But couldn't you now just buy an ad, send them to ihavecancer.example.com and find that in the DNS logs later? You don't even have to own the domain. You could

But couldn't you now just buy an ad, send them to ihavecancer.example.com and find that in the DNS logs later? You don't even have to own the domain. You could use that exact one and then just find the failed resolution in the logs.