Improve DNS output: distinguish NS resolution failures from domain errors (#20)

internal/traverse/response.go

@@ -20,6 +20,11 @@ const (
 	RespNOTIMPL
 	RespCNAMELoop
 	RespError
+	// RespNSResolutionFailed indicates that the traversal could not resolve the
+	// IP address of an in-bailiwick nameserver.  The domain may still be
+	// reachable in practice (e.g. via glue records held by the registry), but
+	// the iterative traversal could not complete that path.
+	RespNSResolutionFailed
 )
 
 func (rt ResponseType) String() string {
@@ -44,6 +49,8 @@ func (rt ResponseType) String() string {
 		return "cname_loop"
 	case RespError:
 		return "error"
+	case RespNSResolutionFailed:
+		return "ns_error"
 	default:
 		return "unknown"
 	}
@@ -231,7 +238,7 @@ func (r *Response) resolveGlue(child *Referral) {
 
 func (r *Response) IsTerminal() bool {
 	switch r.Type {
-	case RespAnswer, RespNODATA, RespNXDOMAIN, RespSERVFAIL, RespREFUSED, RespNOTIMPL, RespCNAMELoop, RespError:
+	case RespAnswer, RespNODATA, RespNXDOMAIN, RespSERVFAIL, RespREFUSED, RespNOTIMPL, RespCNAMELoop, RespError, RespNSResolutionFailed:
 		return true
 	default:
 		return false