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Some software on SHARCNET system requires you to establish a graphical connection, so you can interact with it through a graphical user interface (GUI).

If such a graphical connection is not enabled, the program which requires it (in this example DDT), will generate an error message:

[ppomorsk@orc-login2:~] module load ddt
[ppomorsk@orc-login2:~] ddt
ddt: cannot connect to X server

Resolving this issue requires that a graphical connection is set up. There is a number of ways to do this.

MobaXterm (Windows users)

This package is available as a free download from A convenient tutorial on its use is given on that page. To enable a graphical connection, make sure the box "X11-Forwarding" is checked.

NOTE: once you login to a SHARCNET system, if you want to login into a development node, use the -X flag in your ssh command to keep the graphical connection enabled.

Example: after you are logged in on orca, to get to the development node 1 with graphical connection enabled to:

[ppomorsk@orc-login2:~] ssh -X orc-dev1


VNC, or Virtual Network Computing, allows one to access a graphical interface on a remote computer system across a network. VNC is the primary remote GUI access tool in SHARCNET. Sessions are not persistent. When you logout or are disconnected, they end.

Web Browser (noVNC)

noVNC is a HTML5 VNC client that runs inside a web browser. It enabled connecting to any of the visualizations stations directly from any modern browser (one that supports HTML5). Just click the blue icon next to the workstation name on our systems page. If your machine is under heavy load, you may receive a timeout message instead of connecting successfully.

Note that one cannot cut-and-paste between their local host and the noVNC session. If one desires this functionality it is recommended that they use a stand-alone VNC client.

Local Client (TigerVNC, gvncviewer, and ssvnc)

The local client provides a slightly faster solution that may integrate better with your system (e.g., pass through certain keystrokes that would otherwise be captured by your browser and do full screen mode better). While any vencrypt enabled VNC viewer should work, we have experienced difficulties with many of them and therefore recommend the above three.

Also note that connecting to the visualization stations via the local clients should be done to the <station> address (e.g., and not that standard <station> address or else your client will not be able to properly verify the authenticity of the remote machine.

TigerVNC (Windows, MacOS, and Fedora)

For MacOS and Windows, tigervnc viewer can be downloaded from the tigervnc sourceforce download site. Pick the most recent version and download either the TigerVNC dmg file for MacOS or the vncviewer exe file for Windows.

For Fedora, the tigervnc client is available in the default packages

sudo yum install tigervnc

You will need at least version 1.1 or later (for vencrypt support). If you have an older version than this, you will have to update to a newer version of Fedora.

Although strictly not required, it is a good idea to provide tigervnc with a link to the system certifications so it can verify it is really connecting to our visualization stations

mkdir -p ~/.vnc
ln -s /etc/pki/tls/certs/ca-bundle.crt ~/.vnc/x509_savedcerts.pem

Connecting to a viz stations is then done by running

vncviewer <station>

where <station> is substituted with the appropriate viz station name.

gvncviewer (Ubuntu and Debian)

Ubuntu and Debian do not package tigervnc by default, so we instead recommend installing the gvncviewer application

apt-get install gvncviewer

The gvncviewer program will not prompt you for connecting to machines it cannot verify (it just quits), so it is necessary to provide it with a link to the system certificates

mkdir -p ~/.pki/CA
ln -s /etc/ssl/certs/ca-certificates.crt ~/.pki/CA/cacert.pem

Connecting to a viz stations is then done by running

gvncviewer <station>

where <station> is substituted with the appropriate viz station name.

ssvnc (Windows, MacOS, and Linux)

We have also tested the ssvnc client on Windows, MacOS, and Linux.

For Linux, download a unix_only version, unpack it, and then run the command

/<path to your ssvnc bin>/ssvnc_cmd -proxy vencrypt://<station> <station>

where <path to your ssvnc bin> is substituted with the path to the ssvnc_cmd binary and <station> is substituted with the appropriate viz station name.

