> but port 53 wasn't blocked, so a commercial VPN resulted in unlimited internet
The VPN would have to accept a connection over 53 though, right? This also seems like a great way to possibly bypass VPN blocking via DPI, which I've been hit with before on airlines when going over 443.
Yeah, but it doesn't cost much to accept connections on all ports (AIM did it in the 90s).
DPI should be able to easily detect and block non-DNS traffic on port 53, as well as IP over DNS. Just a matter of configuration effort; but lots of networks lack configuration effort, so it's worth a try.
The VPN would have to accept a connection over 53 though, right? This also seems like a great way to possibly bypass VPN blocking via DPI, which I've been hit with before on airlines when going over 443.