During or after the install process or the first time they login to the admin page display a warning to the end user to check if port 53 is open to the internet.
I really like this idea. There was a Reddit post that showed how many PiHoles were exposed on the net and it was scary. Reddit post
I don’t know how tough this would be to implement but I do think it is a great idea.
I can’t think of any DNS server package that does display any kind of warnings when installed so my vote would be to not show any message
However, we do have a small disclaimer on the Settings page. We have also discussed open resolvers across many threads on different sites. Our community is pretty good at also pointing out the problems as people bring them up.