Merge branch '2982-servfail-servestale-duplicate-queries-v9_18' into 'v9_18'

[v9_18] Don't enable serve-stale on duplicate queries

See merge request isc-projects/bind9!6642

CHANGES

@@ -1,3 +1,7 @@
+5936.	[bug]		Don't enable serve-stale for lookups that error because
+			it is a duplicate query or a query that would be
+			dropped. [GL #2982]
+
 5935.	[bug]		Fix DiG lookup reference counting bug, which could
 			be observed in NSSEARCH mode. [GL #3478]
 

doc/notes/notes-current.rst

@@ -40,4 +40,7 @@ Feature Changes
 Bug Fixes
 ~~~~~~~~~
 
-- None.
+- Fix a serve-stale bug, where BIND would try to return stale data from cache
+  for lookups that received duplicate queries or queries that would be dropped.
+  This bug resulted in premature SERVFAIL responses, and has now been resolved.
+  :gl:`#2982`

lib/ns/query.c

@@ -7575,6 +7575,14 @@ query_usestale(query_ctx_t *qctx, isc_result_t result) {
 		return (false);
 	}
 
+	if (result == DNS_R_DUPLICATE || result == DNS_R_DROP) {
+		/*
+		 * Don't enable serve-stale if the result signals a duplicate
+		 * query or query that is being dropped.
+		 */
+		return (false);
+	}
+
 	qctx_clean(qctx);
 	qctx_freedata(qctx);