A part of what I do at Taos involves interviewing prospective consultants for our Unix/DevOps practice via a thorough technical assessment. Our technical interview spans virtually the entire breadth of topics that encompass the practice of systems administration.
One focus area that I like to spend a bit of time on is DNS. This essential service acts as the underpinning behind almost everything else a system does; when DNS goes away, your system is likely to be very, very unhappy. Despite this, few people tend to have a grasp upon how DNS works underneath the hood. I went searching for a decent write-up that explains the name resolution process, and struggled to find anything succinct that hit the points I felt were important.