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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?

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Kinda like this, but not done for evil. – Zoredache Mar 12 '12 at 7:49

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. 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.

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I have also seen it using dns, so the first dns query will be resolved to the login/policy page. – Niko S P Mar 12 '12 at 1:19
Are you looking to set one up? There are some quick, easy and secure linux distros like pfSense that you can deploy in less than an hour. – G Koe Mar 12 '12 at 2:39
How does the client side work? Windows 10, upon getting such a wifi connection, will launch default browser to go to the portal. Android 5 phones will launch a Sign-in to Network app (not the default browser) which basically just show that portal page. Is there a new protocol involved? What are its specifications? – Old Geezer Jan 4 at 14:49
I know it's an old post, but how do you intercept juts the first web request (or rather web requests until the user has authenticated by some means like clicking a button) ? – Dominik Feb 11 at 0:57

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