What about the doubleclick domains and adwords domains which feed the same data into the same system too? And what about when they launch a new domain? (Would be difficult to keep up with those changes)
For example, do you have an httpd listening on 127.0.0.1? Do you bind any other daemons to 127.0.0.1 or the broadcast address?
If you operate your own root you can reassign the authoritative nameservers for doubleclick.net to nameservers you control. You may or may not choose to return "A" records.
Hopefully nothing is on 127.0.0.1:80/443, that way webkit gets a RST back, which I assume is faster (and less error prone) than serving a dummy page. I'll add a note to the README.
It could also be less than a page. It could be a dummy resource. For example, in the case of an ad server and a smartphone app that has some screen space reserved for ads. You might want your own resource to appear in that space instead.
Another example is reverse engineering API's and protocols for popular web services, social media, storage, etc. In that case you might want a "dummy server" that serves certain responses.