BUG
使用 sudo service ssh start 啟動 ssh 服務，提示：
* Restarting OpenBSD Secure Shell server sshd
Could not load host key: /etc/ssh/ssh_host_rsa_key
Could not load host key: /etc/ssh/ssh_host_ecdsa_key
Could not load host key: /etc/ssh/ssh_host_ed25519_key
[ OK ]
然后使用 powershell 連接，提示 refused:
PS C:\Users\suyua> ssh ryan@localhost
ssh: connect to host localhost port 22: Connection refused
FIX
卸載重裝 openssh-server:
sudo apt-get purge openssh-server # purge 是卸載並刪除配置文件
sudo apt-get install openssh-server
然后修改 /etc/ssh/sshd_config 的下列幾行參數：
#Port 22 # 如果端口沖突，就需要修改這個
ListenAddress localhost # 只接受本地請求
PasswordAuthentication yes # 允許密碼登錄
重啟 ssh 服務：
sudo service ssh restart
然后連接：
PS C:\Users\suyua> ssh ryan@localhost
ryan@localhost's password:
Welcome to Ubuntu 18.04.2 LTS (GNU/Linux 4.4.0-17763-Microsoft x86_64)
* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/advantage
......
問題解決。