用xshell客户端连接主机出现错误：
ssh 用户名@ip_address [port]
官方文档上的提示：
"Could not connect to 'hostname' (port 22): Connection failed."
Check if the host machine is ready for connection and the port is reachable. If you are connecting to a SSH server, make sure that SSH daemon is running on the remote host.
然后意识到可能是没有安装ssh服务。于是sudo apt-get install openssh-server
出现如下错误：
解决方法：安装所有安全更新和重要更新
问题解决。