Home / server / Questions / 1165854
Accepted
Ade
Asked: 2024-09-28 16:39:52 +0800 CST

503 response to prefetch requests, nginx + PHP

  • 772

I have a PHP application / website served by nginx. When I navigate around the website using Google Chrome with the dev tools network panel open I can see just prior to navigating to a new page URL page there is a prefetch request, that the server returns with code 503 (not available).

CloudFlare is being used as a proxy.

I'm concerned about these 503 errors. Is an nginx configuration required to handle prefetch requests properly?

Screenshot showing double request, one being a prefetch request Screenshot showing response headers to prefetch request

nginx

1 Answers

  1. Best Answer

    Hello fellow developers!

    I had the same issue and upon further investigation I found out that this is a Beta feature in Cloudflare, called "Speed Brain". It instructs the browser to initiate a pre-fetch request on any future navigation.

    Cloudflare Speed Brain settings

    I haven't figured out how to fulfill the pre-fetch request for it to be successful, so I disabled the feature for now since it isn't important to me at this point. There are probably additional headers to be set on your servers response to instruct Cloudflare to cache the document.

    While Cloudflare states the following:

    Prefetch requests will never reach origin servers. Prefetch requests only serve content that is stored in Cloudflare’s Cache. If the content is not in Cache, the prefetch request will not continue to origin servers. Without this safeguard, origin server state could be modified despite the prefetch response not being rendered in the browser. An example of this could be a prefetch GET request to a sign-out URL inadvertently triggering a sign-out action on the server.

    i saw in my logs 2 consecutive requests in the same second, the first one being from Cloudflare which had the header "sec-purpose: prefetch", indicating that the request somehow got through. Maybe I don't understand everything or this is a result of the feature still being Beta (bugs).

    The request caused issues on our side because we mistook the pre-fetch for an actual customer request.

    In PHP you can catch such requests by looking the globals in $_SERVER for the header like this:

    if(isset($_SERVER['HTTP_SEC_PURPOSE']) && $_SERVER['HTTP_SEC_PURPOSE'] === 'prefetch') {
    echo "This is the fetch request coming through, do nothing?";
}

    In our code, we also bypassed the problematic sections triggered by the pre-fetch request, since it's enabled by default on free plans. I believe this isn't ideal for a beta feature. However, you can always disable this option, as shown in the picture.

    More on this:

    https://developers.cloudflare.com/speed/optimization/content/speed-brain/

    https://www.cloudflare.com/press-releases/2024/cloudflare-speed-brain-make-millions-of-web-pages-faster/

    Regards!

    • 1