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MATLAB running via VNC.

It is often necessary to remotely start the graphical user interface for complex software packages such as MATLAB. The most common way to do this is with SSH and X11 forwarding. However the performance of SSH+X11 is often too slow similar to MobaXTerm or Putty. A much better alternative is to use VNC to connect to a remote desktop.


First you will need to install a VNC client on your machine to connect to the VNC server. We recommend using TigerVNC. A TigerVNC package is available for Windows, MacOS and most Linux distributions. The following shows how to download, install and configure TigerVNC securely for each operating system. The certificate configuration steps are only required for connecting to VDI nodes so the signing authority of the certificate presented by the vncserver is known. If a popup about a certificate issue occurs, either you have not configured it properly or you are not connected to our server and should not enter your password.


Download and run the latest stable vncviewer64-x.y.z.exe version package installer from the official download page ( for example vncviewer64-1.12.0.exe). Make sure you download the viewer and not the server. To create secure tunnels from your desktop to the vncserver as described in the sections below, you will need to open a terminal window and run the SSH command. This may be done using PowerShell standard on Windows 10 since the 1809 update.


Install the latest stable DMG package by going to the official download page and click the green Download Latest Version button for TigerVNC-1.12.90.dmg (as of January 2023). Once the download is complete double click the DMG file to open it. A TigerVNC Viewer icon should appear in a popup window along with a LICENSE.TXT and README.rst file. To complete the installation, drag the tigervnc icon that appears into the Applications folder and/or the lower app dock. To remove the popup you will need to unmount the DMG file. To do this open a New Finder Window, verify View->ShowSidebar is selected, click the small up arrow beside TigerVNC-1.12.90 in the left side menu and lastly close the finder window. If you are running macOS Monterey 12.2 and TigerVNC crashes then you must upgrade to this latest version.


First install TigerVNC viewer with the package manager for your Linux version:

Linux Version Install Command
Debian, Ubuntu sudo apt-get install tigervnc-viewer
Fedora, CentOS, or RHEL sudo yum install tigervnc
Gentoo emerge -av net-misc/tigervnc

Next, start TigerVNC by either finding it in the applications menu or running vncviewer on the command line. In the "VNC Viewer: Connection Details" window that appears click "Options -> Security" then tick all boxes except "Encryption None" and enter one of the following paths in the "Path to X509 CA Certificate" field.

Linux Version Path to X509 CA Certificate
Debian, Ubuntu /etc/ssl/certs/ca-certificates.crt
Fedora, CentOS, or RHEL /etc/pki/tls/certs/ca-bundle.crt
Gentoo /etc/ssl/certs/ca-certificates.crt

To save the settings click OK and then click Connect. If Connect is not clicked, the settings will not be saved.


Now you need a VNC server to connect to. This can be either a persistent vncserver running on dedicated VDI nodes which are part of Graham, or a temporary vncserver you start on a cluster compute node. VNC is not a heavyweight server, so you can certainly run lightweight sessions on cluster login nodes.

Virtual Desktop Infrastructure (VDI) nodes

Graham has two dedicated VDI nodes collectively known as gra-vdi. These nodes provide a remote desktop environment equipped with accelerated OpenGL. They are intended for the most demanding and complex interactive graphical tasks. The VDI nodes share Graham's /home, /project, and /scratch filesystems. As a result, any data files or directories created on graham by running jobs in the queue will immediately be accessible on gra-vdi for visualization and post-processing purposes without the need to transfer them over.

To connect to gra-vdi, start VNC viewer (tigervnc) on your laptop and enter VNC server: This will bring up a login screen where you can enter your Alliance credentials. A desktop session will then be started on either gra-vdi3 or gra-vdi4 using a round robin algorithm.

