[dsl]

You said originally "there's no way for a site to know what DNS servers I use." I proved that is false.

Is it used to geolocate users? No. Is it used to route traffic in most major CDNs? Yes. The two are completely different use cases.

I think this is getting way out of scope for HN. If you are still curious how this stuff works I can email you directly if you'd like.

[gojomo]

OK, a typical website acting alone can't know what DNS servers my local machine is configured to contact, and furthermore doesn't use such DNS sensing to geo-localize its content (the claim I was responding to).

But, with the technique you've described, a website coordinating with a DNS server can probe to learn one of the DNS servers that gets consulted (directly or indirectly) by my machine. Got it. Neat and useful trick.