Hello! I recently tried NextDNS and noticed that is detects my current DNS resolved on the go. I just opened its website and it immediately showed my current resolver:
When I tried changing private DNS to Cloudflatein settings it instantly showed my new resolver.
But how exactly it works? Does the browser send used DNS server to website? Or it is done somehow via JavaScript?
And also: So every website can know what I am using now? Can it be used for fingerprinting?
Without looking at it it’s probably making a unique request to a resource on a NextDNS subdomain and watching where the request comes from. Like pulling an image from (unique _string).check.nextdns.com. This requires nothing special on the client, it’s making a standard request, and as part of that it needs to do a DNS lookup.
If the source of the and your IP are similar then it’s likely the same network, otherwise it can correlate the source with known resolvers.