Getting all Domains in the same Server

I am currently working on an application that will be scanning a huge number of IP addresses, I was wondering if there was a way to get all the domains in each one of these addresses. I know there are websites that provide reverse Iplook up but they are quite expensive for a lone developer, and provide really low number of queries. So I was wondering how do they get the information, or if there is some free toom/api, or a database that I might query.

Thanks a lot.