Many people like to make use of the data plans from their carriers on more devices than their smartphones. The process by which they do this, called tethering, allows them to use their phone's data plan to access the internet from their laptops or other mobile devices. Although users enjoy this capability, carriers do not like people tethering because it usually results in more data use and more taxing of their networks.
People who want to tether their phones for free usually rationalize that because they have an unlimited data plan, they should be able to use as much data as they want. Carriers don't share the same sentiment. They usually attempt to charge users for unlimited data and restrict their device's tethering capabilities. In order to unlock those capabilities, users are supposed to pay the carriers more for a "tethering plan."
Unsurprisingly, adept users have found ways to circumvent these charges with both iPhones and Android devices. Most users feel safe tethering without the carrier's permission; after all, they are paying for unlimited data plans. Plus, the data is all the same to the carrier, right? There is no way they would know.
Well, it turns out that the carriers do know.
Reddit user (and smartphone tetherer) kehrol received the email above from AT&T, which notes, "Our records show that you use this [tethering] capability, but are not subscribed to our tethering plan." The email also requests that he either stop tethering or pay for a more expensive tethering data plan.
Users trying to figure out how AT&T knew about the tethering have concluded that it has something to do with TTL numbers of sent packets. However, it does not really matter how they know, the fact is they do. Now anyone who makes use of their unlimited data plan on other devices will have to be on the lookout for trouble from their carriers, especially if they use AT&T.
Whether you think it is ethical to tether your phone for free or not, it is still interesting to know that your carrier may be watching you closer than you think.[reddit]