Preserve authenticated DNS responses for SMTP policy

src/libs/xpl/dns.c

@@ -416,24 +416,35 @@ XplDns_Result *
 _XplDns_Query(const char *domain, XplDns_RecordType type)
 {
 	XplDns_Result *result;
+	struct __res_state resolver;
 	unsigned char answer_buffer[4096];
 	int answer_len = -1;
+	int resolver_error = 0;
 
 	result = MemMalloc(sizeof(XplDns_Result));
 	result->list = NULL;
 	result->status = XPLDNS_FAIL;
 	result->dnssec_authenticated = 0;
 
-	// FIXME: is this re-entrant _everywhere_ ?
 	/* DNS mail records are Internet-class data.  ns_c_any is class 255,
 	 * not an instruction to accept any result, and authoritative servers
-	 * are not required to answer MX/A/TXT questions sent with that class. */
-	answer_len = res_query(domain, ns_c_in, type, answer_buffer, 4096);
+	 * are not required to answer MX/A/TXT questions sent with that class.
+	 * Use a per-call resolver state because SMTP queue workers run in
+	 * parallel.  glibc strips the AD bit unless RES_TRUSTAD is explicitly
+	 * enabled, while RES_USE_DNSSEC requests the DNSSEC records needed by
+	 * a validating resolver. */
+	memset(&resolver, 0, sizeof(resolver));
+	if (res_ninit(&resolver) != 0) return result;
+	resolver.options |= RES_USE_DNSSEC | RES_TRUSTAD;
+	answer_len = res_nquery(&resolver, domain, ns_c_in, type,
+	                        answer_buffer, sizeof(answer_buffer));
+	resolver_error = resolver.res_h_errno;
+	res_nclose(&resolver);
 
 	if (answer_len < 0) {
 		// didn't get a sensible answer
-		if (h_errno) { 
-			result->status = (XplDns_ResultCode) h_errno;
+		if (resolver_error) {
+			result->status = (XplDns_ResultCode) resolver_error;
 			return result;
 		} else {
 			return result;