I will talk about ssh forwarding in this blog. The main content contains local forwarding and remote forwarding. I will take the coder_server as an example in the end.

Local Forwarding

SSH local forwarding is to forward a port from local machine to the remote machine. The local machine is where you set up SSH connection. Local forwarding is set up with the -L option. The command to set up local forwarding is:

# Run this command in your machine (local machine).
ssh user_name@ssh_connected_ip_address -L  local_port_number:localhost:remote_port_number

This will forward any connection to local_port_number of your local machine to the remote_port_number of the remote machine, which is user_name@ssh_connected_ip_address in this case. For example, if someone connects to your computer with a web browser, he/she gets the response of the web server running on remote machine if applicable (the port number is set according and the web server is running in the remote machine)

In the above example, the localhost is the remote machine. They can also be different.

# Run this command in your machine (local machine).
ssh user_name@ssh_connected_ip_address -L  local_port_number:forwarded_host:forwarded_port_number

The above command set up the ssh connection between the local machine and the remote machine (user_name@ssh_connected_ip_address). It will forward any connection to the local_port_number of the local machine to forwarded_port_number of the forwarded_host, which can be reached by the remote machine.

Below is a screenshot demonstrating the above concept. The remotehost in the image is actually the remote machine in our code above. Take care of the arrow in the image. It means the direction of forwarding.

Remote Forwarding

SSH remote forwarding is to forward a port from remote machine to the local machine. The local machine is where you set up SSH connection. Remote forwarding is set up with the -R option. The command to set up remote forwarding is:

# Run this command in your machine (local machine).
ssh user_name@ssh_connected_ip_address -R  remote_port_number:localhost:local_port_number

This will forward any connection to remote_port_number of your remote machine (user_name@ssh_connected_ip_address) to the local_port_number of the local machine. For example, if someone connects to the remote machine with a web browser, he/she gets the response of the web server running on the local machine if applicable (the port number is set according and the web server is running in the local machine).

In the above example, the local host is directly the local machine. They can be different as well.

# Run this command in your machine (local machine).
ssh user_name@ssh_connected_ip_address -R  remote_port_number:forwarded_host:forwarded__port_number

The above command set up the ssh connection between the local machine and the remote machine (user_name@ssh_connected_ip_address). It will forward any connection to the remote_port_number of the remote machine to forwarded_port_number of the forwarded_host, which can be reached by the local machine.

Below is a screenshot demonstrating the above concept. The remotehost in the image is actually the remote machine in our code above.

Example with Code Server

Code server is a popular open source work to run VS Code in the remote server, which is accessible by a web browser in different devices.

Here is how I configure coder server:

Reference

1. https://unix.stackexchange.com/questions/115897/whats-ssh-port-forwarding-and-whats-the-difference-between-ssh-local-and-remot