OpenSSH Secure Shell Server

OpenSSH (OpenBSD Secure Shell) is a free and open-source suite of secure networking utilities based on the Secure Shell (SSH) protocol, providing encrypted communication over unsecured networks. Originally developed as a fork of SSH 1.2.12 by the OpenBSD project (led by Theo de Raadt) in 1999, OpenSSH was created to provide a free, secure, and audited alternative to the commercial SSH implementations. The portable version extends OpenSSH beyond OpenBSD to Linux, macOS, and other Unix-like systems. Key features: sshd: the SSH daemon (server) that accepts incoming connections, authenticates users, and provides secure shell access, file transfer, and port forwarding. sshd listens on port 22 by default and supports both SSH protocol version 2 (recommended) and version 1 (deprecated). ssh: the SSH client that connects to remote servers, providing interactive shell access, command execution, and X11 forwarding. Supports SSH agent forwarding for transparent key-based authentication across hops. scp/sftp: secure file transfer utilities. scp provides a simple interface for copying files between hosts. sftp provides an interactive file transfer session with a command set similar to FTP. ssh-keygen: generates, manages, and converts SSH key pairs (RSA, ECDSA, Ed25519, DSA). Supports key generation with custom bit lengths, comment fields, and passphrase protection. ssh-agent: a background process that holds private keys in memory, eliminating the need to re-enter passphrases for each connection. Keys are added with ssh-add. Authentication methods: public key (RSA, ECDSA, Ed25519), password, keyboard-interactive, host-based, and GSSAPI (Kerberos). Port forwarding: local (-L), remote (-R), and dynamic (-D, SOCKS proxy) port forwarding for tunneling arbitrary TCP connections through the encrypted SSH channel. FIDO/U2F hardware key support (since OpenSSH 8.2). Cross-platform: Linux, macOS, BSD, Windows, Solaris. BSD-style license.