Just to make one thing clear first. I am not an expert when it comes to DNS, nor really how it works in Erlang. I know some things, but not all. This is how it works to the best of my knowledge.
The tweet I was referring to was https://twitter.com/davidfowl/status/1365232349584052227?s=20
Erlang provides both a UDP-based implementation and “native” implementation. You can configure which is used here: Erlang -- Inet Configuration.
The UDP-based one is written completely in Erlang with the pros and cons of that. You can see the implementation here. This can be faster than the native one or not depending on what you are doing. There is work ongoing to make it better.
The “native” implementation is a port program called inet_gethost. You can see the implementation here. This program starts a pool of threads that call gethostbyname and communicates to erlang via stdin/stdout. inet_gethost was written long before nifs became a thing, which is why it is not a nif.
Both of these have been around for about 20 years with tweaks along the way, but the main point is the same. Sometimes you want the OS name resolution and something you don’t. It depends on what it is that you are doing.
