For example, if your app watches SCNetworkReachability for the kSCNetworkReachabilityFlagsIsWWAN to monitor when Wi-Fi is available, it may not behave correctly under cellular fallback.

If an app uses this method and then attempts to connect, thinking it will get a connection over Wi-Fi, the app may inadvertently end up with a connection over the cellular network. The lack of the kSCNetworkReachabilityFlagsIsWWAN is not a guarantee that your connection will not go over cellular. If your code needs to enforce that a connection not go over cellular, it needs to specify that when making a networking request.

Instead, for CFNetwork code using CFSocketStream , CFHTTPStream , or similar APIs, set the kCFStreamPropertyNoCellular property to true on the CF object before connecting.

For NSURLRequests , call [request setAllowsCellularAccess: false] .

In addition to making sure your code will never be connected over cellular because of cellular fallback, this call ensures that your code will never be connected over cellular because of the VPN or convoluted routing problems (connecting to IPv6 over cellular because only IPv4 is available over Wi-Fi, etc.).

To verify that your app is behaving properly, you should test against a broken Wi-Fi network. The easiest way to set up a broken Wi-Fi network is to set up an Apple AirPort Base Station or other Wi-Fi access point to hand out a DHCP address and leave the access point disconnected from the Internet. Another simple way of testing is to unplug the cable or DSL line from your cable or DLS modem.