Yes, phones request a particular (plain http) URL from Google or Apple, which returns a 204 response. If they don't get a response, the network is assumed to ha

Yes, phones request a particular (plain http) URL from Google or Apple, which returns a 204 response. If they don't get a response, the network is assumed to have a connectivity issue. If they get redirected due to captive portal, they detect that and can prompt the user to log in. https://github.com/leoleozhu/android-captive-portal https://github.com/leoleozhu/android-captive-portal