Web Application Client DNS Server Detection

On the opendns.org website, their content changes based on if you are using their DNS servers or not. If you are not, then there is a callout to the user to change their DNS settings. If y开发者_高级运维our machine is setup to use their DNS servers then the callout is a link to manage your settings. How are they able to detect what the client's DNS servers are set to?

A method and a demo are currently available at https://gdns.re/edns-demo.

In my words:

• they host an authoritative DNS server
• they make your browser resolve a new unique domain name managed by their DNS server
• your recursive DNS server finds their DNS server, and they associate the requested domain name with the IP of your DNS server
• your browser connects to the resolved domain, and tells it the unique domain name in the Host header
• they retrieve the IP associated with the unique domain name, and respond appropriately

There isn't a way to find out the client's DNS server specifically. However, you do have the clients IP. With that you can check if it's in a database that you have - as Brian pointed out - and if it's not in there you could still resolve the IP to a host name and lookup what the name servers are for that domain. These are the two ways that it could be figured out - which is slightly different than positively detected.