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

internal/traverse/response_test.go

@@ -246,6 +246,7 @@ func TestResponseIsTerminal(t *testing.T) {
 		{RespNXDOMAIN, true},
 		{RespSERVFAIL, true},
 		{RespError, true},
+		{RespNSResolutionFailed, true},
 		{RespReferral, false},
 		{RespCNAMEFollow, false},
 	}
@@ -272,6 +273,7 @@ func TestResponseTypeString(t *testing.T) {
 		{RespNXDOMAIN, "nxdomain"},
 		{RespSERVFAIL, "servfail"},
 		{RespError, "error"},
+		{RespNSResolutionFailed, "ns_error"},
 	}
 
 	for _, tt := range tests {

internal/traverse/traverser.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"strings"
 	"sync"
 	"time"
 
@@ -231,10 +232,15 @@ func (t *Traverser) processReferral(ctx context.Context, ref *Referral, cache *I
 			ref.State = StateResolved
 		} else {
 			addrs, err := t.ResolveNS(ctx, ref.Name, cache, visitedCopy, t.depth)
+			nsName := strings.TrimSuffix(ref.Bailiwick, ".")
+			if nsName == "" || nsName == "." {
+				nsName = strings.TrimSuffix(ref.Name, ".")
+			}
 			if err != nil {
 				return &Response{
-					Referral: ref,
-					Type:     RespError,
+					Referral:     ref,
+					Type:         RespNSResolutionFailed,
+					ErrorMessage: fmt.Sprintf("nameserver %s could not be resolved", nsName),
 				}
 			}
 			if len(addrs) > 0 {
@@ -242,8 +248,9 @@ func (t *Traverser) processReferral(ctx context.Context, ref *Referral, cache *I
 				ref.State = StateResolved
 			} else {
 				return &Response{
-					Referral: ref,
-					Type:     RespError,
+					Referral:     ref,
+					Type:         RespNSResolutionFailed,
+					ErrorMessage: fmt.Sprintf("nameserver %s could not be resolved", nsName),
 				}
 			}
 		}
@@ -360,7 +367,7 @@ func (t *Traverser) ResolveNS(ctx context.Context, nsName string, cache *InfoCac
 			break
 		}
 
-		if resp.Type == RespSERVFAIL || resp.Type == RespError {
+		if resp.Type == RespSERVFAIL || resp.Type == RespError || resp.Type == RespNSResolutionFailed {
 			lastErr = fmt.Errorf("server error resolving %s: %s", nsName, resp.Type)
 			continue
 		}