Previously, I did not have IPv6 upstream, so even a wrong configuration
worked. Now it uses a different routing table for IPv4 and IPv6, so it
also works on dual-stack upstreams.
However, how it worked without IPv6 forwarding enabled, is still a
mystery to me.
The old provider was doing weird stuff with DNS that I wasn’t able to
debug well.
However, apparently, the old provider did MSS clamping on their side.
Therefore, it is now required that I do this on my side.
Simon Bruder