Skadns is a kind of an Asynchronous DNS client software:
- Kind of: it's small. Really small. But it just works.
- Asynchronous: all DNS operations are non-blocking.
- DNS client software: it's a DNS client, what you may know 
  as a "stub resolver". To perform full DNS resolution, 
  you will still need a full resolver like dnscache.

The skadns library offers a simple API to make asynchronous DNS queries.
The skadnsip, skadnsmx and skadnsfilter commands are examples 
of how to use that API. They are mainly there for their source code.
The skadnsd daemon, usable as a child process or as a separate service, 
handles the grunt work of DNS querying and makes the network 
completely invisible to the client.

WWW:	http://www.skarnet.org/software/skadns/