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 limited subnet which forces users to submit their credentials on a form in order to get full access.

My app is automating the authentication process, and therefore it is important to know that full internet access is not available before starting the logon activity.

The question is not about how to check that the network interface is up and in a connected state. It is about making sure the device has unrestricted internet access as opposed to a sandboxed intranet segment.

All the approaches I have tried so far are failing, because connecting to any well-known host would not throw an exception but return a valid HTTP 200 response code because all requests are routed to the login page.

Here are all the approaches I tried but they all return true instead of false for the reasons explained above:

1:

InetAddress.getByName(host).isReachable(TIMEOUT_IN_MILLISECONDS);
isConnected = true; <exception not thrown>

2:

Socket socket = new Socket();
SocketAddress sockaddr = new InetSocketAddress(InetAddress.getByName(host), 80);
socket.connect(sockaddr, pingTimeout);
isConnected = socket.isConnected();

3:

URL url = new URL(hostUrl));
URLConnection urlConn = url.openConnection();
HttpURLConnection httpConn = (HttpURLConnection) urlConn;
httpConn.setAllowUserInteraction(false);
httpConn.setRequestMethod("GET");
httpConn.connect();
responseCode = httpConn.getResponseCode();
isConnected = responseCode == HttpURLConnection.HTTP_OK;

So, how do I make sure I connected to an actual host instead of the login redirection page? Obviously, I could check the actual response body from the 'ping' host I use but it does not look like a proper solution.

Answer 1

For reference, here is the 'official' method from the Android 4.0.1 AOSP code base: WifiWatchdogStateMachine.isWalledGardenConnection(). I am including the code below just in case the link breaks in the future.

private static final String mWalledGardenUrl = "http://clients3.google.com/generate_204";
private static final int WALLED_GARDEN_SOCKET_TIMEOUT_MS = 10000;

private boolean isWalledGardenConnection() {
    HttpURLConnection urlConnection = null;
    try {
        URL url = new URL(mWalledGardenUrl); // "http://clients3.google.com/generate_204"
        urlConnection = (HttpURLConnection) url.openConnection();
        urlConnection.setInstanceFollowRedirects(false);
        urlConnection.setConnectTimeout(WALLED_GARDEN_SOCKET_TIMEOUT_MS);
        urlConnection.setReadTimeout(WALLED_GARDEN_SOCKET_TIMEOUT_MS);
        urlConnection.setUseCaches(false);
        urlConnection.getInputStream();
        // We got a valid response, but not from the real google
        return urlConnection.getResponseCode() != 204;
    } catch (IOException e) {
        if (DBG) {
            log("Walled garden check - probably not a portal: exception "
                    + e);
        }
        return false;
    } finally {
        if (urlConnection != null) {
            urlConnection.disconnect();
        }
    }
}

This approach relies on a specific URL, mWalledGardenUrl = "http://clients3.google.com/generate_204" always returning a 204 response code. This will work even if DNS has been interfered with since in that case a 200 code will be returned instead of the expected 204. I have seen some captive portals spoofing requests to this specific URL in order to prevent the Internet not accessible message on Android devices.

Google has a variation of this theme: fetching http://www.google.com/blank.html will return a 200 code with a zero-length response body. So if you get a non-empty body this would be another way to figure out that you are behind a walled garden.

Apple has its own URLs for detecting captive portals: when network is up IOS and MacOS devices would connect to an URL like http://www.apple.com/library/test/success.html, http://attwifi.apple.com/library/test/success.html, or http://captive.apple.com/hotspot-detect.html which must return an HTTP status code of 200 and a body containing Success.

NOTE: This approach will not work in areas with restricted Internet access such as China where the whole country is a walled garden, and where most Google/Apple services are blocked or filtered. Some of these might not be blocked: http://www.google.cn/generate_204, http://g.cn/generate_204, http://gstatic.com/generate_204 or http://connectivitycheck.gstatic.com/generate_204 — yet these all belong to google so not guaranteed to work.