The documentation says what settings you need to enable for it to work. It also says that it takes the remote user in the X-Remote-User header. I use Authelia and it puts the remote user in the Remote-User instead, so i added a middleware to traefik that renames the header to the one expected by radicale. The only problem remaining is that radicale presents the auth page anyway, and you have to login with the same username as the auth header but with any password