What is the SSH protocol? What are its main functions and how does it work?

SSH (Secure Shell) is an encrypted network protocol used for secure remote login and other network services over unsecured networks.

Core Functions

1. Remote Login: Allows users to securely log in to remote servers through encrypted channels
2. Command Execution: Execute commands on remote servers
3. File Transfer: Securely transfer files via SFTP, SCP, and other protocols
4. Port Forwarding: Create encrypted tunnels to forward network traffic
5. X11 Forwarding: Run graphical applications remotely

How It Works

SSH uses a client-server architecture:

• Server: Listens on port 22 (default), waiting for connection requests
• Client: Initiates connections and performs authentication
• Encrypted Channel: All communication is transmitted through an encrypted tunnel

Authentication Methods

1. Password Authentication: Uses username and password
2. Public Key Authentication: Uses public/private key pairs, more secure
3. Host Key: Verifies server identity to prevent man-in-the-middle attacks

Security Features

• All data transmission is encrypted
• Supports multiple encryption algorithms (AES, ChaCha20, etc.)
• Provides integrity verification (HMAC)
• Supports key exchange algorithms (Diffie-Hellman, ECDH, etc.)

Common Commands

bash
# Basic connection
ssh user@hostname

# Specify port
ssh -p 2222 user@hostname

# Use key authentication
ssh -i /path/to/key user@hostname

# File transfer
scp file.txt user@hostname:/path/to/destination

SSH has become the standard tool for remote management of Linux/Unix systems and is widely used in server management, automated deployment, CI/CD pipelines, and other scenarios.