MINOR: dns: no expected DNS record type found

Some DNS responses may be valid from a protocol point of view, but may
not contain any information considered as interested by the requester..
Purpose of the flag DNS_RESP_NO_EXPECTED_RECORD introduced by this patch is
to allow reporting such situation.

When this happens, a new DNS query is sent with a new query type.

For now, the function only expect A and AAAA query types which is enough
to cover current cases.
In a next future, it will be up to the caller to tell the function which
query types are expected.

include/types/dns.h

@@ -199,6 +199,7 @@ enum {
 	DNS_RESP_CNAME_ERROR,		/* error when resolving a CNAME in an atomic response */
 	DNS_RESP_TIMEOUT,		/* DNS server has not answered in time */
 	DNS_RESP_TRUNCATED,		/* DNS response is truncated */
+	DNS_RESP_NO_EXPECTED_RECORD,	/* No expected records were found in the response */
 };
 
 /* return codes after searching an IP in a DNS response buffer, using a family preference */

src/dns.c

@@ -222,6 +222,11 @@ void dns_resolve_recv(struct dgram_conn *dgram)
 			nameserver->counters.truncated += 1;
 			resolution->requester_error_cb(resolution, DNS_RESP_TRUNCATED);
 			continue;
+
+		case DNS_RESP_NO_EXPECTED_RECORD:
+			nameserver->counters.other += 1;
+			resolution->requester_error_cb(resolution, DNS_RESP_NO_EXPECTED_RECORD);
+			continue;
 		}
 
 		nameserver->counters.valid += 1;
@@ -334,12 +339,18 @@ void dns_update_resolvers_timeout(struct dns_resolvers *resolvers)
 int dns_validate_dns_response(unsigned char *resp, unsigned char *bufend, char *dn_name, int dn_name_len)
 {
 	unsigned char *reader, *cname, *ptr;
-	int i, len, flags, type, ancount, cnamelen;
+	int i, len, flags, type, ancount, cnamelen, expected_record;
 
 	reader = resp;
 	cname = NULL;
 	cnamelen = 0;
 	len = 0;
+	expected_record = 0; /* flag to report if at least one expected record type is found in the response.
+			      * For now, only records containing an IP address (A and AAAA) are
+			      * considered as expected.
+			      * Later, this function may be updated to let the caller decide what type
+			      * of record is expected to consider the response as valid. (SRV or TXT types)
+			      */
 
 	/* move forward 2 bytes for the query id */
 	reader += 2;
@@ -540,6 +551,7 @@ int dns_validate_dns_response(unsigned char *resp, unsigned char *bufend, char *
 				/* ipv4 is stored on 4 bytes */
 				if (len != 4)
 					return DNS_RESP_INVALID;
+				expected_record = 1;
 				break;
 
 			case DNS_RTYPE_CNAME:
@@ -551,6 +563,7 @@ int dns_validate_dns_response(unsigned char *resp, unsigned char *bufend, char *
 				/* ipv6 is stored on 16 bytes */
 				if (len != 16)
 					return DNS_RESP_INVALID;
+				expected_record = 1;
 				break;
 		} /* switch (record type) */
 
@@ -558,6 +571,9 @@ int dns_validate_dns_response(unsigned char *resp, unsigned char *bufend, char *
 		reader += len;
 	} /* for i 0 to ancount */
 
+	if (expected_record == 0)
+		return DNS_RESP_NO_EXPECTED_RECORD;
+
 	return DNS_RESP_VALID;
 }
 

src/server.c

@@ -2120,6 +2120,7 @@ int snr_resolution_error_cb(struct dns_resolution *resolution, int error_code)
 		case DNS_RESP_ANCOUNT_ZERO:
 		case DNS_RESP_TRUNCATED:
 		case DNS_RESP_ERROR:
+		case DNS_RESP_NO_EXPECTED_RECORD:
 			qtype_any = resolution->query_type == DNS_RTYPE_ANY;
 			res_preferred_afinet = resolution->resolver_family_priority == AF_INET && resolution->query_type == DNS_RTYPE_A;
 			res_preferred_afinet6 = resolution->resolver_family_priority == AF_INET6 && resolution->query_type == DNS_RTYPE_AAAA;