Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost

今天给内网的mysql配置frp， 用外网ip连接mysql，报下面连接异常：

Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
  Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost.
  Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost.

用show VARIABLES like 'wait_timeout'，显示时间是28800，没问题。

折腾半天，发现是客户端的frpc.ini配置的mysql端口错了，造成可以连接，但老是报错。