Introduction to X11-forwarding

What’s X11-forwarding?

X11 forwarding is a software that allows for the access of graphical Linux programs remotely through an SSH client.

This tutorial describes how you can get remote Linux GUI applications via SSH to run on your local machine. Thus, you can visualize data without having to download the data from link → Discovery to your local workstation.

To get this to work you basically need an X11 server running on your local workstation. If you are a Linux or Mac user, usually you’d have X11-forwarding installed by default. If you are a Windows user, it’s recommended to download and install VcxSrv X11 server.

Installation steps

Windows Client

  1. Download and install VcxSrv

    • Locate the downloaded file on your local machine and double click on it to start the installation process. Select the Yes button when the User Access Control dialog box pops up.

    • On the next window pane, you can choose to enable or disable shortcuts from being created. Click Next to continue.

    • The "Destination Folder" is where VcXsrv will be installed. Click Install to continue.

    • After the installation has completed, clickClose to exit the window pane.


MacOS Client

  1. Download and install the official X server software XQuartzfor Mac.

Run X11 Server

Windows Client

  1. On your windows start menu, locate and click on the XLaunch icon

  2. Display Settings

    • Change the default value on Display number field from -1 to 0 just to maintain consistency and prevent X11 from assigning values automatically. Click Next to continue.

  3. Extra settings

    • Clipboard and Primary Selection are checked by default and it’s up to you to decide whether you need it or not.

    • Also, uncheck Disable Access Control to prevent other clients on your network from using your X11 server. If enabled, this could be a potential threat where clients could spy on your network and you don’t want that.

      • Disable Access Control

    • Make sure Native opengl is checked to allow GPU acceleration rendering on your local workstation and Click Next to continue.

  4. You can click on the Save Cofiguration button to use your current configuration for subsequent use of X11. Click Finish to complete the configuration.

  5. On bottom right section of your taskbar, you should see the XLaunch icon which indicates that the server is actively running.


MacOS Client

  1. Navigate to Applications > Utiities and run

  2. On the Dock, right click on the XQuartz icon and select Applications > Terminal to launch a new xterm terminal windows.

Connecting to Discovery with X11-forwarding Enabled

There are ways to connect to a remote server to enable and use X11-forwarding.

Connect via putty

  1. Locate and launch Putty on your local workstation.

  2. Set the following parameters:

    • Host Name (or IP address):

      • Port: 22

      • Connection type: SSH

  3. Enable compression. This requests compression of all data sent to and fro the remote server so as to speed up the interactivity between your local machine and Discovery. On the Category: section of the window click on SSH under Connection to display the SSH configuration. Now enable compression checking the checkbox.

    • Enable compression

  4. On the Category: section of the window click on the + button next to SSH under Connection to expand the SSH menu. Now select X11 and enable X11 forwarding by checking the checkbox.

    • Enable x11 forwarding

  5. (Optional) Set the X display location value to is the localhost address where X11 is listening and :0 is the display value.

    You may not need to specify the display parameter but if things aren’t working as expected, specifying it could make it work.

  6. Click Open to connect to Discovery.

  7. On the next prompt give your username followed by your password and hit the Enter key on your keyboard to log in.

  8. Now that you are logged in to Discovery type the command below. SSH will automatically assign the $DISPLAY variable for X11 forwarding.


    $ echo $DISPLAY



    If you got a similar output on your console like the one above localhost:11.0 (the number after the colon could vary. Here it’s 11). It means that X11 is working correctly but if you get a blank output then it means X11 isn’t working correctly. Then, you’d have to repeat the steps in this section from the beginning.

  9. To do a simple test run with xclock, open your console, type xclock and hit Enter .


    $ xclock



    If you got a mini clock window pop up like the one above, then it means things are working the way they should.

Connect via Command Line(CLI)

Aside using putty, you can also connect to Discovery via SSH while enabling X11 forwarding using CLI:

To carry out this step, ensure that X11 Server is running on your local machine.

  1. Set the X11 forwarding DISPLAY environmental variable on your Local workstation/Client

    Before you connect to Discovery you need to make sure that the DISPLAY environmental variable is set via the CLI on your local machine. This is done in order for your SSH session to pick it up and forward it to the remote server correctly (Discovery).

    1. Linux/MacOS Terminal & Windows Command Prompt


      set DISPLAY=
    2. Windows PowerShell


      $env:DISPLAY="" is the localhost IP address where X11 is listening and :0 is the DISPLAY value.

  2. Connect via SSH and enable X11-forwarding

    After setting the DISPLAY as mentioned in Step 1, the next step is to connect via SSH with x11 forwarding enabled.

    syntax: ssh -Y | -X

    Flag Description


    Enables X11 forwarding. (Use -X when possible as it’s more secure)


    Enables trusted X11 forwarding. (Quite Insecure. For windows users, it’s recommended to use PUTTY and using -Y if -X fails for the application you are trying to use.)


    ssh -Y

Enable OpenGL Remote Acceleration

For the final step, the OpenGL Remote Acceleration environmental variable on the remote server needs to be set up on→ Discovery. Hence, whenever your application goes to look for LIBGL, LIBGL will process the environmental variable and do the X11-forwarding redirection.



Fix Font Errors

export LC_ALL=C

Run an Application using X11-forwarding

To show how this works, download the → QFitsView program to your /scratch/<username> file_folder directory on Discovery with a World Coordinate System (WCS) example gls.fits file from NASA’s website. But before testing the application, please make sure that to have all the requirements working.

  • Confirm that the DISPLAY ENV is set


    echo $DISPLAY


  • Confirm that the OpenGL ENV is set




  • Download QFitsView and Sample .fits file

    Download QFitsView


    Download example fits file

  • Show list of downloaded files

    gls.fits  QFitsView_4.0

    As you can see, there are two files in the working space, Next

  • Make the QFitsView binary executable

    chmod ax QFitsView_4.0
  • Launch the QFitsView program

  • Launch the QFitsView program using srun

    Here, 1 CPU, 1 Task, 2GB of Memory for 10 Minutes were requested. Notice the --x11 flag before the program you intend to execute. This informs the compute nodes(in-charge of running the job) that you are running an X11 service and for it to do the necessary forwarding.

    srun -p normal --cpus-per-task 1 --mem-per-cpu 2g --ntasks=1 --time=0-0:10:00 --x11 QFitsView_4.0

    You can also run a login shell on a compute node and execute commands interactively.

    srun -p normal --cpus-per-task 1 --mem-per-cpu 2g --ntasks=1 --time=0-0:10:00 --x11 --pty bash
  • Program Output

  • Open and view the gls.fits file that’s downloaded earlier

    On the menu bar navigate toFile > Open > Computer >/ > scratch > yourusername > gls.fits

  • Visualizing the data


Permanently Add Display Settings

You can permanently add the OpenGL and Font ENV variables to the .bashrc or .bash_profile file in your home directory. Hence, if you login next time, you don’t need to set it again. Add the code below at the end of your .bashrc or .bash_profile file.


# X11-forwarding display settings
if [ -n "$DISPLAY" ]; then
  export LC_ALL=C