Using X-windows (X11) and secure shell (SSH) to connect to a remote UNIX server (host)

X-windows (X11) and SSH software let you run certain types of software on a remote UNIX server (host) while displaying and controlling the software's graphical user interface (GUI) on your local desktop system. In a cluster computing environment, X11 and SSH are typically used to connect to the cluster's head node as illustrated below.
cluster computing environment

This document tells you how to get started with X11 and SSH on local Windows desktop systems.

Windows desktop use

Windows users must:

(1) Install a secure shell program (SSH) to establish a remote connection.
(2) Install an X-Windows server program.
(3) Generate a public/private key pair.

We strongly recommend PuTTY for the SSH program and Xming for the X-Windows server program. To use PuTTY effectively and securely, we also recommend that you set up your public/private key pair (digital identity) to use with all remote host connections..

PuTTY installation

  1. Download PuTTY Installation package. Look for the file named putty-x.xx-installer.msi (e.g., putty-0.76-installer.msi).
  2. Double-click the downloaded putty-x.xx-installer.msi file to begin the PuTTY installation. If prompted by the firewall, click Unblock.
  3. Follow the recommended (default) choices except for the following:
    • At Completing the PuTTY Setup Wizard, clear the View README.txt check box before clicking Finish.

Xming installation

  1. At the Xming distribution site, download and save the latest versions of Xming and the Xming-fonts. Look for the files named Xming-x-x-x-x-setup.exe (e.g., Xming-6-9-0-31-setup.exe) and Xming-fonts-x-x-x-x-setup.exe (e.g., Xming-fonts-7-5-0-70-setup.exe).
  2. Double-click the downloaded Xming-x-x-x-x-setup.exe file to begin the Xming installation. If prompted by the firewall, click Unblock.
  3. Follow the recommended (default) choices except for the following:
    • At Select Components, select
        Don't install an SSH client
    • At Select Additional Tasks, select
        Create a desktop icon for Xming
        and
        Create a Quick Launch icon for Xming        .
    • At Completing the Xming Setup Wizard, uncheck
        Launch Xming before clicking Finish.
  4. Double-click the downloaded Xming-fonts-x-x-x-x-setup.exe file to begin the Xming fonts installation.
  5. Follow the recommended (default) choices with one exception:
    • At Select Components, select all of the listed fonts.

Generate your public/private key in PuTTY

  1. Click PuTTYgen in the All Programs\PuTTY folder to start PuTTYgen and display the PuTTY Key Generator window.
  2. Click Generate to generate a public/private key pair. You must move the mouse around in the empty area of the PuTTY Key Generator window to add randomness to the key. (This will take a few seconds.)
  3. The Key comment will appear as a hint whenever you are prompted for the passphrase for this key.
    1. Change the value in the Key comment text box to a more meaningful hint (e.g. Windows desktop system name and date).
    2. Type an easy-to-remember phrase for this private key in the Key passphrase text box.
    3. Type the same phrase in the Confirm passphrase text box.

  4. Click Save private key
  5. Change the location to your desktop and type a file name to associate with your Key comment (e.g., jdoe).
  6. Close the PuTTY Key Generator window by clicking the X in the upper-right corner.

Configure PuTTY for remote host connections

Double-click the PuTTY desktop icon to open the PuTTY Configuration window. For each remote host name (e.g., compute.example.edu), create a saved session name (e.g., compute) and then define and save its configuration settings as described below.

  1. Click Session.
  2. Type the remote Host Name (or IP address) (compute.example.edu) and Saved Sessions name (compute) for this saved session.

  3. Click Window then click Translation.
  4. In the Translation window: Under Character set translation, change Remote character set: to UTF-8 from the pull-down menu.

  5. Click Window then click Colours.
  6. In the Colours window: Under Select a colour to adjust: use the scroll bar and click on ANSI Blue. Change RGB value: for Green to 128 and Blue to 192.

  7. Click Connection then click Data.
  8. In the Data window:
    • Under Login details, type your compute.example.edu username (e.g., jdoe) in the Auto-login username text box.
    • Under Terminal details, change Terminal-type string from xterm to xterm-color.
      Note: If the remote host name is copland.udel.edu or strauss.udel.edu keep the Terminal-type string as xterm.

  9. Click Connection then double-click SSH and click Auth.
  10. In the Authentication window:
    • Under Authentication parameters, select the Allow agent forwarding check box.
    • Click Browse... and select the private key file you just created and saved on your desktop (e.g. jdoe.ppk). Important Note: Since PuTTY 0.78, the location to enter the Private key file for authentication has been moved to a sub entry under Auth in the panel hierarchy, so click Auth then click Credentials to browse and select the private key file.

