ext-Dns is a core library, an extensible forward DNS server, and some additional tools designed to create DNS server solutions which are able to do any additional operation at the resolution level (running commands, rewritting replies, gathering stats, and so on), without any limit. It provides a ready-to-use solution in case you want to build a DNS server which takes additional actions at the resolution level in an easy way. That is, its design is focused on allowing people to implement whatever they like when a request is received, freeing them from most of the details which involves handling DNS protocol.
s6-dns is a small, efficient, complete, and IPv6-ready DNS client library, with synchronous as well as asynchronous APIs. It is designed to replace libresolv in projects that need to perform DNS resolution. It also comes with small command-line utilities to make common and not so common DNS queries.