Users can also connect directly to either machine by specifying VNC server: or enter VNC server: This may be useful if you find one machine is overloaded (oversubscribed) and thus not very responsive. Or if you consistently want to use the local $SCRATCH directory (defined as /local/tmp/$USER) on one server but not the other. Notice that $SCRATCH is defined differently on the clusters (as /scratch/$USER) where it is shared by all nodes. Similar to the clusters however, any data left on $SCRATCH on gra-vdi will eventually be deleted since the disc space is limited. Please do not plan to store any files on $SCRATCH for more than 60 days!

Lastly, please keep in mind the VDI nodes are a shared resource and intended for visualization tasks. If you need to perform long-running computations within an application which uses a GUI (graphical user interface), then please log out of gra-vdi and instead connect to a compute node on any cluster as described in the Compute Nodes section below. This will ensure the memory and CPU resources on gra-vdi remain fully available for other users to conduct their own simultaneous graphical work without any noticeable performance impacts.

Login nodes

If you want to run a lightweight application in a remote VNC desktop (one that does not require much memory, cputime or a gpu) you may start a VNC server on a cluster login node and then connect to it with the following procedure:

[laptop:~] ssh

1) Specify a 1hr (3600 sec) time limit for your server (or more as required):

[gra-login2:~] vncserver -MaxConnectionTime 3600
 Log file is /home/username/.vnc/gra-login2:3.log

2) Determine the listening port (5903 in this example):

[gra-login2:~] grep port /home/username/.vnc/gra-login2:3.log
 vncext: Listening for VNC connections on all interface(s), port 5903
[gra-login2:~] exit

3) Open a terminal window on your desktop and start a SSH tunnel to the VNC server:

 [laptop:~] ssh -L 5901:gra-login2:5903

4) Open another terminal window on your desktop and connect with vncviewer:

 [laptop:~] vncviewer localhost:5901

Mac or Windows users should click the TigerVNC Viewer application icon on their desktop and enter the localhost:port information in the "Connection Details" dialogue box that appears. Keep in mind that strict memory and cputime limits apply on cluster login nodes. On Graham, these are 8GB and 1 cpu-hour per process according to ulimit -t -v. If you require more resources, then run your VNC server on the VDI nodes or compute nodes instead as described above and below respectively.

Compute nodes

Where VDI login nodes are unavailable you can start a VNC server on a compute node, and with suitable port forwarding, connect to it from your desktop. This gives you dedicated access to the server, but does not provide a full graphical desktop or hardware-accelerated OpenGL.

1) Start a VNC server

Before starting your VNC server, log in to a cluster (such as Cedar) and create an allocation on a compute node using the salloc command (a 24hr maximum time limit applies). For example, to request an interactive job using 4 CPUs and 16GB of memory you could use the command:

[username@cedar5:~/project] salloc --time=1:00:00 --cpus-per-task=4 --mem=16000 --account=def-username
salloc: Pending job allocation 20067316
salloc: job 20067316 queued and waiting for resources
salloc: job 20067316 has been allocated resources
salloc: Granted job allocation 20067316
salloc: Waiting for resource configuration
salloc: Nodes cdr768 are ready for job

Once your interactive job has started, one environment variable must be set in order to avoid some repetitive desktop errors:

[username@cdr768:~/project] export XDG_RUNTIME_DIR=${SLURM_TMPDIR}

Then, start a VNC server with vncserver. Take note of which node your job is running on. If unsure, you can use the hostname command to check. The first time you do this you will be prompted to set a password for your VNC server. DO NOT LEAVE THIS BLANK. You may change it later using the vncpasswd command. Continuing with the example:

[username@cdr768:~/project] vncserver
You will require a password to access your desktops.
Would you like to enter a view-only password (y/n)? n
xauth:  file /home/username/.Xauthority does not exist
New ' (username)' desktop is
Creating default startup script /home/username/.vnc/xstartup
Creating default config /home/username/.vnc/config
Starting applications specified in /home/username/.vnc/xstartup
Log file is /home/username/.vnc/

Determine which port the VNC server is using by examining the log file:

[username@cdr768:~/project] grep -iE "\sport|kill" /home/username/.vnc/
 vncext:      Listening for VNC connections on all interface(s), port 5901

2) Set up a SSH tunnel to the VNC server

Once your VNC server has been started, create a "bridge" to allow your local desktop computer to connect to the compute node directly. This bridge connection is created using an SSH tunnel. SSH tunnels are created on your computer in a new local terminal window using the same SSH connection command as usual, with an extra option added which follows the format ssh user@host -L port:compute_node:port.

