SSH

What is SSH?

Secure Shell (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network. The best known example application is for remote login to computer systems by users.

Install OpenSSH Server

Ubuntu

> sudo apt update
> sudo apt install openssh-server

Configuration

> sudo nano /etc/ssh/sshd_config

Service Management

> sudo systemctl start ssh
> sudo systemctl stop ssh
> sudo systemctl restart ssh
> sudo systemctl status ssh

SSH Tunnel

To use an Ubuntu server as a bridge for SSH to access the target server (which accepts SSH only from the office server), you can set up an SSH tunnel or proxy. Here’s how:

Run the following command on your local machine:

> ssh -L <local_port>:<target_server_ip>:22 <bridge_server_username>@<bridge_server_ip> -p <bridge_server_port>

• -L: Create a local port forwarding.

Then, you can SSH to the target server using the following command:

> ssh -p <local_port> <target_server_username>@localhost

If you need to run it in the background:

> ssh -f -L <local_port>:<target_server_ip>:22 <bridge_server_username>@<bridge_server_ip> -p <bridge_server_port>

• -f: Runs in the background.

If you need to run it and prevent executing commands, keeping only the tunnel:

> ssh -N -L <local_port>:<target_server_ip>:22 <bridge_server_username>@<bridge_server_ip> -p <bridge_server_port>

• -N: Prevents executing commands, keeping only the tunnel.

Auto SSH Tunnel

To automatically establish an SSH tunnel, you can install autossh:

Linux

> sudo apt install autossh

macOS

> brew install autossh

Then, run the following command:

> autossh -M 0 -f -N -L <local_port>:<target_server_ip>:22 <bridge_server_username>@<bridge_server_ip> -p <bridge_server_port>