On my system the ipFire router is set as DNS server and as it looks and I understand it, my PC tries to request the shown external IP addresses as DNS server.
Is this correct and what can I do now? The messages always come directly after the start of the PC and then no longer.
You can try to do a REDIRECT of DNS (if you want).
if I may comment on that one: This is a good thing to do anyways, since it ensures your clients’ DNS traffic needs to pass a properly configured, DNSSEC-validating resolver.