Set ConnectionIdMismatchNul error default log level to debug

tracker/udp/client.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/anacrolix/dht/v2/krpc"
+	"github.com/anacrolix/log"
 	"github.com/protolambda/ctxlock"
 )
 
@@ -224,7 +225,11 @@ func (cl *Client) request(
 		} else if dr.Header.Action == ActionError {
 			// udp://tracker.torrent.eu.org:451/announce frequently returns "Connection ID
 			// missmatch.\x00"
-			err = ErrorResponse{Message: string(dr.Body)}
+			stringBody := string(dr.Body)
+			err = ErrorResponse{Message: stringBody}
+			if stringBody == ConnectionIdMissmatchNul {
+				err = log.WithLevel(log.Debug, err)
+			}
 			// Force a reconnection. Probably any error is worth doing this for, but the one we're
 			// specifically interested in is ConnectionIdMissmatchNul.
 			cl.connIdIssued = time.Time{}