To generate an SSH key pair on Linux or MacOS, we use the inbuilt ssh-keygen. Open your terminal and:

1. Run the command

   ssh-keygen -t ed25519 -C "HPCGATE,HPCLOGIN"

   • -t ed22519 → uses a modern, secure key type (recommended)
   • -C "HPCGATE,HPCLOGIN" → mandatory comment for proper key placement in our system.

2. Choose where to save the key

   You will see a prompt like:

   Enter file in which to save the key (/home/user/.ssh/id_ed25519):

   Press Enter to confirm the default or provide an alternative.

3. Set a passphrase of your choice

   Use a passphrase!A passphrase is a sequence of words or text, longer and more secure than a single-word password, making it memorable yet strong (e.g., ‘MogonClusterSecureAccess2025’ or ‘correctHorseBatteryStaple’)… Hint: Don’t use these examples, create one yourself. An empty one is a serious security concern. Note: Characters are maskedWhile typing your passphrase, characters are masked for security — no visible input appears, so just type normally and press Enter. while typing.

   Enter passphrase (empty for no passphrase): [Type a passphrase]
   Enter same passphrase again: [Type passphrase again]

4. Done!

   This created two files:

   • ~/.ssh/id_ed25519 → private key (keep this file secret!)
   • ~/.ssh/id_ed25519.pub → public key (save to share)

5. Add key to an SSH agent (optional)

   To make the ssh-agentThe ssh-agent is a background program that securely stores and manages your SSH private keys, caching passphrases to avoid re-entering them during sessions. aware of your new key run the following command:

   eval "$(ssh-agent -s)"
   ssh-add ~/.ssh/id_ed25519

   This way, you don’t need to retype your passphrase multiple times within a session.

6. Copy public key (for next step)

   Print the file to screen using:

   cat ~/.ssh/id_ed25519.pub

Modifying existing SSH Keys#

If your existing key lacks the HPCGATE,HPCLOGIN comment, add it with:

ssh-keygen -c -C "HPCGATE,HPCLOGIN" -f ~/Path/To/Your/PrivateKey

Creating an SSH Configuration File#

To simplify remote logins we recommend to use a SSH configuration fileThe SSH configuration file (~/.ssh/config) customizes your SSH client settings, such as host aliases, usernames, key files, and proxy options, making connections simpler and more efficient.. (The lines ForwardX11 yesX11-forwarding enables graphical apps from MOGON on your local display. are optional.) For this purpose edit ~/.ssh/config (create if needed) with:

nano ~/.ssh/config

Paste the configuration content below into the text editor’s window. Adjust the fileReplace <YOUR_JGU_USERNAME> with your actual JGU username (without the < > brackets) and update ~/Path/To/Private/Key with the full path to your SSH private key file. The default would be: ~/.ssh/id_ed25519, then save with Ctrl + o (confirm the filename with Enter), and exit with Ctrl + x.

# MOGON jump host
Host hpcgate
    HostName hpcgate.zdv.uni-mainz.de
    User <YOUR_JGU_USERNAME>    
    ForwardX11 yes    
    IdentityFile ~/Path/To/Private/Key

# for access to MOGON NHR and MOGON KI:
Host mogon-nhr
    HostName mogon-nhr-01
    User <YOUR_JGU_USERNAME>
    ProxyJump hpcgate    
    ForwardX11 yes    
    IdentityFile ~/Path/To/Private/Key

# MOGON jump host
Host hpcgate
    HostName hpcgate.zdv.uni-mainz.de
    User <username>
    IdentityFile ~/Path/To/Private/Key

# for access to MOGON NHR and MOGON KI:
Host mogon
    HostName mogon-nhr-01
    User <username>
    IdentityFile ~/Path/To/Private/Key
    ProxyCommand ssh -W %h:%p hpcgate

Using X11-forwarding on macOS#

In order to use the X11-forwarding, an X11-server should be installed on your system. X11 is no longer included with Mac, but X11 server and client libraries are available from the XQuartz project.