fix error propagation for nxdomain

app/dns/server.go

@@ -241,13 +241,13 @@ func (s *Server) lookupIPInternal(domain string, option IPOption) ([]net.IP, err
 		if len(ips) > 0 {
 			return ips, nil
 		}
-		if err == dns.ErrEmptyResponse {
-			return nil, err
-		}
 		if err != nil {
 			newError("failed to lookup ip for domain ", domain, " at server ", client.Name()).Base(err).WriteToLog()
 			lastErr = err
 		}
+		if err != context.Canceled && err != context.DeadlineExceeded {
+			return nil, err
+		}
 	}
 
 	return nil, newError("returning nil for domain ", domain).Base(lastErr)