Building Secure Servers with Linux

Secure Shell Background and Basic Use

A few years ago, Finnish programmer Tatu Ylönen created a terrifically useful application called the Secure Shell, or SSH. SSH is a suite of tools that roughly correspond to Sun’s rsh, rcp, and rlogin commands, but with one very important difference: paranoia. SSH lets you do everything rsh, rcp, and rlogin do, using your choice of libertarian-grade encryption and authentication methods.

But there was a catch: SSH Version 1 relied heavily on RSA — an excellent but, until very recently, encumbered (patented) technology whose owners required that any application that used it be licensed unless used in noncommercial settings. (Even in noncommercial use, SSH’s legality was murky, especially in the U.S.). But wait, RSA’s U.S. patents expired in September 2000 — problem solved, right?

Almost: Tatu’s got to earn a living, so by the time RSA became less encumbered, SSH itself had become more so as his company, SSH Communications Security, tightened the licensing reins. In fact, beginning with SSH Version 2.0, unlicensed/free commercial use (irrespective of RSA issues) was no longer permitted. All this despite Tatu’s sincere desire that SSH become an Internet standard, one of the requirements of which is that at least one free implementation be available.

SSH Communications Security eventually reloosened the licensing reins with SSH v.2.3, making it free even for commercial use if run on Linux, FreeBSD, NetBSD, and OpenBSD, and returning the right to free ...