If we were to ask a dozen senior system admins to check all their mission-critical systems, most of them would start with checking their DNS servers. Regardless of the size of the server farm, if the DNS fails, everything will go down in a matter of seconds. In this guide, we will start with the basics and explore the behaviors of the DNS system. This guide is about designing a highly available and secure DNS cluster to support millions of users. The guide includes related configuration parameters but does not include step-by-step instructions.