We have several physical web servers. There are also two virtual hosts on the same physical server, running on different ports.
We have a "welcome" web application, that is a "router" to other web applications on these web servers so that a user could easily find the necessary application. Not all "directions" are immediatly visible to a "guest" user. To see all the links to all "directions" available to him or her, a user must log in, using a login form. All works well when a user logs in and gets to a web application deployed on the same server.
However, when an authenticated user follows a link to a restricted application, which is only visible to authorized users, on another physical server, the login page from that server appears again. This means that user has to authenticate himself again...
So the question is: how, if that is possible, to achieve a single sign on among several web servers, just like it works among serveral web applications on the same virtual host? If this is a non-standard feature, which web servers support it?