MFA Guide#

MFA without an agent#

The following steps are applicable when you do not have an ssh agent running and you want to log in to the cluster with an ssh client like PuTTY or MobaXTerm. It also applies when using the ssh command in a terminal on Linux, Mac, or WSL.

  1. Optional Visit the VSC firewall page in a browser and login with your institution credentials. This step is optional when connecting from a white-listed IP address (among which are the KU Leuven address range, Flemish universities, most Belgian Internet providers,…). It is mandatory for other IP addresses (for instance when you are connecting from abroad).

  2. Launch your ssh client and choose your private VSC key for authentication as usual.

  3. If the private key is accepted, your ssh client will now prompt you with an URL. Visit this URL in a browser and authenticate with your institution credentials. Note that this link is dynamically generated, you cannot use it more than once.

The last step is the second factor in the authentication process. When following that link, you will be asked to choose your university/association and log in. If you already did this in the same browser session (for instance when accessing another university-related web sites or during step 1), the login will proceed automatically. If the login was successful, you will be redirected to a page with the message that your VSC identity was confirmed.

Your ssh connection is now completed!


For login[-tier1] only, a successful connection will white list your IP address for 90 days. Within that time frame, the first step mentioned above becomes optional.


The above method works fine to create the connection through MobaXTerm. The included file explorer will show you the files, but opening, downloading and uploading files from here does not work without the agent. If you would like to use the file explorer, have a look at Authentication with an ssh agent .


If you use PuTTY to login, highlighting the URL with your mouse/cursor will copy it to your clipboard, and make it ready to paste into your browser. Therefore, do not use the Ctrl+C combination on your keyboard, or it will cancel your login attempt.

GUI applications with SSH connection in the background#

Some applications such as FileZilla provide a Graphical User Interface (GUI) that will setup an ssh connection under the hood. This leads to a problem with step 3 from the last section, as those applications will typically not prompt you the firewall URL (some applications like WinSCP will do this however). To work around this, you should first login to the KU Leuven cluster with an ssh-client on your machine as explained in the previous section. As long as you keep this connection open, you can connect with the other apps as well. This extra step can however also be avoided by using an ssh agent, which will be explained in the next part.


This method will not always work for NX. It is highly recommended to use the method with an ssh agent when using MFA with NX.


It is currently not possible to connect to NX when using a ED25519 keytype. The RSA4096 keytype does allow you to connect. As this is the recommended keytype for connections to the HPC clusters, this should not be an issue for most users.

Authentication with an ssh agent#

In order to circumvent the annoyance of multiple MFA prompts or connecting to the cluster with an ssh client before being able to use certain apps like FileZilla, you can use an agent. This agent will store a certificate that contains the identity verification you did when following the firewall link. This way, you will only be asked to verify your identity once. Of course this certificate does not live forever. When using the built-in ssh-agent of Linux and Mac this will be as long as your agent lives.

Authentication with an agent on Windows#

For Windows users, the recommended agent is Pageant which is automatically installed together with PuTTY. Make sure you install version 0.78 or later.

Pageant stores the certificates together with your private SSH keys, which allows you to use VSC facilities whenever you are prompted for your identity. To setup your Pageant, please refer to Using Pageant.

Authentication with an agent on Linux/Mac/WSL#

For Linux, Mac, and WSL you can use the SSH agent included in the OpenSSH package by following these instructions to configure it correctly.


Before starting out with the built-in SSH agent, you should know that using an SSH agent poses a possible security threat when you are not using this with care. When using an SSH agent for a connection to a remote server, all certificates that are stored in your agent are visible for root users on the remote server. So be sure to only use the agent for trusted servers. If you would prefer more secure set-ups, have a look at Secure your SSH agent set-up .

First of all, verify that your agent is running. You can do this by executing:

ssh-add -l

If the agent is not running, you will get a Could not open a connection to your authentication agent. message. In this case you can start an instance of the agent with:

eval $(ssh-agent)


If you start your agent in this way, it is only accessible within the context of your current shell. If you want to connect with NoMachine NX, you should also start your NoMachine client from within this shell via the nxplayer command. Otherwise it will not be able to access the certificate stored in your agent.

