Most domains don't allow downloading the entire zone file for security reasons. If you are using your nameserver and your domain, then it might allow it. In that case, you probably have access to the original file.

Also, you need to distinguish between names pointing to the name IP address, and aliases pointing to the same name. A records map name to IP address; you can have multiple names with the same IP address. CNAME records map names to other names. There can be multiple CNAME records with the same destination name.

Finally, this will only find names in the domain you are looking at. It will not search other domains or all the domains in the world. For example, there are a couple of domains that say they are hosted on our nameservers but are not. There is no way to search for these.