SSH - Left Tunnel (Left Port Forwarding) - Local to server

About

When you need to make a SSH connection to connect to a server, you can create a left tunnel (a left port forwarding) to reach the server as of it was direct reachable.

Illustration: Image Credit: How does reverse SSH tunneling work? from Erik

Usage

• When you need to make a connection to a private database (in a private network), you can a SSH tunnel and reach it as of the database was local.

Steps

Create the tunnel

You can create the tunnel with any client such as:

• ssh
• or jsch

Ssh

Create the tunnel: From the client host:

ssh -N -T -l loginName -L8881:farAwayServer:8888 sshServerHost

where

• localhost is the host seen from the ssh client
• The local port is 8881 (The port of your machine)
• farAwayServer is the server to reach (it can be the sshServerHost)
• The server port is 8888
• The loginName is loginName
• N means no remote command
• T disables pseudo-tty allocation (No terminal)

Jsch

Jsch is a java library.

JSch jsch = new JSch();
Session session = jsch.getSession(user, host, port);
localPort = 4321;
remoteHost = "localhost";
remotePort = 3306;
session.setPassword(password);
session.setConfig("StrictHostKeyChecking", "no");
System.out.println("Establishing Connection...");
session.connect();
int assigned_port = session.setPortForwardingL(localPort, remoteHost, remotePort);