For MacOS, download a no_windows version, unpack it, and then in the (found under /Applications/Utilities) run the commands

export UNAME=Darwin.i386
/<path to your ssvnc bin>/ssvnc_cmd -proxy vencrypt://<station> <station>

where <path to your ssvnc bin> and <station> are as before.

For Windows, download a windows_only version and unpack it. Navigate to ssvnc\Windows and run sshvnc. When a window opens up, press Options, and then press Mode. Switch the mode from the default SSH-Only (sshvnc) to SSVNC.

A newer larger configuration window will open. Set VNCHost:Display to <station> and Proxy/Gateway to vencrypt://<station>, where <station> is substituted with the appropriate viz station name. Check Use SSL and uncheck Verify All Certs., Press Connect.

Xming Software (Windows users)

This is an older alternative to MobaXterm, described above.

What is Xming?

Xming is an X11 server that can be run on your local Windows desktop computer - it will allow you to run graphical applications from remote Unix and Linux computers such as the SHARCNET clusters, when used in combination with an SSH client.

Where to get Xming

Xming is available from, where a variety of versions can be downloaded. For a fee, you can download the latest version, or a slightly older, but still functional version is avaliable for free. Of the two free versions, one is the basic "Xming" server, and the second, "Xming-mesa", allows you to make use of 3D graphics if they are used by the remote application. This tutorial will work under the assumption that you have downloaded the free version, labeled "Xming-mesa", since it will do all of the work that the basic Xming program does, and in situations where a program requires 3D graphics support, it is better to have it already, rather than finding out when your program will not run.

How to Install and Use Xming

To install Xming, first download the Xming-mesa public domain from The Website, and run it. For the majority of the setup options, you should accept the default, and just click "Next >". When asked to Select Components, make sure "Normal PuTTY Link SSH client" is selected, and turn on all checkboxes, as below:


Once this has been completed, you can click "Next" until you reach the "Setup" window, and turn on the desktop icons for Xming and XLaunch if you wish. After this, click "Next" until done, and click the final "Install" button to complete the installation.

The first time you install Xming, you may also be asked for additional configuration options for the server. In each of these conditions, the default setting is the best choice for using Xming with the SHARCNET clusters.

Right after installation, the Xming server will already be running, but if you log out or reboot your computer, you will need to click the "Xming" icon to start it up again if you wish to use it. If you chose not to install the desktop icon, the server can also be found in the Programs menu under "Xming". To verify that Xming is installed, check the taskbar of your computer to see if the "X" logo is present, like this:


PuTTy SSH client

Next, if you do not already have a copy of the PuTTY SSH client, you can get one from here - download the putty.exe program file, and place it on your desktop for easy access, no installation is needed for this program. Once it is downloaded, run it, and you will be presented with the configuration screen. For this example, we will connect to the system, but the same instructions can be used for any of the clusters, simply by changing the Host Name you use.

In the first configuration screen, we need to do a few things. We enter our Host Name ( and a name for our session (Rainbow X11) and then click "Save" - we should then see what is shown below:


Next, click "X11" in the category menu on the left. (It should be near the bottom) and turn on the checkbox beside "Enable X11 forwarding", like this:


Last, return to the "Session" category at the very top of the menu on the left, and click the "Save" button again to save the X11 settings. Now the setup is complete, and you can click "Open" to log in. The next time you start PuTTY, all of the settings should be in place already if you double click the "Rainbow X11" entry in the Saved Sessions list. You can now log into the cluster using your regular SHARCNET username and password.

Now that everything is installed, the next time you come to your computer to use X11 programs on SHARCNET, you will only need to follow three steps:

  1. Start Xming (if the Xming logo is not in the tool tray, double click the Xming icon to start it.)
  2. Start PuTTY
  3. Double-click the session you wish to log into in PuTTY and log in with your username and password.

Once you are logged into the cluster, if you wish to test to verify that X11 is working, type the command "xlogo" and hit enter. A small window that looks like this should appear somewhere on your screen:


You can now run any program that requires X11 to function while on the clusters.