An example of an SSH tunnel command run on your computer to connect to a VNC server running on Cedar's cdr768 node and port 5901 would be the following:

[name@local_computer]$ ssh -L 5902:cdr768:5901

The SSH tunnel operates like a normal SSH session: You may run commands over it, etc. However, keep in mind that this SSH session is also your connection to the VNC server. If you terminate the SSH session, your connection to the VNC server will be lost! For more information, please see SSH tunnelling.

3) Connect to the VNC server

If you have a Linux desktop, open a new local terminal window and tell your VNC client to connect to localhost:port. The following example uses the TigerVNC vncviewer command to connect to the running VNC server on cdr768. You will be prompted for the VNC password that you set up earlier in order to connect.

[name@local_computer]$ vncviewer localhost:5902
TigerVNC Viewer 64-bit v1.8.0
Built on: 2018-06-13 10:56
Copyright (C) 1999-2017 TigerVNC Team and many others (see README.txt)
See for information on TigerVNC.

Tue Jul 10 17:40:24 2018
 DecodeManager: Detected 8 CPU core(s)
 DecodeManager: Creating 4 decoder thread(s)
 CConn:       connected to host localhost port 5902
 CConnection: Server supports RFB protocol version 3.8
 CConnection: Using RFB protocol version 3.8
 CConnection: Choosing security type VeNCrypt(19)
 CVeNCrypt:   Choosing security type TLSVnc (258)

Tue Jul 10 17:40:27 2018
 CConn:       Using pixel format depth 24 (32bpp) little-endian rgb888
 CConn:       Using Tight encoding
 CConn:       Enabling continuous updates

If you are on a Mac or Windows desktop, click the TigerVNC Viewer application icon and enter the localhost:port information. For our example it becomes: thumbMac Tiger VNC Viewer Connection Details Dialogue Box

Please note the port number in localhost:port specified above (5902) must match the local port (the first number) you specified when you set up the SSH tunnel. The default VNC port is 5900. If you specified 5900 for the local port of the SSH tunnel, you could omit it when you invoke vncviewer. However, Windows users may find that they cannot set up an SSH tunnel on local port 5900. Once connected, you will be presented with a Linux MATE desktop. To launch a terminal, click on the top menu on "Applications -> System Tools -> MATE Terminal". You may also add a shortcut to the top menu by right-clicking on "MATE Terminal" and by clicking on "Add this launcher to panel". Finally, to launch a program, invoke the command as you would normally within a bash session, for example xclock. To start a more complicated program like MATLAB, load the module and then run the matlab command.

More information

Vncserver password

To reset your VNC server password, use the vncpasswd command:

[gra-login1:~] vncpasswd
Would you like to enter a view-only password (y/n)? n

Optionally you can completely remove your VNC configuration (including your password) by deleting your ~/.vnc directory. The next time you run vncserver you will be prompted to set a new password.

Killing vncserver

If a vncserver is no longer needed, terminate it with vncserver -kill :DISPLAY# as shown here:

[gra-login1:~] vncserver -list

TigerVNC server sessions:

:44		      27644

[gra-login1:~] vncserver -kill :44
Killing Xvnc process ID 27644

If you have multiple vncservers running on a node, you may kill them all by running pkill Xvnc -u $USER.

