I know, this question is very specific. After a whole day, I gave up and installed a clean Debian 10. Because on a system that got upgraded from 8 to 10, it was impossible for me to find the reason, why a container connected to a custom bridged network, is no longer able to talk to the outside world. In other words:
On both servers:
docker network create test
On the migrated server:
docker run --rm busybox ping google.com ----> able to ping
docker run --rm --network test busybox ping google.com ----> unable to ping
On a fresh installed Debian 10:
docker run --rm busybox ping google.com ----> able to ping
docker run --rm --network test busybox ping google.com ----> able to ping
And after spending many many hours, searching for every tiny hint out there, comparing configs, iptables, everything, i had to give up.
I am now going with the fresh system. But because i spent so much time debugging this, I still want to leave a footprint here (and a warning) - and maybe someone out there knows WHY.
In that case, I would be happy to learn something.
I compared everything between the migrated and the new server, everything looks the same.
I even installed the same docker-version, reinstalled docker several times, cleaned the iptables,
added custom dns servers, tried everything you can find on the first 5 pages of a google search.
