ssh into WSL2 ubuntu

wsl_docker_ssh.md

Summary

The problem addressed here is the challenge of accessing a WSL (Windows Subsystem for Linux) instance from an external network. By default, WSL instances are not directly accessible from outside the host machine. This guide presents a solution using Docker and SSH proxying to allow SSH access to the WSL instance from an external machine via the host machine's Windows IP address.

Prerequisites

• Windows 10/11
• WSL2
• Docker Desktop with WSL support enabled
• Ubuntu (or another Linux distribution)

Concept

As WSL machines are not directly exposed to the external network (although it's possible to run in bridge mode, which I opted not to do), the idea is to utilize Docker to proxy SSH into WSL Linux. Instead of proxying ports (as it's a bit tricky), I've chosen to configure my SSH daemon (sshd) to listen to a socket file and proxy that instead.

Setup

1. Install OpenSSH (apt install openssh).
2. Modify /lib/systemd/system/ssh.socket to make it listen to a socket instead of a port: ListenStream=/run/sshd/sshd.sock.
3. Reload the daemon: systemctl daemon-reload.
4. Enable ssh.socket (not ssh.service): systemctl enable --now ssh.socket.
5. Run a Docker container for the proxy:

   docker run -d -v /run/sshd/sshd.sock:/run/sshd/sshd.sock -p 22:22 alpine/socat 'TCP-LISTEN:22,reuseaddr,fork UNIX-CLIENT:/run/sshd/sshd.sock'

Alternatively, you can use the following Docker Compose file:

version: '3.7'
services:
  sshd:
    restart: unless-stopped
    container_name: sshd_proxy
    image: alpine/socat
    ports: 
      - "0.0.0.0:22:22"
    command: TCP-LISTEN:22,reuseaddr,fork UNIX-CLIENT:/run/sshd/sshd.sock
    volumes:
      - /run/sshd/sshd.sock:/run/sshd/sshd.sock

That's it! You can now SSH into your WSL Linux from an external machine using the Windows IP address.