VNC

The Virtual Network Computing (VNC) is a graphical desktop sharing system that uses the Remote Frame Buffer protocol (RFB) to remotely control another computer. It transmits the keyboard and mouse updates back in the other direction, over a network.

Vnc-based connections are faster (require less network bandwidth) then X11 applications forwarded directly through ssh.

The recommended clients are TightVNC or TigerVNC (free, open source, available for almost any platform).

Create VNC Password

Note

Local VNC password should be set before the first login. Do use a strong password.

[username@login2 ~]$ vncpasswd
Password:
Verify:

Start Vncserver

Note

To access VNC a local vncserver must be started first and also a tunnel using SSH port forwarding must be established.

See below for the details on SSH tunnels. In this example we use display number 61.

You can find display numbers which are already occupied on the login2. Here you can see that displays " /usr/bin/Xvnc :79" and " /usr/bin/Xvnc :60" are occupied.

[username@login2 ~]$ ps aux | grep Xvnc
username    5971 0.0 0.0 201072 92564 ?        SN   Sep22   4:19 /usr/bin/Xvnc :79 -desktop login2:79 (username) -auth /home/vop999/.Xauthority -geometry 1024x768 -rfbwait 30000 -rfbauth /home/username/.vnc/passwd -rfbport 5979 -fp catalogue:/etc/X11/fontpath.d -pn
username    10296 0.0 0.0 131772 21076 pts/29   SN   13:01   0:01 /usr/bin/Xvnc :60 -desktop login2:61 (username) -auth /home/vop999/.Xauthority -geometry 1600x900 -depth 16 -rfbwait 30000 -rfbauth /home/vop999/.vnc/passwd -rfbport 5960 -fp catalogue:/etc/X11/fontpath.d -pn
.....

Choose free port e.g. 61 and start your VNC server:

[username@login2 ~]$ vncserver :61 -geometry 1600x900 -depth 16

New 'login2:1 (username)' desktop is login2:1

Starting applications specified in /home/username/.vnc/xstartup
Log file is /home/username/.vnc/login2:1.log

Check if VNC server is started (in this example display number is 61):

[username@login2 .vnc]$ vncserver -list

TigerVNC server sessions:

X DISPLAY #     PROCESS ID
:61              18437

Another command:

[username@login2 .vnc]$  ps aux | grep Xvnc

username    10296 0.0 0.0 131772 21076 pts/29   SN   13:01   0:01 /usr/bin/Xvnc :61 -desktop login2:61 (username) -auth /home/vop999/.Xauthority -geometry 1600x900 -depth 16 -rfbwait 30000 -rfbauth /home/username/.vnc/passwd -rfbport 5961 -fp catalogue:/etc/X11/fontpath.d -pn

Note

The vncserver runs on port 5900 + display number. You get your port number simply as 5900 + display number (in this example 61), so the result is 5961.

To access the VNC server you have to create a tunnel between the login node using TCP port 5961 and your machine using a free TCP port (for simplicity the very same, in this case). See examples for Linux/Mac OS and Windows.

Note

The tunnel must point to the same login node where you launched the VNC server, eg. login2. If you use just cluster-name.it4i.cz, the tunnel might point to a different node due to DNS round robin.

Linux/Mac OS Example of Creating a Tunnel

At your machine, create the tunnel:

local $  ssh -TN -f username@login2.cluster-name.it4i.cz -L 5961:localhost:5961

Issue the following command to check the tunnel is established (please note the PID 2022 in the last column, you'll need it for closing the tunnel):

local $ netstat -natp | grep 5961
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        0      0 127.0.0.1:5961          0.0.0.0:*               LISTEN      2022/ssh
tcp6       0      0 ::1:5961                :::*                    LISTEN      2022/ssh

Or on Mac OS use this command:

local-mac $ lsof -n -i4TCP:5961 | grep LISTEN
ssh 75890 sta545 7u IPv4 0xfb062b5c15a56a3b 0t0 TCP 127.0.0.1:5961 (LISTEN)

Connect with the VNC client:

local $ vncviewer 127.0.0.1:5961

In this example, we connect to VNC server on port 5961, via the ssh tunnel. The connection is encrypted and secured. The VNC server listening on port 5961 provides screen of 1600x900 pixels.

You have to destroy the SSH tunnel which is still running at the background after you finish the work. Use the following command (PID 2022 in this case, see the netstat command above):

kill 2022

Windows Example of Creating a Tunnel

Use PuTTY to log in on cluster.

Start vncserver using command vncserver described above.

Search for the localhost and port number (in this case 127.0.0.1:5961).

[username@login2 .vnc]$ netstat -tanp | grep Xvnc
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        0      0 127.0.0.1:5961              0.0.0.0:*                   LISTEN      24031/Xvnc

On the PuTTY Configuration screen go to Connection->SSH->Tunnels to set up the tunnel.

Fill the Source port and Destination fields. Do not forget to click the Add button.

Run the VNC client of your choice, select VNC server 127.0.0.1, port 5961 and connect using VNC password.

Example of Starting TigerVNC Viewer

In this example, we connect to VNC server on port 5961, via the ssh tunnel, using TigerVNC viewer. The connection is encrypted and secured. The VNC server listening on port 5961 provides screen of 1600x900 pixels.

Example of Starting TightVNC Viewer

Use your VNC password to log using TightVNC Viewer and start a Gnome Session on the login node.

Gnome Session

You should see after the successful login.

Disable Your Gnome Session Screensaver

Open Screensaver preferences dialog:

Uncheck both options below the slider:

Kill Screensaver if Locked Screen

If the screen gets locked you have to kill the screensaver. Do not to forget to disable the screensaver then.

[username@login2 .vnc]$ ps aux | grep screen
username     1503 0.0 0.0 103244   892 pts/4    S+   14:37   0:00 grep screen
username     24316 0.0 0.0 270564 3528 ?        Ss   14:12   0:00 gnome-screensaver

[username@login2 .vnc]$ kill 24316

Kill Vncserver After Finished Work

You should kill your VNC server using command:

[username@login2 .vnc]$  vncserver  -kill :61
Killing Xvnc process ID 7074
Xvnc process ID 7074 already killed

Or this way:

[username@login2 .vnc]$  pkill vnc

GUI Applications on Compute Nodes Over VNC

The very same methods as described above, may be used to run the GUI applications on compute nodes. However, for maximum performance, proceed following these steps:

Open a Terminal (Applications -> System Tools -> Terminal). Run all the next commands in the terminal.

Allow incoming X11 graphics from the compute nodes at the login node:

$ xhost +

Get an interactive session on a compute node (for more detailed info look here). Use the -v DISPLAY option to propagate the DISPLAY on the compute node. In this example, we want a complete node (24 cores in this example) from the production queue:

$ qsub -I -v DISPLAY=$(uname -n):$(echo $DISPLAY | cut -d ':' -f 2) -A PROJECT_ID -q qprod -l select=1:ncpus=24

Test that the DISPLAY redirection into your VNC session works, by running a X11 application (e. g. XTerm) on the assigned compute node:

$ xterm

Example described above: