DNS is the backbone of the Internet. When one is not able to access to a content (such as a website), the first thing to do is to verify the DNS connectivity. This talk will provde an overview of an open source DNS checking tool called "Zonemaster" (https://github.com/dotse/zonemaster), developed and maintained by Afnic (www.afnic.fr) and IIS (www.iis.se). The talk will further delve into the architecture, different components (Engine, API, GUI, CLI) and its usages for different end-users such as general users, Companies operating DNS, Companies having a DNS portfolio of domain names and DNS geeks.