"Invalid response" when returning a HTTP 404 page via Cloudflare

Question

My website is using Cloudflare in order to enable SSL (free SSL is not available on my host, Hostgator). Everything seems to be working fine, except when I return a 404 page. That results in

This site can’t be reached

The webpage at https://example.com/example-url might be temporarily down or it may have moved permanently to a new web address.

ERR_INVALID_RESPONSE

If I send a HTTP 200 response code instead of 404, everything works as normal - even for the same URL. I also tried sending HTTP 500, and that works fine too! So it's definitely something about the 404 response code that's causing the problem.

The reason I believe the problem has to do with Cloudflare is that

1. It works fine on my local development environment (not using SSL).
2. If I run curl https://example.com/example-url I get the correct 404 page back, and the HTTP status is 404 as expected.

I have other sites using Cloudflare as well, but they don't seem to have any problems returning a 404 response, which makes me think it's some configuration problem on Cloudflare for this one site.

Any ideas?

Answer 1

It turns out that the problem was how I was generating the response headers.

I'm using the Slim PHP framework, and a custom error handler middleware. It looked like this:

class ErrorResponseHandler {

    protected $view = null;

    function __construct(\Slim\Views\Twig $view) {
        $this->view = $view;
    }


    public function __invoke(Request $request, Response $response, $next) {
        $response = $next($request, $response);

        if (404 === $response->getStatusCode() ) {
            // Pass an empty response to the 404-handler in MainController
            return (new MainController($this->view))->notFound404($request, new Response());
        }

        // Any other request, pass on current response
        return $response;
    }

}

I'm creating a new Response object in order to get an empty body without the default 404 HTML generated by Slim. However, creating a new Response object will apparently set the Content-Type header to application/x-httpd-php5, which is not understood by browsers (I should have seen this when using curl --head, but I missed it).

The solution is to set the correct Content-Type header, like this:

    if (404 === $response->getStatusCode() ) {
        // Pass an empty response to the 404-handler in MainController
        $new_response = (new Response())->withHeader('Content-Type', 'text/html');
        return (new MainController($this->view))->notFound404($request, $new_response);
    }