Configuring an SSH Server on the Windows Subsystem for Linux

Install openssh

No need to go on too much about this; first you need to install openssh-server and openssh-client.

Modify the sshd-config configuration

The three lines you need to pay attention to are these.

UsePrivilegeSeparation no         #因为wsl没有实现chroot
PasswordAuthentication yes
ListenAddress 0.0.0.0                 #这一项在我的发行版里缺省为注释行。

The rest can be modified according to your needs.

ListenAddress 0.0.0.0                  # 要让其他计算机能连接上需要加上这一行，或者把经常需要连接的计算机IP加入
PermitRootLogin yes                   # 允许root用户登录。

Disable Windows built-in SSH

I didn't know this originally; only after looking it up did I learn that newer versions of Windows come with an SSH service, though I haven't yet figured out how to use it. You can use PowerShell to check this service.

PS C:\Users\user> Get-Service -Name ssh*
Status   Name               DisplayName
------   ----               -----------
Stopped  SshBroker          SSH Server Broker
Stopped  SshProxy           SSH Server Proxy

Just find these two services in the Services console and stop them. Note that you need to stop sshproxy first before you can turn off sshbroker.

Start SSH in debug mode

/usr/sbin/sshd -d

-d indicates that the service is started in debug mode; in this case, errors will be displayed on the console.

Note that even if you start the ssh server this way, it is still only a child service that exists while a bash window is open. Once the last bash window is closed, this ssh server shuts down as well, which is clearly not what we want. Next, let's see how to run the ssh server as a Windows service or a background process.

Start SSH normally

service ssh start

Automatically start the SSH service

Currently WSL does not support running the ssh server as a service. We need to use Windows scheduled tasks and a script so that this service runs automatically when Windows starts.

set ws=wscript.createobject("wscript.shell")
ws.run "C:\Windows\System32\bash.exe -c '/usr/sbin/sshd -D'",0

Save this file as vbs, and add a startup task in Task Scheduler with the trigger set to at system startup. However, a prerequisite for using this method is that your WSL default user is root; if the default user is not root, you must start it using sudo. But! When executing sudo, it will prompt for a password, and at that point there is no way to receive user input. To solve this problem, you need to allow sudo to execute commands without a password.

Enter the following command in bash:

sudo visudo

#includedir /etc/sudoers.d
username ALL=(ALL) NOPASSWD: /usr/sbin/sshd -D

Just change username to your own username.

References

hbaaron.github.io

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