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

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
 		}