WIF Authentication

Hi, I have several applications that use identity foundation for authentication. The applications authenticate to a custom sts using user/pass credentials. All works well in this scenario but i have the following requirement. I need to change the custom sts to support windows integrated authentication in parallel with user/pass authentication. Basically the applications will be accessed both over the internet and also in a local intranet domain. For the users that access the applications inside the local domain the custom sts must log them automatically without credential challenge and for the users that access the applications over the internet the custom sts must show the login page where they can enter the credentials. Can you please give me some ideas on how this scenario can be achieved? Thanks in advance! ...