Thanks! You can request access to zone files from the domain authorities. I take the raw zone file and load it into a series of Bloom filters backed by Redis. There are a lot of domains to check against, and a probabilistic data structure is the only way to store it in memory, which you need for performance. This has the small caveat that the tool will accidentally think a domain is not available about 0.1% of the time. However, it shouldn't ever report domains as available when they are not. So that works quite well for a tool like this.