How do mobile phone companies detect if you are sharing your 3G/4G internet connection?
Asked Answered
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I just spoke with a phone company in Denmark, that offer "Free 3G/4G", on the phone, but they have a limitation if you create a hotspot and share the connection, this limitation is on 50GB.

How do they detect if your mobile device is sharing the connection? Do they intercept some special headers that the computer/other phones send over the connection?

Graiae answered 29/8, 2016 at 9:49 Comment(0)
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MAC addresses are used at a lower layer than IP and relate to each hop or leg of an end to end communication so they don't really help with this issue.

Detecting tethered devices is quite a complicated task and there are special solutions to do exactly this. They tend to look at multiple things to try to determine if other devices are using the mobiles connection, even if the devices are spoofing or manipulating headers etc.

Examples of things a solution will look at:

  • number of simultaneous sessions
  • HTTP user-agent headers
  • device type
  • device screen size
  • TCP Timestamp
  • TCP Source Port
  • TCP Sequence Number
  • Application-based correlation to TCP flows
  • Node-pair correlation to TCP flows

See here for an example solution and more details of the above (this is just one example solution):

Grease answered 31/8, 2016 at 6:22 Comment(0)
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We put the question to Three, which not too surprisingly declined to answer. However, if you think about it logically, it should be fairly simple to detect tethering. Every device with a network connection (including Wi-Fi) has a unique hardware identifier called a MAC address. Assuming that the operator can trace the final destination of the data packets, it should be able to determine that the final MAC doesn't match your phone's. source

Although the post is about UK operators, it should apply in Denmark.

Lox answered 29/8, 2016 at 10:9 Comment(1)
Would it not be relatively simply to spoof the computers mac address, but i assume it would collide with the phone if they were the same?Graiae

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