There used to be a service at myresolver.info, which worked by redirecting to a unique hostname and associating the two requests at the backend. That supported

There used to be a service at myresolver.info, which worked by redirecting to a unique hostname and associating the two requests at the backend. That supported any combination of (IPv4, IPv6) x (DNS, HTTP). This approach (listen on a /96 and encode the resolver's IPv4 address into the HTTP server's IPv6 address) can be stateless, but it only works for the special case of IPv4 DNS and IPv6 HTTP.