I have always admired the skills of people who are really good at penetration testing. Trying to gain access to a machine through not-so-obvious ways is something that requires a high degree of technical knowledge, proficiency with tools and a good dose of creativity. I have never been in a situation where I was commissioned to pentest a box myself, but I've dabbled enough with it lately.
My first exposure to it came when I took the SANS 504 course on Hacker Techniques, Exploits and Incident Handling. The class ends with a capture-the-flag session that I enjoyed a lot.
Since then, I've been keeping an eye open for some other challenges, and I found one at The Last HOPE. One of the speakers mentioned that www.de-ice.net hosts some bootable CD images that are used to teach people pentesting skills. They author of the CD's did a nice job and grouped them in different levels of difficulty. The de-ice CD's are designed to be breakable with the tools included on the Backtrack Live CD.
After downloading the images, I was hooked.
Unfortunately there are only three CD's out at the moment, but I am proud to say that I managed to win all three challenges. I also admit that I needed some help getting the last one; I was unfamiliar with one of the tools used and needed a little hint. With that last hint, I was able to solve the third and final challenge.
If you're into pentesting, or if you would like to get started, I wholeheartedly recommend taking a look at www.de-ice.net. While all the hackable images are Linux-based (due to licensing), they are very informative and fun to do. Don't bang your brains too hard; they progressively get more difficult!