I've seen one site start doing that (or at least proxying the ads through their own servers) - all ads come [random 8 letter domain].ay.[domain].com (which I im

I've seen one site start doing that (or at least proxying the ads through their own servers) - all ads come [random 8 letter domain].ay.[domain].com (which I immediately reversed because they started using it to serve auto-playing video). If it was hosted on their main domain, I wouldn't have a chance (at least without more tools).