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

I believe preventing redirection for your connection will work.

URL url = new URL(hostUrl));
HttpURLConnection httpConn = (HttpURLConnection)url.openConnection();

/* This line prevents redirects */
httpConn.setInstanceFollowRedirects( false );

httpConn.setAllowUserInteraction( false );
httpConn.setRequestMethod( "GET" );
httpConn.connect();
responseCode = httpConn.getResponseCode();
isConnected = responseCode == HttpURLConnection.HTTP_OK;

If that doesn't work, then I think the only way to do it is to check the body of the response.