?? A Never-Ending Story of SSH – An Old Protocol Still Ruling Worldwide!

Introduction to SSH

Secure Shell (SSH) is a cryptographic network protocol used for securely accessing and managing remote computers over an unsecured network. It provides a secure channel for communication between an SSH client and an SSH server, ensuring data integrity, confidentiality, and authentication.

How an SSH Connection is Established

An SSH connection is established through multiple steps, involving authentication and encryption to ensure security. Below are the key steps involved:

1. TCP Connection Establishment

• The SSH client initiates a connection to the SSH server using the Transmission Control Protocol (TCP), typically on port 22.
• This is the first step that establishes a communication channel between the client and the server.

2. Version and Algorithm Negotiation

• The client and server exchange information about supported SSH protocol versions.
• They negotiate supported cryptographic algorithms, including encryption, key exchange, and authentication methods.

3. Key Exchange and Encryption Setup

• Both parties generate a session key using key exchange algorithms (e.g., Diffie-Hellman, ECDH).
• The SSH client sends a public key to the server.
• The server calculates the session key and encrypts a random number using the client's public key.
• The client decrypts this number using its private key, verifying the server's authenticity.
• The session key is used for encrypting the entire SSH session.

4. Authentication Process

• The client initiates a login request.
• The server verifies the request by checking authorized keys stored in ~/.ssh/authorized_keys.
• If a match is found, the server encrypts a challenge message using the client’s public key.
• The client decrypts it using its private key and sends back a response.
• If authentication is successful, the client gains access.

5. Encrypted Communication Begins

• The client can now send encrypted commands to the server.
• The server executes the commands and sends back encrypted results.
• Both the client and server use the session key to decrypt messages throughout the session.

Authentication Methods in SSH

SSH supports multiple authentication mechanisms:

1. Password-based Authentication
2. Public Key Authentication (Recommended)
3. Two-Factor Authentication (2FA)

Common SSH Commands

Once connected via SSH, users can execute various commands:

Command Description ssh user@server Connect to a remote server scp file user@server:/path Securely copy files to a server sftp user@server Start a secure file transfer session ssh -L local_port:remote_address:remote_port user@server SSH Local Port Forwarding ssh -R remote_port:local_address:local_port user@server SSH Remote Port Forwarding exit Close the SSH session

SSH Tunneling and Port Forwarding

SSH allows secure tunneling of traffic through port forwarding:

1. Local Port Forwarding

• Redirects local traffic to a remote server via a secure tunnel.

Example: ssh -L 8080:remote_server:80 user@server        

2. Remote Port Forwarding

• Allows the remote server to access a local port securely.

Example: ssh -R 9090:localhost:3000 user@server        

3. Dynamic Port Forwarding

• Acts as a SOCKS proxy to forward multiple connections.

Example: ssh -D 1080 user@server        

Conclusion

SSH is an essential tool for secure remote access, command execution, and file transfer. Understanding its connection process, authentication methods, and commands helps in maintaining a secure and efficient workflow. Using key-based authentication and secure tunneling techniques ensures enhanced security in remote communications.