Figure 1 - Sample /etc/sshd/sshd_config file

AllowTcpForwarding yes
X11Forwarding      yes
X11DisplayOffset   10
X11UseLocalhost    yes

Although I have selected these four settings for manipulation, your sshd.config file may contain more settings.

Figure 2 - Initial PuTTY configuration

Here, we show the steps necessary to give our Windows captive access to an external Linux system.

Figure 2a: Initial (empty) PuTTY Configuration
First, we start off with the initial (empty) PuTTY configuration menu, and fill in the IP address of our Linux "safe haven" system. We also give this profile a name in the Saved Sessions window.

Figure 2b: Addressing the Linux "Safe haven"
Next, we save the profile by clicking the "Save" button, which saves our settings for future use.

Figure 2c: Saved "Safe Haven" address

Figures 3a-3e - Using PuTTY to escape to Linux

Here, we show how PuTTY connects from our Windows system to the ssh server on our Linux "Safe Haven".

Figure 3a: PuTTY asks if you trust the target system
Once we click the "Open" button on the PuTTY configuration window, PuTTY performs an initial conversation with our Linux system. If you trust the connection, you answer "Yes" and PuTTY will continue on with your Linux login.

Figure 3b: Logging on to Linux - your user ID please
The first thing Linux will ask is your user ID. Enter it at the prompt and the login will continue.

Figure 3c: And your password, if you mind
Next, Linux will ask for your password. Enter it at the prompt and the login will complete.

Figure 3d: Past the security perimeter, and into the commandline
Now, with the logon complete, you are ready to enter console commands into your Linux system.

Figure 3e: Doing useful work at the Linux commandline

Figure 4 - Windows Icon for Xming X server

Figure 4: Xming icon

Figures 5a-5c - Escaping to Linux GUI

We still have a minor setup to complete in order to permit GUI access to Linux. We have to enable X11 forwarding on our PuTTY session.

Figure 5a: Enable PuTTY X11 forwarding
With X11 forwarding enabled, PuTTY and OpenSSH will set the Linux $DISPLAY environment variable to point at the SSH tunnel to our Xming X server.

Figure 5b: Linux recognizes X11 forwarding
And, with $DISPLAY set properly, we can start up a Linux X application and have it display on our Windows system

Figure 5c: Linux GUI application, now showing here