SSH Implementations

Two major implementations of SSH exist for Linux:

SSH The original SSH was developed by SSH Communications Security ( This product is commercial, but source code is available with some packages. As I write, the latest version is 3.2.

OpenSSH This product was originally affiliated with the OpenBSD OS, but versions for many other OSs, including Linux, have since emerged. This is an open source re-implementation of the SSH protocols, and it is compatible with the original SSH product. The latest version is 3.6.1. You can learn more at

Most Linux distributions ship with OpenSSH. Debian calls its OpenSSH package ssh. Mandrake and Red Hat both split the software into three packages: openssh, openssh-clients, and openssh-server. Slackware and SuSE both ship the software in a single package called openssh.

Note Because the original SSH is commercial and isn't included with any major Linux distribution, whereas OpenSSH is open source and comes with all major Linux distributions, this chapter uses OpenSSH as a model. Most of the information applies to the commercial version, though. For brevity, I usually refer to the software as SSH, except when the distinction between the commercial SSH and OpenSSH is important.

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