Admin interface not available

Since the Tutor upgrade (/ open edX upgrade to Koa), the Django admin interface at https://learn.totem-project.org/admin/ is not available and always returns a 403.

The reason for this is that we have an nginx IP address filter on that page. The Tutor upgrade put Caddy between nginx and the LMS. As a result nginx only sees requests that come from Caddy, and filters based on that.

The nginx logs confirm this:

nginx_1            | 2021/02/24 09:18:51 [error] 10#10: *9050 access forbidden by rule, client: <not my IP address>, server: learn.totem-project.org, request: "GET /admin/ HTTP/1.1", host: "learn.totem-project.org"

2 solutions possible:

1. Move the IP filter to caddy
2. Make sure nginx gets a forwarded IP address from Caddy. The nginx logs do seem to include IP addresses from users, so it seems like Caddy even already forwards something.