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Did you ever sit at the airport or at a cafe and there was a unencrypted wireless access point nearby, but whenever you wanted to visit a site their website would pop up asking for a fee to use the internet through their AccessPoint (aka. Captive Portal)?

In most cases you are able to look up arbitrary hostnames, ie. google.com. That is because if you cannot resolve a host name your browser won’t display any site. So these providers usually allow to look up hostnames to then filter whether they are allowed to access the site or not. In the latter case, their pay-to-get-access site pops up.

But you can use the fact that you can resolve arbitrary hostnames to gain free connection to the internet. Not a very fast one, though, but still a free internet connection.

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