Different web servers

[Moe Elonon]

We have two web servers, Netscape (Perl) and IIS5
(ASP.Net), users must login thru Netscape server first,
and then they have the option to go to the other IIS
server for other functionalities, and I want to make sure
that the users get authenticated thru Netscape server
before they access the IIS sites.

I tried to use "Request.UrlReferrer" to check where the
users came from, but it returns the following
error "Exception Details: System.NullReferenceException:
Object reference not set to an instance of an object".

So what is it that my missing? Or is there any better way
to deal with this issue?

Thanx