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Info Services > Computing Guide > Cluster Processing > Appendix > Accessing the cvpost cluster with VNC

Accessing the cvpost cluster with VNC

Accessing the cvpost cluster with VNC

While ssh will work fine if you are on the internal NRAO network, if you are trying to display things from a remote site we recommend using VNC.

NOTE UPDATED: In the past, cluster nodes in Charlottesville could not access the bulk.cv.nrao.edu web server directly (where value added data products are often accessed), due to fundamental network architecture internals.  That has now changed with an upgrade to said bulk server; please file a helpdesk ticket if you find otherwise.

Connect to the NRAO

From your local machine, login to the ssh portal login.cv.nrao.edu with your username (e.g. cv-4386).  Skip this section if you are physically at the NRAO.

For Linux and Mac machines

ssh cv-4386@login.cv.nrao.edu

N.B.: in macOS 10.14, it may be necessary to include the option "IPQoS=throughput" when using ssh to connect successfully, e.g.:

ssh -o IPQoS=throughput cv-4386@login.cv.nrao.edu

Failing to specify the IPQoS in 10.14 results in a "packet_write_wait:...Broken pipe" error.


For Windows machines

Install PuTTY, fill in the Host Name field and click Open.


Start the VNC server

From the ssh portal, or some other NRAO machine, login to the node assigned to you (e.g. cvpost050)

ssh cvpost050

and start a VNC server with the following command


The first time you run this, it should prompt you to set a password.  Do not use the same password as your username.  The system should then return something like:

New 'cvpost050:1 (cv-4386)' desktop is cvpost050:1

The 1 in this example is your session.  You will need this number later when you use your VNC client.


Connect to the VNC server

The VNC Client used to connect to the VNC server is different depending on the OS you are using (Linux/RHEL, Linux/Ubuntu, MacOS)

Linux (RHEL, CentOS, SL, OEL, Debian)

If your local machine is an RHEL or Debian derivative, use vncviewer to start the VNC connection like so (assuming the session number is 1):

vncviewer -via cv-4386@login.cv.nrao.edu cvpost050 :1

If you are physically at the NRAO, skip the "-via" syntax like so

vncviewer cvpost050 :1


Linux (Ubuntu)

If your local machine is Linux/Ubuntu, use remmina to start the VNC connection like so (assuming the session number is 1)

Launch the remmina program and select Connection -> New

Set the Name to something descriptive like NRAO Cluster, change the Protocol to VNC - Virtual Network Computing, set the Server to the node assigned to you followed by a colon and the session number (e.g. cvpost050:1), set the User name (e.g. cv-4386).  If you see Repeater, leave it blank. Then select the SSH tab

Check the box for Enable SSH tunnel, select Custom and set it to ssh.cv.nrao.edu, set the User name (e.g. cv-4386), click on Save.  The window will disappear (Ubuntu 16+) so then right-click on the entry for this connection in the main Remmina window and then choose Connect.



The primary way of connecting on a Mac is:

Built-in screen sharing (or TigerVNC)

This assumes you are connecting from the outside.  First, establish a tunnel to the relevant node in a terminal window:

ssh -L 590X:cvpost050:5901 login.cv.nrao.edu

Where "X" is a number greater than 1 that you choose. Don't forget: the 1 in 5901 in this example corresponds to the VNC session number on the remote machine. Leave that terminal in the background.  Then in the Finder, pull down the GO menu and choose Connect to Server. For the server address, specify:


You will be prompted for your VNC password.

If you are going to be running mpicasa:

replace using Screen Sharing in the last step with the TigerVNC client. Sessions tend to hang when using Screen Sharing while running mpicasa. TigerVNC generally lives on github: https://github.com/TigerVNC/tigervnc/releases



WARNING - the content below no longer works, as newer versions of tightvnc no longer have the ssh tunneling option.  A ssh tunnel needs to be set up manually (see above) if you use tightvnc on Windows.  Versions of TigerVNC may still have the ssh tunneling option.

If your local machine is Windows, use a VNC client like the Java Viewer from TightVNC with the following setup.  The port number can be found by adding 5900 to the session number.  So in the above example, with a session number of 1, the port will be 5901.  If you are physically at the NRAO, leave the "SSH Server" line blank.


End the VNC server

Commands that are run in this VNC session will continue to run even after closing your local VNC client. Once all processes are done, you should close your VNC server by connecting via ssh again to the nmpost cluster node and running (assuming the session number is 1)

vncserver -kill :1

Common difficulties

Black screen on RHEL8

Since the move to RHEL8, it has become more common for users to encounter a black screen when attempting to connect to a VNC session.  This usually happens when more than one VNC server session is started.  The default xstartup file provided with RHEL 8 does not work well.  As a workaround, please replace the contents of your ~/.vnc/xstartup file with the following:

dbus-launch --exit-with-session gnome-session

This file can be accessed from any linux machine you are logged into onsite or via VPN/SSH access.  For example, if you connect to cvpost050 as per the above example, before you launch vncserver, you can edit this file with:

vi ~/.vnc/xstartup

Also make sure the permissions on the ~/.vnc/xstartup file are 0755. 


Other RHEL8 issues

Other issues may arise from the upgrade to RHEL8.  Many of these can be solved by exiting VNC and making sure it is no longer running (e.g. vncserver -kill :1), removing your ~/.vnc/xstartup, restarting VNC.


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