From e7e29278a8041fe4dd734c61402b679e0a8cee3d Mon Sep 17 00:00:00 2001
From: Mark Andrews <marka@isc.org>
Date: Mon, 8 May 2023 17:39:51 +1000
Subject: [PATCH] Handle FORMERR on unknown EDNS option that are echoed

If the resolver received a FORMERR response to a request with
an DNS COOKIE option present that echoes the option back, resend
the request without an DNS COOKIE option present.

(cherry picked from commit f3b24ba789bed8e561519909709e1671bb290c7f)
---
 lib/dns/include/dns/message.h |  1 +
 lib/dns/resolver.c            | 42 ++++++++++++++++++++++++-----------
 2 files changed, 30 insertions(+), 13 deletions(-)

diff --git a/lib/dns/include/dns/message.h b/lib/dns/include/dns/message.h
index d769fe94dc..b5d9a5a168 100644
--- a/lib/dns/include/dns/message.h
+++ b/lib/dns/include/dns/message.h
@@ -279,6 +279,7 @@ struct dns_message {
 	unsigned int free_saved	      : 1;
 	unsigned int cc_ok	      : 1;
 	unsigned int cc_bad	      : 1;
+	unsigned int cc_echoed	      : 1;
 	unsigned int tkey	      : 1;
 	unsigned int rdclass_set      : 1;
 	unsigned int fuzzing	      : 1;
diff --git a/lib/dns/resolver.c b/lib/dns/resolver.c
index 0dddb43de7..f2c2e1cace 100644
--- a/lib/dns/resolver.c
+++ b/lib/dns/resolver.c
@@ -8380,12 +8380,18 @@ rctx_opt(respctx_t *rctx) {
 				    memcmp(cookie, optvalue,
 					   CLIENT_COOKIE_SIZE) == 0)
 				{
-					query->rmessage->cc_ok = 1;
-					inc_stats(fctx->res,
-						  dns_resstatscounter_cookieok);
-					addrinfo = query->addrinfo;
-					dns_adb_setcookie(fctx->adb, addrinfo,
-							  optvalue, optlen);
+					if (optlen == CLIENT_COOKIE_SIZE) {
+						query->rmessage->cc_echoed = 1;
+					} else {
+						query->rmessage->cc_ok = 1;
+						inc_stats(
+							fctx->res,
+							dns_resstatscounter_cookieok);
+						addrinfo = query->addrinfo;
+						dns_adb_setcookie(
+							fctx->adb, addrinfo,
+							optvalue, optlen);
+					}
 				} else {
 					query->rmessage->cc_bad = 1;
 				}
@@ -10061,13 +10067,23 @@ rctx_badserver(respctx_t *rctx, isc_result_t result) {
 		add_bad_edns(fctx, &query->addrinfo->sockaddr);
 		inc_stats(fctx->res, dns_resstatscounter_edns0fail);
 	} else if (rcode == dns_rcode_formerr) {
-		/*
-		 * The server (or forwarder) doesn't understand us,
-		 * but others might.
-		 */
-		rctx->next_server = true;
-		rctx->broken_server = DNS_R_REMOTEFORMERR;
-		log_formerr(fctx, "server sent FORMERR");
+		if (query->rmessage->cc_echoed) {
+			/*
+			 * Retry without DNS COOKIE.
+			 */
+			query->addrinfo->flags |= FCTX_ADDRINFO_NOCOOKIE;
+			rctx->resend = true;
+			log_formerr(fctx, "server sent FORMERR with echoed DNS "
+					  "COOKIE");
+		} else {
+			/*
+			 * The server (or forwarder) doesn't understand us,
+			 * but others might.
+			 */
+			rctx->next_server = true;
+			rctx->broken_server = DNS_R_REMOTEFORMERR;
+			log_formerr(fctx, "server sent FORMERR");
+		}
 	} else if (rcode == dns_rcode_badvers) {
 		unsigned int version;
 #if DNS_EDNS_VERSION > 0