How to use SSH tunnels and SSH port forwarding (2024)

SSH tunnels, much like real tunnels, serve to connect two points. The first of these points is a computer that is usually located on an unsecured network. The target point is a server or web address that you can’t or don’t want to access from your network. SSH tunnels work as links between different servers and connect the TCP ports on two computers with each other. Any TCP port can be forwarded using SSH tunneling, which is why the process is also called SSH port forwarding or SSH forwarding.

You can use pretty much any server as an SSH server. For example, a Raspberry Pi SSH server would work.

There are various use cases for secure shell port forwarding. In most cases, SSH port forwarding is used to create an encrypted connection between a local computer (the local host) and a remote computer. The use of this virtual network allows certain restrictions on access to be bypassed. It will look like you are on this network, when you are in reality just accessing it using the SSH tunnel. This is similar to a Virtual Private Network (VPN) but is nonetheless different — try not to mix the two up.

If you’re transporting data from services that use an unencrypted protocol, you can use SSH forwarding to encrypt the data transfer. A SSH File Transfer Protocol, SFTP for short, will be used for this. SSH tunnels also offer increased security when you’re surfing on unfamiliar networks, for example in a hotel or coffee shop. SSH keys use asymmetric encryption and provide an even higher level of security.

It’s important to note that SSH tunneling is frequently used by hackers, who build backdoors in internal networks so that attackers can easily access internal data.

The most common method for setting up an SSH tunnel is local port forwarding. This involves accessing local network resources from your local computer by forwarding a port from your computer to a port on the SSH server. The port can then be forwarded to a port on the target computer.

To set up local port forwarding, use the following terminal command on Unix-based operating systems or macOS:

First you’ll have to define which port you want to forward (local_port). You can use any port number higher than 1024. Ports with smaller port numbers are privileged and can only be accessed by the root. Next, enter the IP address of the target server (remote_address) and your credentials (remote_port).

Let’s apply this to an example. Say you want to connect with a server in your office that’s at the address 123.234.1.111 in the office network. You can access your office’s SSH server using ssh.test.com. Your username is “Testuser”. The command that you would use to set up the SSH tunnel for port 1234 would look as follows:

If you want to listen on a port with port number 8080, for example, and send the data to port number 3000 on your local computer, the command will look as follows:

In the example above, the SSH server has the address remote.host. The data you’re tunneling can now be accessed externally by entering this address in a browser, together with the port number:

A third way of using SSH tunnels involves dynamic port forwarding, which enables you to use a socket on your local computer that will function as a kind of SOCKS proxy. All of the applications that use a SOCKS proxy will then connect with the SSH server and send their traffic through the tunnel. This kind of port forwarding is often used for tunneling web browser traffic.

To set up dynamic port forwarding, use the following terminal command:

You can also set up SSH tunnels on Windows. Since the terminal is less central on Windows, you’ll want to use one of the many practical tools with a user interface, like PuTTY for example. They make setting up an SSH tunnel as easy as pie.

After you’ve downloaded and installed PuTTY, open it up. In the start menu under “Hostname”, enter the IP address of the SSH server you want to connect with. Then navigate to “Connection/SSH/Tunnels”, where you can enter the source port and target port in the fields “Source Port” and “Destination”. Finally, click the “Open” button to establish the connection. You should now be able to access the port in a browser using 127.0.0.1:port_number.

To access computers with a private IP address you’ll need a reverse SSH tunnel. With a reverse SSH tunnel, the remote computer you’re looking to access can create a connection to your local computer. You can then use that connection to create a new connection from the local computer to the remote computer.

The following command will set up an SSH tunnel from the remote computer:

The local computer can then use the following command to connect with the remote computer: