What's the problem with this?
Operators don't like mobile (3G) tethering because they'd rather sell more subscriptions, especially if the one you're tethering from offers unlimited data.
I use this every single day.
iPhone has a separate set of APN settings for tethered data, so there is no need for any fancy detection, it's immediately obvious.
My point about not bothering was in reference to TCP/IP fingerprinting.
Sounds like a very good reason to go for a Nokia or something, if one is going to do this sort of thing...
OTOH there is an apparent compatibility issue with Joikuspot and IOS 6.x wifi clients... JS works fine with everything else.
I use a mifi router. It works with every data enabled sim, even my phone sim.
Apparently, the UK "Three" network has been detecting tethering even on unlocked phones for over a year.
Interesting...
yup - I can confirm that too. I am on a Three contract with 'All You Can Eat' data including tethering. When I'm in the UK it's, quite obviously, no issue for me to have devices connected to the iPhone for internet.
However, when I'm in the EU and paying the £5 for All You Can Eat daily data then I would also receive the text message when it's detected that I have tethered a device. Interestingly, I got away with about 10 hours of tethered use before they messaged me.
I still think that "3" are detecting tethering the same way as the Iphone-contract retailers i.e. they set up the SIM card to tell the phone to set up a separate APN, to be used when tethering is used.
I would bet there is NO way for a phone manufacturer to get a phone into the high street shops in volume (which usually means branded by a network) unless they agree to support such a feature.
Otherwise, as far as I can tell, if you ran a VPN from the phone to your home or office, the telco could see absolutely nothing.
I think Joikuspot (for Nokias) prevents tethering being detected, which is why it was banned by the US networks. Nokia have to mod US-market-customised phones so Joikuspot cannot be installed.
I have seen some suggestions that the network can detect an unusual reduction in the TTL field of the TCP/IP packets.
As the tethered device contacts the network's first router via the tethering device , I suppose the TTL value would be one less than expected. Of course the network would have to own that first router, which might not work overseas, and they would have to be bothered enough to set it up to look.
However, different operating systems use different starting values for TTL, so if you tether a laptop the TTL might be very different from what they expect. Eg windows starts at 128 whereas Android starts at 64.
Or maybe they just look for "excessive" data use. That is what costs them, after all.
Interesting.
There seems no doubt they can do a number of things to indicate tethering, and they could be added up - a bit like one used to do in the early days of (primitive) spam filtering.
But if you run a VPN between the phone and a remote router (which you obviously own), they can't see anything, except packet timings, and any of those could come from an app on the phone. These days, any half decent phone is as powerful as a cheap laptop. One could run all of EuroGA on a £50 Nokia from Ebay... well maybe the slide show on the home page might run a bit slower 
Of course the network would have to own that first router, which might not work overseas,
Funnily enough, AFAIK, when you are doing mobile data abroad, your data packets travel via the phone network, all the way back to the country where your SIM card is provisioned, and they enter the internet only there.
So if you are on say Vodafone UK, and you are in Mongolia, the data travels via the cellular network all the way back to Voda UK. The dynamic IP which your phone is allocated via DHCP is allocated by Voda UK.
It did amaze me to hear this!
But if you run a VPN between the phone and a remote router ...
I don't know VPN protocol well enough to know if it decrements the TTL, as opposed to starting a new one. As I understand it, the TTL decrement is to prevent loops, so it it could be quite reasonable to decrement as normal, even with everything else encrypted. But I don't know.
And that network back to the UK story is amazing!
