Contents

  1. In Windows
  2. In Linux
  3. Tunneling to a cluster node
  4. Screen
    1. Example
    2. More elaborate example
    3. Some useful commands
    4. More info
  5. VNC
    1. Setup
    2. Connecting from elsewhere
    3. SSH Tunneling
    4. Sample bash script

In Windows

If you have a Windows machine, you can open an Command Prompt or Windows Powershell and connect to LST network with: 

  ssh username@login.lst.uni-saarland.de

and then start the application, script of whatever you want to use. (If it's your first time you may confirm the connection to this server once.)

A more convenient approach may the use of an graphical ssh manager like https://www.bitvise.com/ssh-client-download

In Linux

If you have a Linux machine, this is quite easy. Use: 

  ssh -X username@login.lst.uni-saarland.de

and then start the application, script of whatever you want to use. For looking at your email box, you can also configure your email client at home.

If you would like to run compute jobs, please use the compute cluster machines: https://wiki.lst.uni-saarland.de/wiki/index.php?title=User_Doc#Printing.2C_Compute_Cluster_and_other_Services

Tunneling to a cluster node

To connect from the outside directly to a cluster node you could use this: 

 ssh -t login.lst.uni-saarland.de ssh jones-6

Screen

Imagine the following scenario: You're at home, and you would like to use your home internet connection (DSL or whatever) to log into CoLi to check your email, process some data with a script, or whatever else. As you're working, your internet connection is suddenly reset. Everything you were working on that you hadn't saved is lost, or the script process you had running is orphaned (i.e. you can't see what's happening) or even killed. If this sounds familiar, read on.

You can use screen to create a persistent terminal session. This means the session will continue to run even if you are disconnected, and you can even explicitly "detach" from a screen session and re-attach again later (even from somewhere else!). You can even have several terminals (windows) open within the same screen session and switch between them easily.

Example

I want to process some data, and I know it will take my machine several hours to complete this process. I open a terminal, type screen to start a new screen session, and start my processing script. I press Ctrl+a, d to detach from the screen session, then I log out of my machine and go home. A few hours later, I want to know how my script is doing, so I log into my machine (via login from the outside), type screen -r to reattach to the screen session, and there is my script, just as I left it, busily munching away at my data the whole time.

More elaborate example

I want to split my script into two "threads" that will run simultaneously on two different machines (in half the time, obviously). I start my screen session as before, then start the first thread. Then I hit Ctrl+a, c to start a new window within screen, log into the other machine, and start the second thread. Now, within the single screen session, I can switch back and forth between the windows with Ctrl+a, p and Ctrl+a, n, respectively. Detaching from the screen session works just as before.

Some useful commands

Typing screen -ls will show the running screen sessions on the current machine. You can specify which session you want to reattach to by typing the desired screen session's identifier after the -r. You can even attach to the same session from multiple terminals by using -x instead of -r. This way, you could display different windows running in the same screen session simultaneously.

Within a screen session, you can see a list of available windows by typing Ctrl+a, ", and even copy and paste text from one window to the next.

Since the Ctrl+a command is used to prefix screen commands, you'll probably have to use Ctrl+a, a instead of just Ctrl+a to e.g. jump to the beginning of the line at the prompt.

To close a screen window, just type exit (or Ctrl+d) as you would normally. If you close the last window, the screen session is terminated. Be careful not to confuse Ctrl+d with Ctrl+a, d, or you'll end up terminating your screen session instead of detaching from it...

More info

Within screen, type Ctrl+a, ? for a help screen.

Type man screen for the screen manpage.

Go to http://aperiodic.net/screen/ for an entire Wiki (including tutorials) about every aspect of screen.

VNC

Another scenario: You want to work under X Windows from home. You may or may not have figured out how X forwarding works, but if you have, you've suffered the realization that it's slow to the point of uselessness. Again, if your internet connection is suddenly disrupted, your session is lost.

You can use VNC to create a persistent X Windows session (VNC Server) that you can connect to (using a VNC Viewer) from wherever you have internet access (preferably broadband). This works similarly to Microsoft's Remote Desktop in that your input (mouse and keyboard) is forwarded to the VNC Server and the screen display is sent to your VNC Viewer as a series of bitmap images, which are easy to compress and much less complex than the window structure itself (i.e. it's much faster).