  11. Click X11.
    In the SSH X11 forwarding options window: Under X11 forwarding, select the Enable X11 forwarding check box.

  12. Click Session again and click the Save button to save the session configuration named compute.

  13. You may repeat this series of steps to create saved sessions for other remote host connections. Or you may return later to define them.
  14. Close the PuTTY Configuration window by clicking the X in the upper-right corner.

Configure a trusted connection for remote hosts

Your local Windows desktop system is now installed with an SSH program (PuTTY), an X-windows server program (Xming) and public/private key icon (e.g. jdoe.ppk). You also have configured a remote host connection in a saved session (e.g. compute) with host name and login details (e.g. compute.example.edu and jdoe). To complete the process, you need to exchange keys to allow a trusted connection from your Windows desktop system to this saved session.

  1. Double-click the public/private key desktop icon (e.g., jdoe.ppk) to start Pageant and display the Pageant: Enter Passphrase window. Type the passphase for the key comment (e.g. jdoe laptop Aug26) and click OK.

    Note: The Pageant: Enter Passphrase window may not be displayed if you have already successfully entered the passphrase during the current login session. Pageant is running if you see the Pageant icon in the taskbar.

  2. To make your first connection, double-click the PuTTY desktop icon to start PuTTY and display the PuTTY Configuration window, then double-click the saved session (e.g. compute). Click Yes in the PuTTY Security Alert window.

    3. Note: The PuTTY Security Alert window may not be displayed if you previously clicked Yes for the same remote host.

  3. The PuTTY window will be displayed with a message Access denied, followed by a prompt for the password for your username (e.g, jdoe) for the remote host (e.g., compute.example.edu). Follow the prompts and enter your responses as shown in the table below:

    4. Prompt Action
    jdoe@compute.example.edu's password: Type the password for jdoe on compute.example.edu
    Note: What you type will not be displayed on the screen.
    [jdoe@compute ~]$  Type 
    ssh-add -L >> .ssh/authorized_keys

    The following sample session displays all the messages, prompts, and responses for the username jdoe on the remote host centos.css.udel.edu.

    If you see an error .ssh/authorized_keys: No such file or directory similar to the one shown below,

    follow the prompts and type your responses as shown below. Otherwise, proceed to step 4.

    Prompt Action
    [jdoe@compute ~]$  Type ssh-keygen -C "key comment"
    e.g. key comment=jdoe compute Aug26
    Enter file in which to save the key (/home/jdoe/.ssh/id_rsa):   Press return to accept the default key file name shown in parentheses
    Enter passphrase:  Press return
    Enter same passphrase again:  Press return
    [jdoe@compute ~]$  Type ssh-add -L >> .ssh/authorized_keys

  4. Close the remote server window by typing the command exit.
  5. You may repeat steps two and three to configure a trusted connection for other saved sessions. Or you may return later to configure them.

Using PuTTY and Xming

  1. Double-click the Xming desktop icon to start the X-windows server. The Xming server is running if you see the Xming icon in the taskbar.

  3. Double-click the public/private key desktop icon to start Pageant. The Pageant: Enter Passphrase window will only be displayed if you have not successfully entered the passphrase during the current login session.

  4. Double-click the PuTTY desktop icon to start PuTTY and display the PuTTY Configuration window. Double-click the saved session for the remote host you want to make a trusted connection.


    5. Note: hostname and date are two UNIX commands executed on the remote host centos.css.udel.edu.

  5. Repeat step four to make additional connections to the same remote host on your Windows desktop system. If you have other saved sessions configured with a trusted connection, you automatically connect to them as well.

  6. Test the configuration by typing the UNIX command 

     xclock -bg green &
    A green xclock window should appear. Close it by clicking the X in the clock's upper-right corner.

    When you no longer need Xming, right-click the Xming icon in the taskbar and click Exit. Then click Exit again in the Xming - Exit? window. This will close all X-windows and running applications.

Helpful Tips

  1. To copy and paste from one PuTTY window to another PuTTY window.
    • Select the text you want to copy in the source window.
    • Right-click in the destination window, to have the text inserted at the cursor.

  2. Pageant is running if you see the Pageant icon in the taskbar.

    • Right-click the Pagent icon in the taskbar and click View Keys. The keys listed in the Pagent Key List window should be the same as the keys listed if you type ssh-add -l on the remote host.
    • When you no longer need Pageant, right-click the Pagent icon in the taskbar and click Exit. This will stop Pageant, but not close your current remote host connections. Remember to double-click the public/private key desktop icon to display the Pageant: Enter Passphrase window before you start a new remote host connection.