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.
UpdateDD is a command line tool that updates a dynamic DNS record. It supports the following features: Web-based IP-detection, IP address caching, configuration files with C-like syntax, simple interface for managing multiple accounts and hostnames, and status output to syslog. Supported services include: changeip.com, dyndns.org, eurodyndns.org, ovh.net, no-ip.org, ods.org, hn.org, regfish.com, and tzo.com.
Whoisdomaincheckup is a Web-based (PHP) application that can check the registration status of a domain name. In particular, it can check whether a domain name is currently free for registration. It doesn't use any exec or system commands, but instead uses the fopen command to access the Whois servers.
DNS Module for AOLserver is a module that implements a DNS server and proxy. It can serve from its own database or perform forwarding to another DNS server and cache results. The server doesn't support a database directly, instead it provides commands similar to 'route add|del' to add and remove DNS records to/from memory cache.
Host is a complete and powerful command-line based DNS query and testing tool. It has many of the same basic features as other similar tools such as nslookup and dig, but it has more extensive testing and test automation features. Host also has a much more standard (i.e. Unix-based) command-line user interface.