SSH(Secure Shell)

SSH is how you remotely control a headless server via the command line. Always disable password login and use SSH keys, change the default port, and consider tools like fail2ban. SSH also powers secure file transfer (SCP/SFTP) and port tunneling.

Key-based authentication is the single most important hardening step: generate a key pair, copy the public key to the server with ssh-copy-id, then disable password authentication entirely so brute-force attempts become pointless. Beyond that, SSH is a versatile tool — SCP and SFTP move files securely, and local or reverse port forwarding lets you tunnel to a service without exposing it publicly. As a rule, never forward SSH straight to the internet without keys and fail2ban; better still, reach it over a VPN so it isn't exposed at all.