net: dns: swallow packets that have no useful records in them

There are some poorly compliant mdns responders on the network
that will respond with zero counts on the answers and the additional
records (in addition to the qdcount). So, this removes the checks
in the unpack method since this is a "valid" DNS packet but the
logic is already partially handled in the dns_read code already.

An example packet can be seen in this decode

  0000   33 33 00 00 00 fb 9a f8 c3 0c 07 0b 86 dd 60 07   33............`.
  0010   c4 e4 00 14 11 ff fe 80 00 00 00 00 00 00 98 f8   ................
  0020   c3 ff fe 0c 07 0b ff 02 00 00 00 00 00 00 00 00   ................
  0030   00 00 00 00 00 fb 14 e9 14 e9 00 14 f1 64 00 00   .............d..
  0040   84 00 00 00 00 00 00 00 00 00                     ..........

  User Datagram Protocol, Src Port: 5353, Dst Port: 5353
    Source Port: 5353
    Destination Port: 5353
    Length: 20
    Checksum: 0xf164 [unverified]
    [Checksum Status: Unverified]
    [Stream index: 2]
    [Stream Packet Number: 1]
    [Timestamps]
    UDP payload (12 bytes)
  Multicast Domain Name System (response)
    Transaction ID: 0x0000
    Flags: 0x8400 Standard query response, No error
        1... .... .... .... = Response: Message is a response
        .000 0... .... .... = Opcode: Standard query (0)
        .... .1.. .... .... = Authoritative: Server is an authority
                              for domain
        .... ..0. .... .... = Truncated: Message is not truncated
        .... ...0 .... .... = Recursion desired: Don't do query recursively
        .... .... 0... .... = Recursion available: Server can't do
                              recursive queries
        .... .... .0.. .... = Z: reserved (0)
        .... .... ..0. .... = Answer authenticated: Answer/authority
                              portion was not authenticated by the server
        .... .... ...0 .... = Non-authenticated data: Unacceptable
        .... .... .... 0000 = Reply code: No error (0)
    Questions: 0
    Answer RRs: 0
    Authority RRs: 0
    Additional RRs: 0

Signed-off-by: Charles Hardin <[email protected]>

Author: ckhardin

subsys/net/lib/dns/dns_pack.c

@@ -195,8 +195,6 @@ int dns_unpack_response_header(struct dns_msg_t *msg, int src_id)
 {
 	uint8_t *dns_header;
 	uint16_t size;
-	int qdcount;
-	int ancount;
 	int rc;
 
 	dns_header = msg->msg;
@@ -231,16 +229,6 @@ int dns_unpack_response_header(struct dns_msg_t *msg, int src_id)
 
 	}
 
-	qdcount = dns_unpack_header_qdcount(dns_header);
-	ancount = dns_unpack_header_ancount(dns_header);
-
-	/* For mDNS (when src_id == 0) the query count is 0 so accept
-	 * the packet in that case.
-	 */
-	if ((qdcount < 1 && src_id > 0) || ancount < 1) {
-		return -EINVAL;
-	}
-
 	return 0;
 }
 

subsys/net/lib/dns/resolve.c

@@ -295,9 +295,15 @@ static int dispatcher_cb(struct dns_socket_dispatcher *my_ctx, int sock,
 	}
 
 	ret = dns_read(ctx, dns_data, len, &dns_id, dns_cname, &query_hash);
-	if (!ret) {
-		/* We called the callback already in dns_read() if there
-		 * were no errors.
+	if ((ret == 0) || (ret == DNS_EAI_NODATA)) {
+		/* The callback is already called in dns_read() if there
+		 * were no errors indicated by a return of zero
+		 *
+		 * Also, in the case of no data records to process will
+		 * result in bypassing the callback. However, this goes
+		 * out a similar path as success to allow the request to
+		 * timeout or allow another packet to be processed that
+		 * might have records to validate.
 		 */
 		goto free_buf;
 	}
@@ -1138,14 +1144,29 @@ int dns_validate_msg(struct dns_resolve_context *ctx,
 		goto quit;
 	}
 
-	if (dns_header_qdcount(dns_msg->msg) != 1) {
+	if (dns_header_qdcount(dns_msg->msg) < 1) {
 		/* For mDNS (when dns_id == 0) the query count is 0 */
 		if (*dns_id > 0) {
 			ret = DNS_EAI_FAIL;
 			goto quit;
 		}
 	}
 
+	if (dns_header_ancount(dns_msg->msg) < 1) {
+		/* there are no useful records in this message */
+		if (*dns_id > 0) {
+			ret = DNS_EAI_FAIL;
+			goto quit;
+		}
+
+		/*
+		 * Assume another multicast responder might respond
+		 * differently.
+		 */
+		ret = DNS_EAI_NODATA;
+		goto quit;
+	}
+
 	ret = dns_unpack_response_query(dns_msg);
 	if (ret < 0) {
 		if (ret == -ENOMEM) {