What is SSH?

SSH (Secure Shell) is a cryptographic network protocol for securely operating network services over an unsecured network — primarily used for remote command-line access to servers. It replaced the insecure Telnet and rsh protocols. SSH uses port 22 and encrypts all traffic including passwords, commands, and file transfers. Key features: Remote shell access (ssh user@server), file transfer (SFTP and SCP), port forwarding/tunneling (forward local ports through the encrypted tunnel), and X11 forwarding (forward graphical applications). Authentication methods: password (less secure) and public key (more secure — private key on client, public key on server in ~/.ssh/authorized_keys). SSH key pairs are generated with ssh-keygen. OpenSSH is the most widely used implementation. Always disable root SSH login and password authentication in production servers.