only set DF for sockets that can handle it (#3448)

Fixes #3445

sys_conn.go

@@ -30,11 +30,15 @@ func wrapConn(pc net.PacketConn) (rawConn, error) {
 		if err != nil {
 			return nil, err
 		}
+
+		if _, ok := pc.LocalAddr().(*net.UDPAddr); ok {
+			// Only set DF on sockets that we expect to be able to handle that configuration.
 			err = setDF(rawConn)
 			if err != nil {
 				return nil, err
 			}
 		}
+	}
 	c, ok := pc.(OOBCapablePacketConn)
 	if !ok {
 		utils.DefaultLogger.Infof("PacketConn is not a net.UDPConn. Disabling optimizations possible on UDP connections.")