大大，使用FTP为什么连不上？220-You are user number 1 of 50 allowed. 响应：  220-Local time is now 18:43. Server port: 21. 响应：  220 You will be disconnected after 15 minutes of inactivity. 命令：  USER blog 响应：  331 User blog OK. Password required 然后就连接失败 安全组添加了21端口! 请问怎么解决？