I run an Express js page driven heavily by socket.io but I'm planning to setup an Nginx instance (proxy-ing the node js app) to develop the rest of the website.
If a user is authenticated in Nginx how do I pass the credentials to the express js app? Can I share a cookie, or a session? Also take into consideration storing data in Redis. What's the best way of handling this?
If you use cookies to remember authentication, they will be automatically passed to Node by Nginx along with the rest of the request, with the proxy_pass directive.
If you are talking about Basic Auth (user is not strictly speaking "authenticated to Nginx" but Nginx will not serve requests without the correct credentials, you can set up custom headers when you pass requests to Node with the proxy_set_header directive, to tell Node who is authenticated.