🔐 The SSH Protocol Suite

🤔 What is SSH?

SSH (Secure Shell) is a cryptographic network protocol used for secure communication over an unsecured network. It provides a secure, encrypted channel between a client and a server, making it the standard for remote administration.

• Key Features:
  • Encryption: All traffic, including passwords and commands, is encrypted, preventing eavesdropping.
  • Authentication: Verifies the identity of both the user and the server.
  • Integrity: Ensures that the data sent has not been tampered with.
• Default Port: SSH uses port 22 by default.

💻 Remote Terminal Access Using SSH

SSH allows you to open a command-line interface (a “shell”) on a remote machine as if you were sitting right in front of it.

• How to Connect: The basic command is ssh username@hostname_or_ip.
• Example: ssh admin@192.168.1.100
• First-time Connection: The first time you connect to a new server, SSH will show you the server’s public key fingerprint and ask you to verify it. This is a crucial security step to prevent man-in-the-middle attacks.

📂 Transferring Files Using SCP

SCP (Secure Copy Protocol) uses SSH to securely transfer files between a local and a remote computer.

• Syntax:
  • Upload (local to remote): scp /path/to/local/file.txt username@hostname:/remote/path/
  • Download (remote to local): scp username@hostname:/remote/path/file.txt /local/path/

🔑 SSH Public and Private Keys

This is a highly secure alternative to password-based authentication. It uses a pair of cryptographic keys to verify your identity.

• The Key Pair:
  • Private Key: This is your secret key. It is stored on your local computer and must be kept absolutely safe.
  • Public Key: This is the “lock.” It is copied to the remote server and can be shared without risk.
• How it Works: When you connect, the server uses your public key to issue a challenge. Your SSH client uses your corresponding private key to solve the challenge, proving your identity without ever sending your private key over the network.

✈️ Connecting to Multiple SSH Servers (SSH Config File)

Managing connections to many servers can be simplified using an SSH config file, located at ~/.ssh/config on Linux and macOS.

• The Problem: Remembering different hostnames, usernames, ports, and keys is difficult.
• The Solution: You create aliases or “shortcuts” in the config file.
• Example ~/.ssh/config file:
  ~/.ssh/config

  Host work-server
      HostName 10.20.30.40
      User ubuntu
      Port 2222
      IdentityFile ~/.ssh/work_key
   
  Host home-nas
      HostName nas.local
      User admin

• Result: Instead of typing the full command, you can now simply connect by typing ssh work-server or ssh home-nas.