Google (ChromeOS, Android) does a similar thing by checking http://connectivitycheck.gstatic.com/generate_204 http://connectivitycheck.gstatic.com/generate_204

Google (ChromeOS, Android) does a similar thing by checking http://connectivitycheck.gstatic.com/generate_204 http://connectivitycheck.gstatic.com/generate_204 and expecting an HTTP Response Status 204, or else it assume there's a captive portal or something blocking Internet access. Apple (iOS, MacOS) checks for https://captive.apple.com/hotspot-detect.html https://captive.apple.com/hotspot-detect.html and expects a basic HTML page with the body containing "Success".