How do captive portal network connections work?

Question

Internet access at hotels, airports cafes is often gated by a captive portal which forces you to a particular web page on first use, for example a payment page or some page to accept a terms of service or an authentication/authorization page. You see this with both wireless and wired connections.

How does this work?

Answer 1

It certainly varies by vendor of the wireless product but in my experience it usually works something like this:

1. Your laptop makes a wireless connection to an intelligent access point, which may be connected to a centralized management station.
2. Your first web request is intercepted and replied to with a Location: header that redirects you to a login/policy page (e.g. http://hotelwireless.net/login). This might live directly on the intelligent access point, or on a central management station.
3. Once you've completed authentication, your MAC address is added to a list of allowed clients causing future requests to be correctly routed to the Internet, or accessible Intranet resources.

With regards to what to call it, I've heard it referred to as a "captive portal" or "wireless access portal" most frequently.