Years ago I ran a “public” iodine service - anyone could connect, outbound traffic from the server went over Tor (to protect me from abuse requests).

I kept a log of all train/hotel/airport/etc networks that it worked on, and also offered some other tunneling protocols.

Was a fun project for a while. Wish I’d kept the thing running.

These days DNS tunnelling is a bit harder to do due to some DNS servers (notably Google) randomising the case in DNS requests which breaks certain encodings horribly.