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Setting up SSH keypair

Tip

A SSH keypair for password-less access within the UBELIX HPC system is automatically generated for all accounts since 12.11.2024. Note that this keypair is automatically rotated and should therefore not be used for any other purpose.

If you want to use UBELIX from a terminal, we recommend to register a SSH keypair. SSH keypairs serve as a means of identifying a user to a SSH server. When using SSH keys your password will never be send over the network.

UBELIX only accepts SSH keys based on the RSA (4096 bit) or ed25519 algorithms. If possible, we recommend using ed25519.

Generate your SSH keys

If you already have an appropriate SSH key pair that you want to use with UBELIX, you may skip to registering your public key. If not, start by generating an SSH key pair as detailed below.

An SSH key pair can be generated using a Linux, macOS, Windows PowerShell terminal. For example, you can use the following command to generate an ed25519 key:

ssh-keygen -t ed25519 -f $HOME/.ssh/id_ed25519_ubelix

You will be asked for a passphrase. Please choose a secure passphrase. It should be at least 8 (preferably 12) characters long and should contain numbers, letters and special characters. Do not leave the passphrase empty.

After that an SSH key pair is created, i.e. a pair of files containing the public and private keys, e.g. files named id_ed25519_ubelix (the private key) and id_ed25519_ubelix.pub (the public key) in your /home/<username>/.ssh/ directory.

An SSH key pair can be generated with MobaXterm (Tools :octicons-arrow-right-16: MobaKeyGen).

In order to generate your key pairs for UBELIX, choose the option Ed25519. Then, press the Generate button.

Create SSH key pair with windows - step 1

You will be requested to move the mouse in the Key area to generate some entropy; do so until the green bar is completely filled.

Create SSH key pair with windows - step 2

After that, enter a comment in the Key comment field and a strong passphrase. Please choose a secure passphrase. It should be at least 8 (preferably 12) characters long and should contain numbers, letters and special characters. Do not leave the passphrase empty.

Create SSH key pair with windows - step 3

The next step is to save your public and private key. Click on the Save public key button and save it to the desired location (for example, with id_ed25519_ubelix.pub as a name). Do the same with your private key by clicking on the Save private key button and save it to the desired location (for example, with id_ed25519_ubelix as a name).

Key format

To use your key, you may need two different key formats. The instructions above generate a private key in PuTTY format (PPK), which can be used with PuTTY and in a MobaXTerm SSH session created via the MobaXTerm GUI.

However, if you are using the OpenSSH client (the ssh command in a MobaXTerm terminal), you will need a key in the OpenSSH format. To convert your key, go to the Conversions :octicons-arrow-right-16: Export OpenSSH key menu in the key generator tool and save it in the OpenSSH format.

You can convert between these two key formats at any time using the key generator tool. Load a key by using the Conversions :octicons-arrow-right-16: Import key menu, then save it in the desired format:

  • OpenSSH to PPK: load a OpenSSH key, then save it with the Save private key button
  • PPK to OpenSSH: load a PPK key, then save it via the Conversions :octicons-arrow-right-16: Export OpenSSH key menu

Warning

The private key should never be shared with anyone, not even with UBELIX staff. It should also be stored only on your local computer (public keys can be safely stored in cloud services). Protect it with a good password! Otherwise, anyone with access to the file system can steal your SSH key.

Register your public key

To register your ssh key with your UBELIX account the generated public key need to be added to the ~/.ssh/authorized_keys file in your UBELIX account. This step can be done by simply issuing from your terminal:

ssh-copy-id -i ~/.ssh/id_ed25519_ubelix.pub `<alias>`

If the previous steps were successful you can now login to UBELIX using your SSH keypair using 

ssh -i ~/.ssh/id_ed25519_ubelix <user>@submit01.unibe.ch
Note that you will be prompted for the passphrase of your SSH-key instead of your Campus Account password.

Tweaking your SSH config

To simplify the login procedure you can tweak the local SSH configuration by adding a host declaration to ~/.ssh/config on your local desktop/laptop. This way we can omit typing the username and ssh-key from our ssh invocation. Note that you will need to substitute your own Campus Account username:

~/.ssh/config

Host ubelix
    Hostname submit02.unibe.ch
    User <user>
    IdentityFile ~/.ssh/id_ed25519_ubelix
    ServerAliveInterval 60

From now on you can log in to the cluster by using the specified alias:

ssh ubelix

SSH session timeout

In the definition above we silently introduced the ServerAliveInterval 60 option. If a SSH connection goes idle for a specific amount of time (default 10 minutes), you may be confronted with a "Write failed: Broken pipe" error message or the connection is simply frozen, and you are forced to log in again. To prevent this from happening, we configure the client to periodically send a message to trigger a response from the remote server through the ServerAliveInterval option

SSH Agent

By this point you have replaced typing your campus account password by typing your ssk-key passphrase every time you login to UBELIX. But you can use the helper tool ssh-agent to securely save your passphrase, so you do not have to re-enter it all the time.

The behavior of ssh-agent depends on the flavor and version of your operating system. On MacOS your keys can be saved in the system’s keychain. Most Linux installations will automatically start ssh-agent when you log in.

Add the key to ssh-agent:

ssh-add ~/.ssh/id_ed25519_ubelix

You will now have to provide your ssh key’s passphrase once per reboot of your device.