ssh provides a safe way of connecting to a computer, encrypting traffic and avoiding passing passwords across public networks where your traffic might be intercepted by someone else. Yet making a server accessible from all over the world makes that server very vulnerable. Therefore servers are often put behind a firewall, another computer or device that filters traffic coming from the internet.
In the VSC, all clusters are behind a firewall, but for the tier-1 cluster muk this firewall is a bit more restrictive than for other clusters. Muk can only be approached from certain other computers in the VSC network, and only via the internal VSC network and not from the public network. To avoid having to log on twice, first to another login node in the VSC network and then from there on to Muk, one can set up a so-called ssh proxy. You then connect through another computer (the proxy server) to the computer that you really want to connect to.
This all sounds quite complicated, but once things are configured properly it is really simple to log on to the host.
Setting up a proxy in PuTTY
Setting up the connection in PuTTY is a bit more complicated than for a simple direct connection to a login node.
- First you need to start up pageant and load your private key into it. See the instructions on our "Using Pageant" page.
In PuTTY, go first to the "Proxy" category (under "Connection"). In the Proxy tab sheet, you need to fill in the following information:
- Select the proxy type: "Local"
- Give the name of the "proxy server". This is vsc.login.node, your usual VSC login node, and not the computer on which you want to log on and work.
- Make sure that the "Port" number is 22.
- Enter your VSC-id in the "Username" field.
In the "Telnet command, or local proxy command", enter the string
plink -agent -l %user %proxyhost -nc %host:%port(the easiest is to just copy-and-paste this text).
"plink" (PuTTY Link) is a Windows program and comes with the full PuTTY suite of applications. It is the command line version of PuTTY. In case you've only installed the executables putty.exe and pageant.exe, you'll need to download plink.exe also from the PuTTY web site. We strongly advise to simply install the whole PuTTY-suite of applications using the installer provided on that site.
Now go to the "Data" category in PuTTY, again under "Connection".
- Fill in your VSC-id in the "Auto-login username" field.
- Leave the other values untouched (likely the values in the screen dump)
Now go to the "Session" category
For advanced users
If you have an X-server on your Windows PC, you can also use X11 forwarding and run X11-applications on the host. All you need to do is click the box next to "Enable X11 forwarding" in the category "Connection" -> "SSH"-> "X11".
What happens behind the scenes:
- By specifying "local" as the proxy type, you tell PuTTY to not use one of its own build-in ways of setting up a proxy, but to use the command that you specify in the "Telnet command" of the "Proxy" category.
- In the command
plink -agent -l %user %proxyhost -nc %host:%port%user will be replaced by the userid you specify in the "Proxy" category screen, %proxyhost will be replaced by the host you specify in the "Proxy" category screen (vsc.login.node in the example), %host by the host you specified in the "Session" category (login.muk.gent.vsc in the example) and %port by the number you specified in the "Port" field of that screen (and this will typically be 22).
- The plink command will then set up a connection to %proxyhost using the userid %user. The -agent option tells plink to use pageant for the credentials. And the -nc option tells plink to tell the SSH server on %proxyhost to further connect to %host:%port.