i need an adult. I know noone who would understand my worries, so you guys need to be it.

i have a nextcloud running on my raspberry pi. performance is horrible, dont ask, but it works.

i mostly use it to backup the photos of my phone sd card every night when my phone charges. Internally this works good. If i am elseplace it wont for obvious reasons.

In my youthful joy of doom i opened port 443 and forward it to my raspi. I get internet via cable and my ip is pretty much static (it was the same for 10 months). So external access is provided.

Now i thought, its stupid that i cannot sign an ssl certificate cause i dont have a domain. Lets buy domain. But before i do that i did some try runs with duckdns to test the principle.

Some back and forth, it works now. Pretty god, i could even make a cron job on the raspbi to renew (that should work right?). Only problem. randoname.duckdns.org doesnt work internally. Or should not at least.

So i googled a bit and it turns out that my router (a cable fritz!box i bought myself) can be a local network dns. Or cannot. Regardless what i try, it doesnt accept the changed config file.

Now the problem.

It works anyway. randoname.duckdns.org points to my external "static" ip and resolves to that from my internal network..so it works on my phone or laptop. if i traceroute the thing it goes via two hops out and finishes in less than 1ms.

Now to the problem:
I have no fokkin clue why. The expected behaviour would be that it shouldnt work. If i do what i intended todo on pc in the hosts file tracert works correctly, directly pointing to the internal ip.

What i cannot figure out, is it the fritz!box being smart? Is it my ISP being smart?

Reason to rant: i have absolutly NOONE to ask, i know not a single person who would even understand what troubles me. I want to learn, i want to know WHY not just some mindless russian patchwork of "if it works its good enough".

thats depressing.