filter some upstream errors

upstream.go

@@ -36,12 +36,23 @@ func getResponseFromUpstream(msg *dns.Msg, upstreams []*url.URL) (*dns.Msg, erro
 		}(up)
 	}
 
+	var errmsg *dns.Msg
+
+	for i := 0; i < len(upstreams); i++ {
 		select {
 		case <-ctx.Done():
 			return nil, errors.New("time out")
 		case m := <-resch:
+			if m.MsgHdr.Rcode == dns.RcodeSuccess {
 				return m, nil
 			}
+			errmsg = m
+		}
+	}
+	if errmsg != nil {
+		return errmsg, nil
+	}
+	return nil, errors.New("empty result")
 }
 
 func queryUpstream(msg *dns.Msg, upstream *url.URL) (*dns.Msg, error) {