How do cellular ISPs detect tethering? I've already gone back to a tethering plan but I'd really like to know how they blocked me...

I recently tried to move to an unlimited cellular plan with a local cellular provider. (They are pretty much it in our rural area so not alot of choice.) The only problem was that tethering wasn't allowed. "No problem", I thought. I have a rooted phone, how hard can that be to circumvent? Well, it turns out it was harder than I thought. After two days I gave up and went back to my old plan with limited data but that allows tethering.

It's still driving me crazy that I couldn't figure out how they were detecting me. Now granted, I'm not a top of the line programmer by any stretch of the imagination but generally I can get things to work.

Things that I tried...

*PDANet+ with hide tether over wifi (didn't try usb as I couldn't find a linux client) *ClockworkMod usb tether for linux (I use Ubuntu 16.04 and 17.04) *Using Strongvpn on my phone and then an app to route the tether through the VPN (this would work for a bit then the "you are tethered message would show up")
*Modifying the ttl on ubuntu by using this statement "sudo iptables -t mangle -A POSTROUTING -j TTL --ttl-set 065" *Using my asus router to spoof the ttl *modifying the apn on my phone to add "dun" to the apn type *using chrome browser "inspect" to spoof a cell phone *tethering to an ipad using an app to route through the vpn

Additional info: *When tethering my Asus router to my android phone I would get this error message "your isp dhcp is not functioning properly" *Occasionally the "you are tethering warning screen" would popup as a false positive. Happened to hubby a couple of times while he was looking at ham radio information.

Now, it's quite possible that some of the things I tried were really silly but I'd still like to know what methods they use to detect tethering?

Thanks everyone!