Asynchronous DNS library (+ license)

Hello all,

I'm working on a commercial application which will run Linux. The application will be distributed closed-source and for a fee. Now this application will be speed critical and will have to be able to provide domain name lookups - many at the same time. So, we need an asynchronous dns library (or we'll have to code one ourselves).
Is there any DNS library that is compatible for that? I've seen ADNS, but I don't think GPL is compatible.

Also, one side question... The application will use threads. Can we use pthreads? I can't find the license it's released under, but I believe it's part of the compiler/kernel, so I guess it's allowed, right?

Thanks in advance

uhuh google? c-ares seems to be fine wrt. licensing