fix dns decoder for response without question (#843)

README.md

@@ -1,7 +1,7 @@
 <p align="center">
 <img src="https://goreportcard.com/badge/github.com/dmachard/go-dns-collector" alt="Go Report"/>
 <img src="https://img.shields.io/badge/go%20version-min%201.21-green" alt="Go version"/>
-<img src="https://img.shields.io/badge/go%20tests-507-green" alt="Go tests"/>
+<img src="https://img.shields.io/badge/go%20tests-508-green" alt="Go tests"/>
 <img src="https://img.shields.io/badge/go%20bench-21-green" alt="Go bench"/>
 <img src="https://img.shields.io/badge/go%20lines-31679-green" alt="Go lines"/>
 </p>

dnsutils/dns_parser.go

@@ -209,6 +209,8 @@ func DecodePayload(dm *DNSMessage, header *DNSHeader, config *pkgconfig.Config)
 		dm.DNS.Qtype = RdatatypeToString(dnsRRtype)
 		dm.DNS.Qclass = ClassToString(dnsQclass)
 		payloadOffset = offsetrr
+	} else {
+		payloadOffset = DNSLen
 	}
 
 	// decode DNS answers
@@ -242,8 +244,9 @@ func DecodePayload(dm *DNSMessage, header *DNSHeader, config *pkgconfig.Config)
 			return &decodingError{part: "authority records", err: err}
 		}
 	}
+
+	// decode additional answers
 	if header.Arcount > 0 {
-		// decode additional answers
 		answers, _, err := DecodeAnswer(header.Arcount, payloadOffset, dm.DNS.Payload)
 		if err == nil { // nolint
 			dm.DNS.DNSRRs.Records = answers

dnsutils/dns_parser_payload_test.go

@@ -1042,3 +1042,47 @@ func TestDecodePayload_UpdateQuery(t *testing.T) {
 		t.Error("expected no error on decode", err)
 	}
 }
+
+func TestDecodePayload_MDNSResponseWithNoQuestion(t *testing.T) {
+	payload := []byte{
+		0x00, 0x00, 0x84, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x03, 0x0b, 0x5f, 0x67, 0x6f,
+		0x6f, 0x67, 0x6c, 0x65, 0x63, 0x61, 0x73, 0x74, 0x04, 0x5f, 0x74, 0x63, 0x70, 0x05, 0x6c, 0x6f,
+		0x63, 0x61, 0x6c, 0x00, 0x00, 0x0c, 0x00, 0x01, 0x00, 0x00, 0x00, 0x78, 0x00, 0x35, 0x32, 0x53,
+		0x48, 0x49, 0x45, 0x4c, 0x44, 0x2d, 0x41, 0x6e, 0x64, 0x72, 0x6f, 0x69, 0x64, 0x2d, 0x54, 0x56,
+		0x2d, 0x30, 0x66, 0x33, 0x31, 0x30, 0x63, 0x32, 0x63, 0x39, 0x66, 0x62, 0x62, 0x64, 0x61, 0x64,
+		0x64, 0x31, 0x39, 0x39, 0x30, 0x30, 0x38, 0x31, 0x61, 0x61, 0x66, 0x35, 0x37, 0x65, 0x66, 0x38,
+		0x33, 0xc0, 0x0c, 0xc0, 0x2e, 0x00, 0x10, 0x80, 0x01, 0x00, 0x00, 0x11, 0x94, 0x00, 0xd2, 0x23,
+		0x69, 0x64, 0x3d, 0x30, 0x66, 0x33, 0x31, 0x30, 0x63, 0x32, 0x63, 0x39, 0x66, 0x62, 0x62, 0x64,
+		0x61, 0x64, 0x64, 0x31, 0x39, 0x39, 0x30, 0x30, 0x38, 0x31, 0x61, 0x61, 0x66, 0x35, 0x37, 0x65,
+		0x66, 0x38, 0x33, 0x23, 0x63, 0x64, 0x3d, 0x42, 0x35, 0x35, 0x36, 0x39, 0x31, 0x37, 0x42, 0x30,
+		0x36, 0x33, 0x38, 0x46, 0x46, 0x32, 0x43, 0x35, 0x30, 0x45, 0x42, 0x43, 0x33, 0x34, 0x34, 0x41,
+		0x45, 0x35, 0x31, 0x46, 0x39, 0x43, 0x37, 0x03, 0x72, 0x6d, 0x3d, 0x05, 0x76, 0x65, 0x3d, 0x30,
+		0x35, 0x14, 0x6d, 0x64, 0x3d, 0x53, 0x48, 0x49, 0x45, 0x4c, 0x44, 0x20, 0x41, 0x6e, 0x64, 0x72,
+		0x6f, 0x69, 0x64, 0x20, 0x54, 0x56, 0x12, 0x69, 0x63, 0x3d, 0x2f, 0x73, 0x65, 0x74, 0x75, 0x70,
+		0x2f, 0x69, 0x63, 0x6f, 0x6e, 0x2e, 0x70, 0x6e, 0x67, 0x09, 0x66, 0x6e, 0x3d, 0x53, 0x48, 0x49,
+		0x45, 0x4c, 0x44, 0x09, 0x63, 0x61, 0x3d, 0x34, 0x36, 0x33, 0x33, 0x36, 0x35, 0x04, 0x73, 0x74,
+		0x3d, 0x31, 0x0f, 0x62, 0x73, 0x3d, 0x46, 0x41, 0x38, 0x46, 0x46, 0x36, 0x46, 0x30, 0x45, 0x30,
+		0x38, 0x38, 0x04, 0x6e, 0x66, 0x3d, 0x31, 0x09, 0x63, 0x74, 0x3d, 0x30, 0x39, 0x31, 0x43, 0x44,
+		0x34, 0x13, 0x72, 0x72, 0x3d, 0x41, 0x6e, 0x64, 0x72, 0x6f, 0x69, 0x64, 0x4e, 0x61, 0x74, 0x69,
+		0x76, 0x65, 0x41, 0x70, 0x70, 0x0b, 0x72, 0x73, 0x3d, 0x4a, 0x65, 0x6c, 0x6c, 0x79, 0x66, 0x69,
+		0x6e, 0xc0, 0x2e, 0x00, 0x21, 0x80, 0x01, 0x00, 0x00, 0x00, 0x78, 0x00, 0x2d, 0x00, 0x00, 0x00,
+		0x00, 0x1f, 0x49, 0x24, 0x30, 0x66, 0x33, 0x31, 0x30, 0x63, 0x32, 0x63, 0x2d, 0x39, 0x66, 0x62,
+		0x62, 0x2d, 0x64, 0x61, 0x64, 0x64, 0x2d, 0x31, 0x39, 0x39, 0x30, 0x2d, 0x30, 0x38, 0x31, 0x61,
+		0x61, 0x66, 0x35, 0x37, 0x65, 0x66, 0x38, 0x33, 0xc0, 0x1d, 0xc1, 0x53, 0x00, 0x1c, 0x80, 0x01,
+		0x00, 0x00, 0x00, 0x78, 0x00, 0x10, 0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x8c, 0x3e,
+		0x51, 0x15, 0x8e, 0x6d, 0xa2, 0x96,
+	}
+
+	dm := DNSMessage{}
+	dm.DNS.Payload = payload
+	dm.DNS.Length = len(payload)
+
+	header, err := DecodeDNS(payload)
+	if err != nil {
+		t.Errorf("unexpected error when decoding header: %v", err)
+	}
+
+	if err = DecodePayload(&dm, &header, pkgconfig.GetDefaultConfig()); err != nil {
+		t.Error("expected no error on decode", err)
+	}
+}