Redirect to login page which is in another site

I have开发者_JAVA技巧 two sites that are app1 and app2. If a user requests a resource in app2, in order to authenticate, I like to redirect to the login page which is in app1. Once authenticated over there, return back to app2.

Can you please tell what are necessary steps?

An easy way would simply be to use Response.Redirect to point the user to the new site, and place the current URL as a QueryString. If the user is successfully authenticated in the new site, then parse the querystring to get the original URL they came from and redirect them there.

(ps - you should accept answers to previous questions if you expect to get better answers yourself)