A lot of this depends on where we want to go from here, too. If you use traditional UDP DNS, it's one of those things the C10k folks love to show can be handled

A lot of this depends on where we want to go from here, too. If you use traditional UDP DNS, it's one of those things the C10k folks love to show can be handled by a Pentium III. You could probably get a single modern server to handle more than a billion UDP DNS requests per second. But if you want to add a TLS handshake to each request and that sort of thing, not so much.