This would enable non-blocking DNS queries. One use case is during TLS peer certificate verification, we need to perform a reverse DNS lookup to get the peer's hostname. This blocks the event loop thread!
(1) Linux provides getaddrinfo_a, however I don't see an equivalent one for getnameinfo:
(2) A popular library is c-ares (MIT license):
(3) ADNS (GPLv3):
(4) c-ares has a list of other libraries: