One of the many reasons we jailbreak our iOS devices is to add functionality that Apple or its partners don’t want us to have. Internet tethering is one of the most popular, but lately AT&T has started to crack down on such unauthorized usage -- and here’s how they know what you’re up to.

Redmond Pie is reporting that jailbreakers have finally figure out how AT&T is tracking their unauthorized tethering usage, allowing a user to share their 3G data connection with laptops, Wi-Fi only iPads and other devices. You may recall that the carrier recently started warning heavy users of unauthorized tethering via text message that the jig was up, but no one could quite figure out how they knew in the first place.

As it turns out, the answer was quite simple. According to iPhoneDownloadBlog and AndroidPolice, many unauthorized tethering apps make their method known because they work exactly the same as the official tethering from AT&T.

“Jailbroken iPhones typically use the same tethering technique as a standard iPhone, the one that’s already present in iOS,” the report reveals. “This method exposes tethering activity quite readily, because the iPhone, when in tethering mode, sends traffic through an alternate APN (AT&T access point/router) for the express purpose of identifying the traffic as tethered data. This makes it extremely easy for AT&T to identify whether or not an iOS device is utilizing tethering, and just how much of their data is consumed via tethering.

“Some tethering applications for iOS make use of alternative methods and route tethered traffic through the phone’s normal data APN, but by and large, most jailbreakers stick with the stock application because it’s easy to use and doesn’t require any complicated setup,” the report continues. “In fact, many iPhone users jailbreak for the sole purpose of avoiding AT&T’s tethering fees. These are the people AT&T’s is going after.”

One of the most popular jailbreak-only tethering apps is MyWi, and it turns out that it does indeed use the easier method, which means using the app exposes your activity to AT&T -- use it enough and they’ll likely come calling with a text message to get on board the official tethering bandwagon.

As it turns out, there is at least one jailbreak tethering app that actually hides your tethering activity from AT&T -- it appears simply as regular 3G data activity from your iPhone. The name of the app is PDANet, but since its use may be ethically questionable, we’ll leave it to readers to track it down, should you be interested in doing so…

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