Setup

If you've never used VNC before on your machine, you have to setup a VNC password (which is different from your CoLi account's password). To do this, type vncpasswd. You only have to do this once (and later only to change your VNC password).

Start a VNC Server on your office machine by typing vncserver. Note the desktop number given in the output (i.e. the :1 in mymachine:1).

You can now connect to this VNC Server by typing vncviewer mymachine:1 (or whatever that number was). You will be prompted for your VNC password after which you'll have a fully working X session within your normal session. Closing the VNC Viewer does not terminate this session, so you can reconnect to it later.

Connecting from elsewhere

Of course, the point is to be able to connect to this VNC Server from home. To do this, you'll need a local VNC Viewer. Since the flavor of VNC installed at CoLi is RealVNC, it makes sense to use the RealVNC Viewer program for whatever your home computer's operating system is. But you could just as well use a different VNC implementation, such as TightVNC.

For Windows, this is a very small executable (i.e. it doesn't need to be installed), available for free. Download it from http://www.realvnc.com/download.html, choosing the Free Edition, Stand-alone Viewer. Or install the whole package, but to connect to your CoLi machine, you only need the viewer.

For Linux, use either your distribution's package management system to get the viewer, or download the Linux version of the Free Edition.

For MacOS, there are a few freeware VNC clients. JollysFastVNC is fast and stable (despite being in the alpha stage). Chicken of the VNC is another client, which however seems to be rather sluggish.

Alternatively, you can use VNC's built-in webserver and Java applet to connect to your VNC Server via any Java-enabled web browser. To do this, simply point your browser to port 5800 + desktop number on mymachine, i.e. http://mymachine:5801/. This will open an applet which allows you to log into your VNC Server directly.

HOWEVER...

SSH Tunneling

...you will not be able to reach your machine directly. You need to use SSH tunnels to map your machine's relevant port to a port on your home computer, passing through login.

Under Windows, this can be done with e.g. PuTTY in several easy steps:

  1. Start PuTTY

    1. Under Session, Host Name, enter login.lst.uni-saarland.de

    2. Under Connection, tick Enable TCP keepalives

    3. Under Connection, SSH, Protocol Options, tick Enable compression

    4. Under Connection, SSH, Tunnels, Source Port, enter 5900 + desktop number, i.e. 5901, then for Destination, enter mymachine:5901, then click the Add button (substitute your actual machine name for mymachine)

    5. To save yourself the trouble of having to do this every time, go back to Session, enter some name under Saved Sessions, and click the Save button. This way, you can just double-click on that name to connect to login with all tunnel settings, etc. already set

    6. Click the Open button

    7. If you get a "PuTTY Security Alert" window, just do what it says

  2. Back on your home machine, start the VNC Viewer program, and enter localhost: + desktop number, i.e. localhost:1. If you followed all of the above steps, you should be greeted with the opportunity to enter your VNC password, after which you'll be connected to your running VNC Server.

  3. When finished, just close the viewer window

If you want to connect via your web browser, using the Java applet, substitute 5800 for all references to port 5900 in the instructions above, and open the URL e.g. http://localhost:5801/.

Under Linux/OSiX, just type ssh -Cc blowfish -L 5901:mymachine:5901 login.lst.uni-saarland.de to log into login with an SSH tunnel to mymachine as detailed above.

Sample bash script

To easily open or close the tunnel on login, you could use a script like the following: 

 #!/bin/bash
 
 # adjust the following two lines as appropriate:
 port=5901
 host="mymachine"
 
 tunnel="ssh -CfNc blowfish -L $port:localhost:$port $host"
 echo "/$tunnel\$/{print \$2}" > awktempfile
 pid="$(ps -fu $USER | awk -f awktempfile)"
 rm -f awktempfile
 if [ -z $pid ];
 then
    echo "Opening tunnel to $host:$port"
    $tunnel
 else
    echo "Closing tunnel to $host:$port (PID $pid)"
    kill $pid
 fi

public: Working_from_home (last edited 2023-05-08 08:18:41 by superuser)