- My ISP DNS
- My ISP special DNS-forward page for people who doesn't have an account. For every customer that does not have an account and plugs in a computer in the wall socket in my building a sign-up page is presented in the web browser.
- My own router
My best guess so far is alternative number 2. It is a rather complicated feature requiring to catch all packets going to port 53 (I have lost DNS quires for both TCP and UDP). It could also be my router giving me the trouble. But I did not change anything in the router settings since when the problem started.
I have some problem error searching this. For that I need a computer outside my network that I has root access to so I can set up a server on port 53.