Get the Whitepaper called Penetration Testing Linux with Brute Forcing Tool. It discusses using Backtrack, Metasploit, and various brute forcing tools for breaking into a Linux host.
You can download it from the following link: https://packetstormsecurity.com/files/download/102796/pentest-linux.pdf
Source: https://packetstormsecurity.com/files/102796/Penetration-Testing-Linux-With-Brute-Forcing-Tool.html