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This may sound a bit naive questions as i am a beginner. But I do my best to put it into words. You may have experience when sometimes you want to use the internet in a cafe for example (with your wifi device ofcourse), when you click on the desired network, you will be redirected to a log-in page to insert a username and password. Is this a specific app? what is this app (functionality) called? Is it commercial or is there any open-source projects that can be used? Thank you.

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There are any of a number of different ways to do this. Many options, both open-source and proprietary will handle it. I know DD-WRT is one example of an open source router firmware which provides this kind of functionality directly. Behind the scenes, what is basically happening is that any connection that is made to the device gets routed to a special network where any request will resolve to an HTTP redirect to the login page. Once login is done, the router tracks the MAC address of the device that is allowed access and redirects it to the network that is actually connected to the network or Internet.

A similar type of goal can be accomplished for private networks using WPA-Enterprise which would require an actual username and password to be configured in the wifi connection itself, though WPA Enterprise is not what you are directly talking about since it doesn't use an open wifi connection. It is, however, more secure since it gives each user their own user specific encryption and thus multiple users can not eavesdrop on each other.

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You mean that this is DD-WRT should be replaced by the routers default firmware? –  Hossein Jan 2 '13 at 18:21
    
DD-WRT is a replacement firmware that can installed on many routers. I think the majority of systems like the one described in the question require hardware support at some level since they have to be able to redirect users differently based on authentication, though there might be a software system that could run on a computer acting as a gateway as well. –  AJ Henderson Jan 2 '13 at 18:22
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@AJHenderson If you're using the first method, does that mean that someone who doesn't want to pay for Wifi access somewhere can just listen to traffic and change their MAC address to one which is obviously authenticated at that moment? –  us2012 Jan 2 '13 at 19:16
    
@us2012 If they are using the implementation I described (MAC filtering), then yes, it could be bypassed in that manner. I'm not sure what other direct methods the AP could use to identify a unique client. I suppose other options could be a proxy that would look at some cookie or require an encrypted connection. As far as I know this would require a step less transparent to the user though. Another option would be to hand off the connection to another network using WPA Enterprise after login. This would allow for each connection to be uniquely encrypted at the transport (wireless) layer. –  AJ Henderson Jan 2 '13 at 19:24

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