Textbook of Linux

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OpenSSH

A free implementation of the SSH protocol suite.

OpenSSH provides the ssh client and sshd server that almost every Linux administrator uses to log in to remote machines, forward ports, transfer files, and tunnel traffic. The same project also ships ssh-keygen (key generation and signing), ssh-agent (key caching and forwarding), sftp (interactive file transfer), scp (file copy), and ssh-keyscan. OpenSSH is one of those quietly indispensable pieces of software that essentially every Unix system depends on.

The codebase was forked from the last freely licensed version of Tatu Ylönen's SSH at the OpenBSD project in 1999, after the original SSH became proprietary. OpenSSH has since become the dominant SSH implementation on Linux, the BSDs, macOS, and recent versions of Windows (where Microsoft ships an OpenSSH build). The OpenBSD project maintains the canonical "portable" version that other operating systems package.

OpenSSH's defaults have evolved with cryptographic best practices: SHA-1 host keys deprecated, ed25519 preferred over RSA, support for security-key-backed keys (FIDO2 / U2F) added, and recent post-quantum hybrid key exchange options. It is funded primarily by the OpenBSD Foundation through donations.

The client is preinstalled on most distributions. To install
the server:
  Debian/Ubuntu: sudo apt install openssh-server
  Fedora/RHEL:   sudo dnf install openssh-server
  Arch:          sudo pacman -S openssh
Then enable it: sudo systemctl enable --now sshd