A little demo project to detect which DNS provider a visitor is using.
You'll need a remote server setup.
Setup DNS records, an NS and A records (sorry, I didn't setup the use of IPv6 at this time).
First an A record for the server where the DNS and HTTP server will be run.
Next setup an NS record that for subdomain 'dns-observer' that points to the name defined in the A record.
Clone this repository and run it.
There are files which contain the IPv4 ranges of Cloudflare and NextDNS.
Cloudflare's ranges can be found here.
I took the IP ranges for NextDNS from ipinfo.io but the ranges aren't where the requests to the DNS server will come from.
This project was just quickly slapped together for a proof of concept but it wouldn't have been possible without the following: