How to use SSH port forwarding to protect insecure connections

Port forwarding is using an SSH connection to forward another network application connection, as illustrated in the following diagram:

Instead of conncting directly from the application client to the application server, the application client first connects to an SSH session running on the client side, which encrypts the data and forwards to the server's SSH server daemon. The SSH server daemon decrypts the data and forwards them to the application server. Thus, we can use SSH connection to protect insecure connections such as telnet.

1. Local forwarding

And we get the output like this:

We know the telnet session is not secure because all the data are transmitted in clear text. So we build an SSH port forwarding session like this:

And we get the output like this:

We initiated an SSH session just like a regular session, but we also enabled port forwarding. Now the SSH session is also listening on port 3000 on the client and is ready to forward any packets it receives to port 23 (telnet) on the server (10.1.43.52).

From now on, we can initate the telnet connection to local port 3000, which is forwarded to port 23 on the server, which looks like a regular telnet session, except that the data are protected by SSH encryption.

We can use special escape sequence ~ # to show all forwarded connections in the SSH session:

2. Remote forwarding

Local forwarding is initiated from the client. It is also possible to initiate port forwarding from the server, which is called remote forwarding.

Suppose you already log into the server (with SSH), now you can issue the command on the server for remote forwarding:

And we will get the output like this:

Now, on the client, you can issue the telnet command to connect to the localhost side to be forwarded to the server: