T managers face a problem determining what products and policies are best to properly secure their network. The problem with many security products is that it is hard to validate their claims. Penetration testing is a process for testing the security of networks by imitating an attacker. This process typically involves gathering information through Internet searches and other open sources, scanning and mapping the targeted network with tools like Nmap, determining vulnerabilities on targeted computers, and exploiting those computers. Conducting a successful penetration test involves a great deal of knowledge and a wide variety of software, including some of the following open source tools.
http://security.linux.com/security/06/09/14/1644214.shtml