Is it possible to validate an ASP.NET Forms Authentication ticket in ISA/UAG (or similar) and do redirects based on the result?

We have an ASP.NET MVC application that uses Forms Authentication to create and validate the authentication ticket (cookie). The log on flow is very special, not just username/password, but it ends with a call to FormsAuthentication.SetAuthCookie(userId, false) to create t开发者_开发知识库he cookie. So a standard cookie is created, it is just how it is created that is special.

We will also have to integrate with a couple of other web sites that are not neccessarily .NET based or not possible to customize via some kind of Single Sign-On that do not require any modification on the other sites. All sites are behind the same UAG/ISA Server.

Is it possible to setup some kind of filter in UAG/ISA Server that can validate the Forms Authentication cookie created by the ASP.NET MVC application? If it is not OK it should redirect to the log on page in that application? If it is OK it should just let the request through. For example, would it be possible to use a ISAPI filter with UAG that does this? We can use the same machineKeys on the UAG server and the ASP.NET app server (I guess that is required to start with)

I don't know much about UAG and I'm also interested in alternatives to UAG. In fact, what we really need is just something that works as a perimeter protection for all those sites and that can utilize the already existing log in flow/cookie.