Captive Portal

I am writting a portable *bsd captive portal in binary form (cgi) for a project and i need your help.

I have a lot of code ready but i am stuck at a detail, how did you menage to make the captive portal know the original host the user wanted to go ?

i menaged to block the traffic and redirect the user to a local web server that shows the same page (the captive portal login page) when the user tries to access any site, but now i am unable to get the original site the user wanted to go...

thank you in advance.