Skip to content

Logging into PERUN Supercomputer

Introduction

This guide explains how to connect to the PERUN supercomputer using SSH (Secure Shell). You can access PERUN through a terminal-based SSH client or via the web-based Open OnDemand interface.

Prerequisites

Before logging in, ensure you have: - A valid PERUN account with username and password - SSH key pair properly configured (see SSH Keys documentation) - SSH client installed on your computer

1. Connection Methods Overview

PERUN offers two primary methods for remote access:

Method Description Best For
SSH Client Terminal-based command-line access Advanced users, scripting, automation
Open OnDemand Web-based graphical interface Beginners, file management, interactive apps

This guide focuses on SSH client connections. For Open OnDemand instructions, please refer to the dedicated Open OnDemand documentation.

2. Connecting via SSH Client

2.1 Linux, macOS, and Modern Windows

Modern operating systems (Linux, macOS, Windows 10/11) include a built-in SSH client accessible through the terminal.

Basic Connection Command

Open your terminal and enter:

ssh username@login.perun.tuke.sk -i /path/to/private_key

Parameters explained: - username - Replace with your PERUN username - login.perun.tuke.sk - PERUN login server address - -i /path/to/private_key - Path to your private SSH key file

Example

ssh jsmith@login.perun.tuke.sk -i ~/.ssh/id_rsa

2.2 First-Time Connection

When connecting for the first time, you may see a security warning:

The authenticity of host 'login.perun.tuke.sk' can't be established.
ED25519 key fingerprint is SHA256:...
Are you sure you want to continue connecting (yes/no/[fingerprint])?

Response: Type yes and press Enter to add the server to your known hosts.

Security Note

This message appears only on your first connection. If you see it on subsequent connections, contact PERUN support as it may indicate a security issue.

3. Windows-Specific SSH Clients

While Windows 10/11 includes a native SSH client, some users prefer graphical alternatives.

MobaXterm is a feature-rich SSH client with integrated X11 server and tabbed interface.

Connection Steps:

  1. Launch MobaXterm and click the Sessions button in the top menu
  2. Select SSH from the session types
  3. Configure connection settings:
  4. Remote host: login.perun.tuke.sk
  5. Username: Your PERUN username

MobaXterm Basic SSH Settings Figure 1: MobaXterm session configuration showing the Remote host field and SSH session type

  1. Configure SSH key authentication:
  2. Navigate to Advanced SSH settings tab
  3. Check the Use private key option
  4. Click the folder icon to browse for your private key

  5. Select your private key file (e.g., id_rsa)

  6. Save and connect:

  7. Click OK to save the session
  8. Double-click the saved session to connect

Session Management

MobaXterm saves your sessions for quick reconnection. You can organize them into folders for better management.

3.2 PuTTY

PuTTY is a lightweight, traditional SSH client for Windows.

Connection Steps:

  1. Launch PuTTY
  2. Configure basic session:
  3. Go to Session category
  4. Host Name: login.perun.tuke.sk
  5. Port: 22
  6. Connection type: SSH

PuTTY Basic Session Configuration Figure 2: PuTTY main window showing Host Name and Port configuration for PERUN connection

  1. Configure SSH key authentication:
  2. In the left panel, expand Connection → SSH → Auth
  3. Click on Credentials
  4. Click Browse under "Private key file for authentication"
  5. Locate and select your private key file (must be in .ppk format)

PuTTY SSH Key Authentication

Figure 3: PuTTY SSH authentication settings showing where to specify the private key file

  1. Save session (optional but recommended):
  2. Return to Session category
  3. Enter a name in "Saved Sessions" (e.g., "PERUN")

  4. Click Save

  5. Connect:

  6. Click Open to establish connection

PuTTY Key Format Requirement

PuTTY requires SSH keys in .ppk format. If your key was generated in OpenSSH format (Linux, macOS, or Windows command line), you must convert it using PuTTYgen.

Converting OpenSSH Keys to PuTTY Format

If you have an OpenSSH private key (e.g., id_rsa), convert it to PuTTY format:

  1. Launch PuTTYgen (included with PuTTY installation)
  2. Load your OpenSSH key:
  3. Click Load button
  4. Change file filter to "All Files (.)"
  5. Select your OpenSSH private key file
  6. Enter passphrase if prompted
  7. Save in PuTTY format:
  8. Click Save private key
  9. Choose a filename (e.g., perun_key.ppk)
  10. Save the file
  11. Use the .ppk file in PuTTY's authentication settings

4. Troubleshooting Common Issues

4.1 Connection Refused

Problem: Connection refused error when attempting to connect.

Solutions: - Check your network connection - Ensure you're not behind a firewall blocking SSH connections - Verify the hostname is correct: login.perun.tuke.sk - Try connecting to a specific login node

4.2 Permission Denied (publickey)

Problem: Authentication fails with "Permission denied (publickey)" error.

Solutions: - Verify your SSH key is correctly configured - Ensure you're using the correct private key file path - Check that your public key is registered in your PERUN account - Verify the private key has correct permissions: 

chmod 600 ~/.ssh/id_rsa

4.3 Host Key Verification Failed

Problem: Warning about host key mismatch.

Solution: - DO NOT ignore this warning casually - Contact PERUN support to verify if the host key has legitimately changed - If confirmed safe, remove the old key from ~/.ssh/known_hosts: 

ssh-keygen -R "login.perun.tuke.sk"

4.4 Timeout Issues

Problem: Connection times out without establishing.

Solutions: - Check your internet connection - Verify you can reach the server: 

ping login.perun.tuke.sk
- Try connecting to a specific login node - Contact your network administrator about firewall rules

4.5 PuTTY Key Format Error

Problem: PuTTY shows "Couldn't load private key" error.

Solution: - Your key is likely in OpenSSH format - Convert it to .ppk format using PuTTYgen (see section 3.2)

5. Security Best Practices

5.1 Key Management

  • Never share your private SSH key
  • Always use passphrases for private keys
  • Keep backups of your keys in a secure location
  • Rotate keys periodically (every 6-12 months)

5.2 Connection Security

  • Do not use public or untrusted networks without VPN
  • Always verify the host key fingerprint on first connection
  • Log out properly when finished: 
    exit

5.3 Private Key Permissions

Ensure your private key has restrictive permissions:

# Linux/macOS
chmod 600 ~/.ssh/id_rsa
chmod 700 ~/.ssh

Security Warning

If your private key permissions are too open, SSH will refuse to use it for security reasons.

6. Quick Reference

SSH Command Template

ssh username@login.perun.tuke.sk -i ~/.ssh/id_rsa

Common Connection Settings

Setting Value
Hostname login.perun.tuke.sk
Port 22
Protocol SSH
Authentication Public key (SSH key)

MobaXterm Quick Setup

  1. Sessions → SSH
  2. Remote host: login.perun.tuke.sk
  3. Advanced SSH settings → Use private key → Browse to key file
  4. OK → Connect

PuTTY Quick Setup

  1. Session → Host Name: login.perun.tuke.sk
  2. Connection → SSH → Auth → Credentials → Browse to .ppk key file
  3. Return to Session → Save → Open

7. Getting Help

If you encounter issues not covered in this guide:

  • PERUN Documentation: Check the official documentation at your institution's HPC portal
  • Support Email: Contact PERUN support team
  • System Status: Check for maintenance announcements

Document Version: 1.0
Last Updated: December 2025