I used HTTP:Request to retrieve a webpage by url "http://www.xxx.com/".

I found if the WEB Server is IIS and the link "http://www.xxx.com/" points to a html file, I could get the real-url by header "content-location", for example "http://www.xxx.com/index.html",
but if the WEB Server is Apache or the link "http://www.xxx.com/" points to a asp file, I can't get the header "content-location".
my question is how can I get the real url when I retrieve a webpage by url "http://www.xxx.com/" that points a asp file?
thanks