GNU adns is a C library that provides easy-to-use DNS resolution
functionality. The library is asynchronous, allowing for multiple
concurrent calls. The package also includes several command-line
utilities for use in scripts.

The library is written by Ian Jackson and is an official GNU
package. It's homepage is http://www.chiark.greenend.org.uk/~ian/adns
and http://www.gnu.org/software/adns.