Access

Up: Supercomputers

SSH

Interaction with iVEC supercomputers is done remotely, over an encrypted communication channel. Secure Shell version 2 (SSH-2) is used for this. This gives you command-line access to one of the login nodes of an iVEC supercomputer, from which you can run commands to interact with the supercomputer or use a command-line text editor to edit files. SSH can also be used to securely tunnel GUI programs such as GUI text editors and debuggers.

Logging in from Linux and Macs

There are several ways of logging into the different login nodes as long as you use secure shell (SSH-2) protocol. For example, you could login from your linux/unix workstation or your Mac terminal window using the command line interface. You can use any of the following commands to login to EPIC.

ssh [email protected]

Mac users can use a similar command from their xterm windows to login.

To allow remote programs to control your local display, such as being able to open up a new GUI window (such as for a debugger), use:

ssh -Y [email protected]

The –X flag can sometimes be used instead of –Y, and is preferred for improved security.

Logging in from Windows

A typical Windows installation will not include a terminal client, though there are various clients available. You may wish to check with your institution to see if they have a recommended or preferred client. Alternatively, a suitable and free client, called PuTTY, is available for all recent versions of Windows. It can be downloaded and installed following the instructions on the PuTTY website, provided you have the appropriate permissions for your computer. Otherwise, you may need to ask a system administrator to complete the installation for you.

If you wish to run graphical applications – such as an editor, a debugger, or a visualisation tool – on Epic, then you will also need an X Server. Again, this is not usually part of a Windows installation, though there are various options including XMing for which there is both a free and a paid-for version. Putty does not have its own X server.

The process for logging in to Epic will vary from client to client, though you will typically be asked to provide the host name of the login node – choose epic.ivec.org or fornax.ivec.org depending on which supercomputer you want to log in to. You may be asked to specify the connection protocol, which is SSH. Below, we explain step-by-step how to connect to Epic from a PuTTY session.

Start PuTTY using, for example, the icon added to the Start menu during the installation process. This will open a configuration window something like that displayed in Figure 1.

The configuration window may look a little daunting, as it presents the full configurability of PuTTY on a single screen. However, it is relatively straightforward to set up a simple session using the following steps.

1. (If necessary, click on Session in the list of configuration items in the left-hand pane) then enter the hostname (epic.ivec.org or fornax.ivec.org) into the Host Name field on the right and check that the Connection type is set to SSH.
2. If you want to run remote GUI applications, then in the left-hand pane, expand the configuration options for SSH (by clicking on the + symbol immediately to the left of it) and then click on X11. Then make sure that the option “Enable X11 forwarding” in the right-hand pane is checked. This allows you to run graphical clients on your local X server.
3. Return to the Session section (as edited in step 1) and enter a memorable name, such as Epic, in the field immediately below Saved Sessions text. Then click the Save button to add your new session to the list of saved sessions. This means that you will not repeat the configuration process next time you run PuTTY: instead you can simply highlight it in the list of Saved Sessions and click the Load button.
4. Finally, click Open to start the session. The configuration window will disappear and be replaced by a terminal window.
5. Assuming this is first time you have connected to Epic from your computer, you will be asked to confirm that you trust the host (that is, Epic) – see Figure 2. Confirm so by clicking Yes. PuTTY will then remember the fingerprint of Epic and will not prompt you to trust this host again.
   PuTTY Security Alert, which prompts you to confirm you trust a new host
   
   
6. You may now enter your username and password, as prompted, to log in.

You may explore other PuTTY configuration options – for example, setting colours and mouse-interaction options, at your leisure. Make sure to save any changes you make in your session object. At this time, you may wish to check that your X Server is also installed correctly, with the following steps:

1. Start XMing (or your chosen X Server) from the Windows Start menu – typically the X server will start in the background and wait quietly for you to start an application in your terminal client.
2. To do this, change the focus to the Epic terminal and type the command:

   xclock

   at the prompt. If everything is in order, a simple clock application should appear on your desktop. However, your firewall software may pop up a warning, since the X Server sets up a connection to the remote host that many firewalls are not familiar with. You should allow the connection through the firewall in order to run the application.
3. You can close the clock by simply closing the window as you would do for any Windows application.

Windows Alternatives

MobaXterm includes an integrated X server with SSH client. It is very easy to use, and does not require installation, so can be used from a USB stick.

Cygwin is a port of many applications to Windows, including ssh and X.

Common Problems

“Server unexpectedly closed network connection” is usually due to the system denying you access as a result of too many failed connection attempts from your IP address. This is a security feature we have implemented to mitigate against brute force password attacks, and the system thinks that your multiple failed logins (perhaps you typed your password wrong a few times) is such an attack. When this happens, email [email protected] with your username and when you could not log in, and we can work out what IP address you tried and unblock it.

“fork: retry: Resource temporarily unavailable” is due to you having too many processes running on the login nodes. iVEC has limited the number of processes for each username because the login nodes are in high demand from many users, and process limits reduce the chance of the login nodes becoming overloaded and unusable. You cannot get this limitation lifted for you, but there are steps to take to minimize its impact on you:

• Transfer data via the datamover nodes, such as epicdata.ivec.org or fornaxdata.ivec.org. These are different servers and any processes spawned by scp/winscp/filezilla/etc will not affect you on the login nodes.
• When compiling programs, many processes can be spawned by Make, compilers, preprocessors, wrapper scripts, etc. Consider writing a PBS jobscript for compiling and run it via the queue. If you need to be interactive with the compilation, use qsub –I and the debugq.
• Sometimes when you logout processes can get left behind. If you can’t log in to kill them, email [email protected]. You can kill processes via the kill command, and work out the process number to kill by using ps ux.
• Try to limit yourself to two or three login sessions. E.g. one for editing files and another for submitting jobs.