(to kill the agent use eval "$(ssh-agent -k)")

If your agent is running, the ssh-add -l will list the identities that were added to the ssh-agent. If none are added, the output will state The agent has no identities.. You can add your key with:

ssh-add </path/to/your/private/key>

Now ssh-add -l should show your key.

Depending on how ssh is configured, it might be that your key will not be stored by default. It is probably best to verify the following steps before continuing:

  1. Adapt or create a profile for your cluster connection in the config file in your .ssh folder. If you do not have a config file there, create one first. From your home dir:

    touch ~/.ssh/config
    chmod 600 ~/.ssh/config
  2. In this file you can create a profile for each of your connections and add options specifically for that connection. For Tier-2:

      ForwardAgent yes
      PubkeyAuthentication yes
      ChallengeResponseAuthentication yes
      PreferredAuthentications publickey,keyboard-interactive

The indentation is not strictly necessary, but is recommended for readability.

If you now connect to the cluster using your standard ssh command, the certificate will automatically be stored for as long as your agent lives.

If you want to use apps that use ssh in the background (NX, FileZilla), you should also first inject a certificate in your agent before trying to connect. This can be done by connecting to the VSC firewall page with agent forwarding:

ssh -A


You might have to adapt some options in the configuration of your connection profiles in some apps. Have a look at Configuration of ssh-clients and UI apps below.

Secure your SSH agent set-up#

There are ways to protect yourself from possible malicious attempts to use certificates stored in your SSH agent on your local machine. A root user on any remote server can access those certificates, and can use those to connect to the remote servers for which you also have a certificate stored in your agent, and this connection will happen in your name. Luckily, as the agent only lives for a limited time this threat also only exists for a limited time. You also don’t expose your private key.

Before diving into the technical settings, the first step to ensure your security, is vigilance. Think about what connections could potentially pose a risk. Avoid adding those to your agent. When connecting to our cluster you set up the agent to avoid that you need to follow the firewall link every time. If you see no special need for using an agent, it is probably better to avoid it.

If you do wish to use multiple certificates in your agent, you can also add your key to your agent using:

ssh-add -c /path/to/your/private/key

The -c will ask for a confirmation every time you (or in the worst case someone else in your name) tries to use the agent to connect to a remote server. You will manually have to click ok before. You might have to install the package ssh-askpass on your local machine first.

Configuration of ssh-clients and GUI apps#

If you have not yet set up your ssh-client or other apps that use ssh to be able to use an agent, you might have to make some changes in your connection profiles. Different apps will need different changes, but here we shortly show what to do for MobaXTerm, PuTTY and NX:

  • MobaXTerm

    • right-click on the user session you have created to connect to the Tier-2 cluster and choose ‘Edit Session’

    • Select the ‘Advanced SSH settings’ tab

    • Uncheck ‘Use private key’ if selected

    • click ‘Ok’

  • PuTTY

    • Load your profile to connect to the Tier-2 cluster

    • Go to ‘Auth’ under ‘Connection’

    • Be sure that ‘Allow agent forwarding’ is checked

    • If you have a private key file stored under ‘Private key file for authentication’, remove it

  • NX

    • Right-click on the connection to the Tier-2 cluster

    • Click on ‘Edit connection’

    • Select the ‘Configuration’ tab

    • Select ‘Use key-based authentication with a SSH agent’

    • Click ‘Modify’ and verify that ‘Forward authentication’ is checked

Known issues - General remarks#

  • It has happened that users cannot properly load the MFA URL. If that would happen to you, it is worth trying to paste the URL in an incognito browser window. This has only been verified to work in Chrome and does not seem to work in Firefox.

  • If you are using sshfs, no link will be prompted to you as when using ssh. This is intended to be this way. The recommended approach would be to use an ssh agent to store your certificate. This will avoid you having to connect with the MFA link every time when connecting to the cluster.