Update 2021-02-16: I accidentally solved the problem myself, with help by Docker specialist @cwrau using a IPv6 NAT container

I need help with a NAS (running the Unraid operating system) and Docker containers (Docker v19.03.5 at the moment) for various services.

My setup includes

• a nginx reverse proxy
• a NextCloud container
• a GitLab/Gitea container (and maybe others in the future once the problem is solved)
• a manually created Docker network (“proxynet”)

All these services work alright for me. The issue I have is with outgoing IPv6 connections. I don’t have a stable IPv4 but I have a dynamic DNS pointing at my NAS’s IPv6 address. I can connect from computers and mobile devices to NextCloud just fine.

I cannot ping6 google.de from within any Docker container in the proxy network, or the reverse proxy container itself, though. I also cannot connect to other NextCloud instances that are only available via IPv6, rendering the federation/sharing aspect moot.

I tried a couple of things, like running a ipv6nat container next to the others, but I don’t understand enough of the underpinnings and maybe Unraid’s limitations to make this work in reasonable time.

(Call to action removed since this is solved)

Throwing out the reverse proxy is an option, but I’d rather keep it in place and eventually use it to hide the rest of my network behind it as well. (I do have a Raspberry Pi or three in the LAN as well.)