I wish there were better authentication options with Nginx. The ngx_http_auth_request_module is limited: First, it assumes that the authentication agent doesn't need to talk to the user. Second, it doesn't cache the authentication.

Perhaps nginx might instead check all requests for a particular signed cookie, verify the signature, if the signature matches, verify that the cookie isn't too old, and then unpack variables from the cookie that the application server might want, such as REMOTE_USER. It seems nginx would then want to freshen-up the cookie.

If the cookie doesn't exist, signature doesn't match, or the cookie has expired, then, nginx should proxy the request to a delegate... but, it should return the results of that delegation directly to the user agent. It'd be the job of the delegate to set/sign the cookie with the information needed when authentication succeeds.

In this way, the authentication agent has full control over the process (so it doesn't have to be in nginx), and, heavyweight authentication is cached.

EDIT: Thanks mixedbit -- you're correct that nginx will forward 3xx onto the client. However, I recall patches are needed to support headers; and, without 200 going to the client, how do you support LDAP form authentication? Even so, an extra sub-request to authenticate each request is still heavyweight.