How to check for unrestricted Internet access? (captive portal detection)

Question

I need to reliably detect if a device has full internet access, i.e. that the user is not confined to a captive portal (also called walled garden), i.e. a

Answer 1

This is best done here as in AOSP : https://github.com/aosp-mirror/platform_frameworks_base/blob/6bebb8418ceecf44d2af40033870f3aabacfe36e/core/java/android/net/captiveportal/CaptivePortalProbeResult.java#L61

https://github.com/aosp-mirror/platform_frameworks_base/blob/e3a0f42e8e8678f6d90ddf104d485858fbb2e35b/services/core/java/com/android/server/connectivity/NetworkMonitor.java

private static final String GOOGLE_PING_URL = "http://google.com/generate_204";
private static final int SOCKET_TIMEOUT_MS = 10000;

public boolean isCaptivePortal () {

try {
            URL url = new URL(GOOGLE_PING_URL);
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setConnectTimeout(SOCKET_TIMEOUT_MS);
            urlConnection.setReadTimeout(SOCKET_TIMEOUT_MS);
            urlConnection.setUseCaches(false);
            urlConnection.getInputStream();
            return (urlConnection.getResponseCode() != 204)
                    && (urlConnection.getResponseCode() >= 200)
                    && (urlConnection.getResponseCode() <= 399);
        } catch (Exception e) {
            // for any exception throw an exception saying check was unsuccesful
        } finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
        }
}

Please note this will probably not work on a proxy network and something more advanced as in the AOSP url needs to be done