Multiple connections

Any vncserver(s) running under your username can be listed with vncserver -list. To reconnect to a particular vncserver on a login node 1) re-establish a tunnel 2) run the vncviewer command again. It is possible to start multiple remote vncviewer connections to an existing vncviewer desktop, for example to open a second connection from your home machine while leaving the original vncviewer connection running at the office. To do this requires starting your vncserver with a special option vncserver -AlwaysShared, otherwise additional connections will by default close the initial connection to vncserver.

Failures to connect

Repeated failing attempts to establish a new vncserver/vncviewer connection may be due to an old SSH tunnel still running on your desktop tying up ports. To identify and kill any such tunnels, open a terminal window on your desktop and run ps ux | grep ssh followed by kill PID.

Unlock screensaver

If your VNC screensaver times out and requests a password, enter your cluster account password to unlock it (not your vncserver password). If you are running the MATE desktop and the screensaver will not unlock, try running killall -9 .mate-screensaver. This should no longer be a problem on our clusters as the VNC screensaver has been disabled.


The VDI nodes support direct vncviewer connections and GPU-accelerated OpenGL graphics for appropriately-configured software applications. The VDI nodes also provide an extra set of software modules in the SnEnv environment, analogous to the StdEnv environments available on all our clusters.

On regular login nodes, a standard software environment and some default modules are automatically loaded when you log in. This is not so on a VDI node, thus you will see:

[name@gra-vdi4]$ module list
No modules loaded

Therefore, before running any graphical software on gra-vdi you must first manually load one of the following


Most users will find it sufficient to load the StdEnv module on gra-vdi. Doing so will provide access to the same software modules that are loaded by default on the clusters:

[name@gra-vdi4]$ module load CcEnv StdEnv/2023
[name@gra-vdi4]$ module avail


In some rare cases users will need to load a locally installed module(s) on gra-vdi. To do this the SnEnv must first be loaded:

[name@gra-vdi4]$ module load SnEnv
[name@gra-vdi4]$ module avail


Instead of loading SnEnv or StdEnv users may want to load the nix module which provides open-source software that is optimized to use accelerated OpenGL whenever possible. This module is only available on graham and gra-vdi and can be loaded as follows:

[name@gra-vdi4]$ module load nix

The nix and nix-env commands will now be in your path to manage software packages via NIX within your personal nix environment.

Installing software

To install a nix package into your environment, click the black terminal icon on the top menu bar or select Applications -> System Tools -> Terminal. Once a terminal window appears, run module load nix. You can now search for programs using the nix search <regexp> command and install them in your environment using the nix-env --install --attr <attribute> command. As an example, to install QGIS do the following:

[name@gra-vdi4]$ nix search qgis
[name@gra-vdi4]$ nix-env --install --attr nixpkgs.qgis

Your nix environment persists from one login to the next, so you only need to run an install command once. For example:

[name@gra-vdi4]$ module load nix
[name@gra-vdi4]$ qgis

works! In summary whatever software you install today will be available next time you load the nix module.

OpenGL applications

For accelerated OpenGL to work, it is necessary to adjust compiled binaries to pre-load "" from VirtualGL. This level of customization is automatically done for you on gra-vdi when you install any OpenGL based software package with nix. It is however something that must be done manually after you download and install any software from outside of nix. To do this we suggest using the patchelf utility to adjust the final binary. It can be installed into nix with nix-env --install --attr nixpkgs.patchelf if it's not already on the system. Then once you have built your OpenGL application against the system libraries and for example installed it as ~/.local/bin/myglapp you can add the VirtualGL system library /usr/lib64/VirtualGL/ by running:

[name@gra-vdi4]$ module load nix
[name@gra-vdi4]$ patchelf --add-needed /usr/lib64/VirtualGL/ ~/.local/bin/myglapp

Note that it is also possible to preload via the LD_PRELOAD environment variable. This is generally a bad idea as it applies indiscriminately to all binaries, and those that require a different than that set in LD_PRELOAD will then fail. It can be used safely in some cases in wrapper scripts.