How Many DNS Nameservers is enough?

In his first episode of PING for 2025, APNIC’s Chief Scientist, Geoff Huston returns to the Domain Name System (DNS) and explores the many faces of name servers behind domains. Up at the root, (the very top of the namespace, where all top-level domains like .gov or .au or .com are defined to exist) there is a well established principle of 13 root nameservers. Does this mean only 13 hosts worldwide service this space? Nothing could be farther from the truth! literally thousands of hosts act as one of those 13 root server labels, in a highly distributed worldwide mesh known as "anycast" which works through BGP routing. The thing is, exactly how the number of nameservers for any given domain is chosen, and how resolvers (the querying side of the DNS, the things which ask questions of authoritative nameservers) decide which one of those servers to use isn't as well defined as you might think. The packet sizes, the order of data in the packet, how it's encoded is all very well defined, but "which one should I use from now on, to answer this kind of question" is really not well defined at all. Geoff has been using the Labs measurement system to test behaviour here, and looking at basic numbers for the delegated domains at the root. The number of servers he sees, their diversity, the nature of their deployment technology in routing is quite variable. But even more interestingly, the diversity of "which one gets used" on the resolver side suggests some very old, out of date and over-simplistic methods are still being used almost everywhere, to decide what to do.