.0.2
+tcp-only.	300	NS	ns.tcp-only.
+ns.tcp-only.	300	A	10.53.0.4

bin/tests/system/dispatch/tests_tcponly.py

@@ -0,0 +1,33 @@
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0.  If a copy of the MPL was not distributed with this
+# file, you can obtain one at https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+import dns.message
+import pytest
+
+import isctest
+
+pytestmark = pytest.mark.extra_artifacts(
+    [
+        "ans*/ans.run",
+    ]
+)
+
+
+def test_tcponly_not_resolved():
+    """
+    An authoritative server that only answers over TCP is unreachable
+    when its zone is queried over UDP: the resolver does not transparently
+    fall back to TCP after UDP timeouts. (This confirms the expected behavior
+    for this commit; TCP fallback will be restored in the next.)
+    """
+    msg = dns.message.make_query("foo.tcp-only.", "A")
+    res = isctest.query.udp(msg, "10.53.0.2", timeout=15)
+    isctest.check.servfail(res)

lib/dns/resolver.c

@@ -2675,33 +2675,16 @@ resquery_send(resquery_t *query) {
 
 	if (fctx->timeout && (query->options & DNS_FETCHOPT_NOEDNS0) == 0) {
 		isc_sockaddr_t *sockaddr = &query->addrinfo->sockaddr;
-		struct tried *tried;
+		struct tried *tried = triededns(fctx, sockaddr);
 
 		/*
 		 * If this is the first timeout for this server in this
 		 * fetch context, try setting EDNS UDP buffer size to
 		 * the largest UDP response size we have seen from this
 		 * server so far.
-		 *
-		 * If this server has already timed out twice or more in
-		 * this fetch context, force TCP.
 		 */
-		if ((tried = triededns(fctx, sockaddr)) != NULL) {
-			if (tried->count == 1U) {
-				hint = dns_adb_getudpsize(fctx->adb,
-							  query->addrinfo);
-			} else if (tried->count >= 2U) {
-				if ((query->options & DNS_FETCHOPT_TCP) == 0) {
-					/*
-					 * Inform the ADB that we're ending a
-					 * UDP fetch, and turn the query into
-					 * a TCP query.
-					 */
-					dns_adb_endudpfetch(fctx->adb,
-							    query->addrinfo);
-					query->options |= DNS_FETCHOPT_TCP;
-				}
-			}
+		if (tried != NULL && tried->count == 1U) {
+			hint = dns_adb_getudpsize(fctx->adb, query->addrinfo);
 		}
 	}
 	fctx->timeout = false;