Understanding Linux SSH Service: A Complete Guide to Configuration and Usage

SSH is a secure remote management protocol for Linux servers, replacing insecure services like Telnet by ensuring data security through encrypted transmission. Its advantages include high security (default AES/RSA encryption), cross-platform compatibility, and rich features such as file transfer. It consists of the server-side `sshd` (listening on port 22) and the client-side `ssh`. Installation varies: for Ubuntu/Debian, install and start `sshd` via `apt`; for CentOS/RHEL, use `yum`. Verification involves checking service status and port availability. Basic client login is done with `ssh username@IP`. Passwordless login requires generating a key pair and copying the public key to the server. Server configuration is managed via `sshd_config`, allowing modifications such as changing the port, disabling direct root login, or password-based authentication. After configuration changes, restart the service. Common issues require checking service operation, port accessibility, and firewall settings. SSH is a must-have skill for system administrators, requiring mastery of installation, configuration, and basic usage.