WiFi "Logon" screen

I recently took a business trip and, thankfully, the hotel had wireless internet available.  After finding their network, connecting and getting an IP address, I opened Internet Explorer.  Instead of my default page being loaded, I was directed to the hotel's page for their wireless access.  It had information about their service, and a user agreement.  Only after I agreed to their terms could I freely browse the internet.

My question is how did they do that?  Is that a function of their access points, or is is special software on their server?  
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There are a few ways to do that.

I would do it by routing all web traffic (port 80) to an internal server and show the agreement page. After that's been accepted, I could either re-program the router to route traffic normally, or more likely allow the server to proxy serve for external web sites.

There are lots of HTTP proxy servers out there that will do this kind of thing, and I had the very same experience a few weeks ago myself.

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