How to check if a domain exists programmatically?

Question

Let's say I have Java or C or C++ or foo language on a computer with a connection to the world wide web and I want to programmatically check if a domain foo.com, or bar.net has been registered, how could I do it without relying on an outside service?

I'm looking for the principle here, code to make a connection is not needed.

Answer 1

Your best bet is to ask WHOIS. AFAIK a DNS lookup might not always return a result even though the domain might still be registered.

Answer 2

If you need all information, you'll need to implement the whois protocol. I found this wiki for a list of whois servers (dutch wiki, might be less informative than the english)