Situation: One of my servers is located at my home. It’s connected to the internet via two different interfaces at the same time:
- Physical Interface: Connected to Deutsche Telekom ISP via DSL / landline.
- Virtual Wireguard VPN interface: Connected to one of my data center servers, has a public IPv6 address.
The “data center server” acts as a gateway for my home server and routes a static IPv6 address to it. This setup lets me reach my home server via a static and public IP address, although my DSL provider does not assign a static IPv6 subnet to my landline. (But that is subject to another story … ;-) ).
After finishing the setup, I ran into the problem of asymmetric routing: Packets addresses to my static IPv6 address (and thus routed via the Wireguard VPN) did arrive at my home server, but the response packets were not send back the same way: They were routed via my Deutsche Telekom landline and therefore originated from another source IP address, which the original requester did not expect.