Poslib is a library which provides easy access to the Domain Name System. The client part can be used to create DNS client applications, while the server part lets you easily create a multi-threaded DNS server program, portable to many operating systems including Linux, FreeBSD, and Windows.
Network Query Tool (NQT) is a one-stop solution for getting information about a domain or IP address. Instead of manually using different Unix commands or visiting numerous Web sites to investigate a host, just load Network Query Tool and enter the hostname or IP. NQT does the rest. It supports a large number of TLDs and ccTLDs.
PowerDNS Authoritative Server is a high-performance authoritative nameserver with a host of backends. Besides plain BIND configuration files, PDNS reads information from MySQL, Oracle, PostgreSQL, SQLite, MongoDB, and many other databases. Backends can easily be written in any language, and a sample Perl backend is provided. PDNS powers around 30-40% of all domain names in many parts of the world, including >90% of all DNSSEC domains in Europe.
Posadis is a domain name server for Unix and Win32 systems. It can both act as an authoritive and as a caching DNS server, and it supports standard DNS master files, zone transfers, and DNS Notify. Its modular structure allows plug-ins to define new zone types and add new functionality.
pdnsd is a Proxy DNS server for Linux and FreeBSD that is designed to cope with unreacheable nameservers (e.g. because the dial-in link is not up) in a graceful manner to prevent DNS-dependent applications like Netscape from hanging. It has a permanent disk cache (i.e. the cache contents are saved in a file on exit) and supports parallel query and a wide variety of link uptests. It also has the ability to serve some types of locally defined records.