Phase 4.1: Error handling, edge cases, and robustness (#11)

internal/traverse/response.go

@@ -16,6 +16,9 @@ const (
 	RespNODATA
 	RespNXDOMAIN
 	RespSERVFAIL
+	RespREFUSED
+	RespNOTIMPL
+	RespCNAMELoop
 	RespError
 )
 
@@ -33,6 +36,12 @@ func (rt ResponseType) String() string {
 		return "nxdomain"
 	case RespSERVFAIL:
 		return "servfail"
+	case RespREFUSED:
+		return "refused"
+	case RespNOTIMPL:
+		return "notimp"
+	case RespCNAMELoop:
+		return "cname_loop"
 	case RespError:
 		return "error"
 	default:
@@ -41,11 +50,12 @@ func (rt ResponseType) String() string {
 }
 
 type Response struct {
-	Referral *Referral
-	Server   net.IP
-	Cache    *InfoCache
-	Decoded  *dns.DecodedResponse
-	Type     ResponseType
+	Referral     *Referral
+	Server       net.IP
+	Cache        *InfoCache
+	Decoded      *dns.DecodedResponse
+	Type         ResponseType
+	ErrorMessage string
 }
 
 func NewResponse(ref *Referral, server net.IP, cache *InfoCache) *Response {
@@ -59,15 +69,28 @@ func NewResponse(ref *Referral, server net.IP, cache *InfoCache) *Response {
 func (r *Response) Process(msg *miekgdns.Msg) *Response {
 	if msg == nil {
 		r.Type = RespError
+		r.ErrorMessage = "nil DNS response"
 		return r
 	}
 
 	r.Decoded = dns.DecodeResponse(msg)
 	if r.Decoded == nil {
 		r.Type = RespError
+		r.ErrorMessage = "failed to decode DNS response"
 		return r
 	}
 
+	// Synthesize CNAME from DNAME when the server didn't include a synthesized CNAME record.
+	if len(r.Decoded.CNAMEChain) == 0 && r.Referral != nil && len(r.Decoded.DNAMEMappings) > 0 {
+		for _, dm := range r.Decoded.DNAMEMappings {
+			synthesized := dns.SynthesizeCNAMEFromDNAME(r.Referral.Name, dm.Owner, dm.Target)
+			if synthesized != "" {
+				r.Decoded.CNAMEChain = append(r.Decoded.CNAMEChain, synthesized)
+				break
+			}
+		}
+	}
+
 	r.Type = r.classify()
 	return r
 }
@@ -78,6 +101,10 @@ func (r *Response) classify() ResponseType {
 		return RespNXDOMAIN
 	case dns.ResponseSERVFAIL:
 		return RespSERVFAIL
+	case dns.ResponseREFUSED:
+		return RespREFUSED
+	case dns.ResponseNOTIMPL:
+		return RespNOTIMPL
 	case dns.ResponseAnswer:
 		if len(r.Decoded.CNAMEChain) > 0 && !r.hasFinalAnswer() {
 			return RespCNAMEFollow
@@ -94,7 +121,11 @@ func (r *Response) classify() ResponseType {
 
 func (r *Response) hasFinalAnswer() bool {
 	for _, rr := range r.Decoded.Answers {
-		if _, ok := rr.(*miekgdns.CNAME); ok {
+		switch rr.(type) {
+		case *miekgdns.CNAME, *miekgdns.DNAME, *miekgdns.RRSIG:
+			// CNAME and DNAME are redirect records, not final answers.
+			// RRSIG is a DNSSEC signature record — it covers the CNAME/DNAME
+			// but is not itself the answer to the original question type.
 			continue
 		}
 		return true
@@ -200,7 +231,7 @@ func (r *Response) resolveGlue(child *Referral) {
 
 func (r *Response) IsTerminal() bool {
 	switch r.Type {
-	case RespAnswer, RespNODATA, RespNXDOMAIN, RespSERVFAIL, RespError:
+	case RespAnswer, RespNODATA, RespNXDOMAIN, RespSERVFAIL, RespREFUSED, RespNOTIMPL, RespCNAMELoop, RespError:
 		return true
 	default:
 		return false