Molte delle promozioni per la navigazione web dal proporio smartphone precisano che l’accesso ad Internet e’ consentito *solo dal device*, e non attaccando il device ad un computer attraverso il tethering (wifi o usb). Mi sono sempre chiesto come facessero i carrier a capire quando stavo navigando nell’uno o nell’altro modo, e grazie a questo commento a questo post ho avuto la risposta, almeno per Android:

The built-in tether support acts as a WiFi hotspot and router. This does a USB tether, and acts as a network proxy. Very different.

The biggest difference, from the carrier’s end, is that routing is pretty easy to detect; each packet that goes through the router, the time-to-live (TTL) field in the IPv4 header is decremented. In addition, there is NAT going on, and there are heuristics to detect that. There are ways around this using Linux’s iptables that can masquerade the routing/NAT, but I’m unaware of any mainstream implementations of this on Android, and it would require root. This app doesn’t; I’m like 90% sure it’s all done over ADB (Android Debug Bridge), which every Android device supports.

With the USB tether acting as a network proxy, there is simply no routing/NAT going on, so there is no way for the carrier to detect what’s going on. It just appears to be data coming from the phone.

Ottimo, penso anche che il meccanismo sia estendibile anche ad altre piattaforme, non si finisce mai di imparare dai commenti! Prossima mossa, vedere se veramente il tethering USB non viene scoperto dai carrier.