The extended test on https://www.dnsleaktest.com/ https://www.dnsleaktest.com/ does that. There’s also various tests that reveal EDNS subnet leakage. It’s pret

The extended test on https://www.dnsleaktest.com/ https://www.dnsleaktest.com/ does that. There’s also various tests that reveal EDNS subnet leakage. It’s pretty easy to implement; somehow don’t respond to a request, but do respond to a second. (If you’re clever you can probably do it without server side state, e.g. encode a deadline in the custom hostname.)