Connecting to a Linux System via SSH

Introduction

SSH (Secure Shell) is the standard protocol used to securely access remote Linux systems. It provides:

  • Encrypted communication over the network
  • Command-line access to remote systems
  • Secure file transfer capabilities

Instead of passwords, many systems use SSH keys for authentication. A key pair consists of:

  • A private key (kept on your machine, must be protected)
  • A public key (stored on the server)

When you connect, SSH verifies that your private key matches the public key on the server.

Basic SSH Connection

The standard command is:

ssh -i <path-to-private-key> <user>@<hostname>

Example:

ssh -i mykey.pem root@myhost.domino-lab.net

Notes:

  • <user> is the remote user (e.g. root, ubuntu, admin)
  • <hostname> is the server name or IP address
  • The -i option specifies your private key

macOS and Linux (Recommended Starting Point)

Both systems include a built-in SSH client.

Set Key Permissions

SSH requires that your private key is only accessible by you:

chmod 600 mykey.pem

If permissions are too open, SSH will refuse to use the key.

Connect 

ssh -i mykey.pem user@hostname

Windows with MobaXterm (Recommended Option)

MobaXterm provides an easy graphical interface.

There is a MobaXterm Home edition which provides a portable edition, which does not need installation. It just needs to be extracted and started.

Steps:

  1. Start MobaXterm
  2. Click “Session” → “SSH”
  3. Enter your hostname
  4. Specify the remote user
  5. Enable “Use private key” and select your key file
  6. Connect

MobaXterm uses the standard OpenSSH key file PEM based format.

Windows with OpenSSH (Command Line)

Modern Windows versions include OpenSSH:

ssh -i mykey.pem user@hostname

Set Key Permissions (Important) 

icacls ed25519 /inheritance:d
icacls ed25519 /remove "Authenticated Users" "Users" "Everyone"
icacls ed25519 /grant:r "%USERNAME%":R

This ensures only your user can read the key.

Windows with PuTTY / WinSCP (Advanced)

PuTTY

PuTTY requires its own key format (.ppk).

Convert Key

Use PuTTYgen to convert your key:

  • Load your .pem file
  • Save as .ppk

Configure Connection

  1. Open PuTTY
  2. Enter hostname
  3. Go to Connection → SSH → Auth
  4. Select your .ppk file
  5. Connect

WinSCP

WinSCP is used for file transfers.

Steps:

  1. Enter hostname and username
  2. Select your private key file
  3. Connect using SCP or SFTP

Common Issues

Wrong Username

Always specify the remote user:

ssh user@hostname

Key Rejected

Check:

  • Correct key file is used
  • Permissions are restricted
  • Public key is installed on the server

Permission Warning

If you see:

WARNING: UNPROTECTED PRIVATE KEY FILE!

Fix permissions as described above.

Summary

Typical command:

ssh -i mykey.pem user@hostname

Key points:

  • SSH provides secure remote access
  • Use key-based authentication when possible
  • Ensure private key permissions are properly restricted