Topics: Security, X11

Double X11 forwarding

Everybody is usually quite familiar with how to open an X11 windows GUI on a Windows PC. It involves running an X-server on the PC, for example Xming. Install this with all default settings. Make sure you have PuTTY installed on your PC before installing Xming. Then on your PC run Xlaunch, and make sure to set your DISPLAY to a higher value, for example "10" and to check "No Access Control".

Log in to the UNIX host through PuTTY, and before starting the session to your UNIX host, go to "Connection" -> "SSH" -> "X11" in PuTTY and select "Enable X11 forwarding", and then click "Open". Once logged in, set the DISPLAY variable to the IP address of your PC and set the correct display, for example:

# export DISPLAY=""
And then, to test, run xclock or xeyes:
# xeyes
The program xeyes should open on your window.

Now, how do you get around opening an X window if you have to go through a jumpserver first to get to the correct UNIX server, where you would like to start an X-based program? That's not too difficult also. After logging in on the UNIX jumpserver, following the procedure described above, issue the following command:
# ssh -X -Y -C otherunixhost
Of course, replace "otherunixhost" with the hostname of the UNIX server you'd like to connect to through your jump server. Then, again, to test, run "xeyes" or "xclock" to test. It should open on your PC. Now you have X11 forwarding from a UNIX server, to a jumpserver, and back to your PC, in fact double X11 forwarding.

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