Error 521 is a Cloudflare error code that you may encounter when working on a WordPress website, or browsing the internet. in this article, we will discuss the common causes of every 521, and how to fix the Cloudflare issue.

A Word of Warning

Do not attempt to replicate an error in any of the solution videos using the terminating URL. Comments in these videos and blogs may point to malware.

Notification

The Cloudflare error page will pop up with the Error 521 notification indicating a Ray ID (the error log) and UCT date-time stamp of the error. You can use this to trace the error or report it to your internet service provider. The error will show you that the error is with the final URL server, that your browser is working, and that the Cloudflare content delivery network hub (CDN) is acceptable.

The notification will also tell you what happened. The webserver is not returning a connection. As a result, the web page is not displaying. At the bottom right, the notification will tell you what you can do. If you are visiting, please try again in a few minutes. If you are the website owner, contact your Internet service provider, letting them know that your web server is not responding, and then there's a link to troubleshooting information.

The two most common causes of 521 errors are:

Offline origin web server application

The most common reason for an Error 521 is that the hosting service for your domain is down. If you are using an Internet service provider, then notify them of the error. If your hosting is self-managed, either with AWS, Google Cloud, Azure, or a sleuth of other self-hosting providers, then the problem is yours to check. It may require resetting your instance.

Blocked Cloudflare requests

Cloudflare, Inc. is a web infrastructure and website security company based in the United States specializing in content delivery networks and DDoS mitigation. Cloudflare's services function as a reverse proxy for websites, sitting between a website's visitor and the Cloudflare customer's hosting provider.

As their website security company, their reputation is dependent on providing exceptional security. If Cloudflare does not authorize your SSL (Secure Sockets Layer) certificate, an Error 521 will also occur.  SSL is a security protocol that creates an encrypted link between a web server and a web browser. 

Error 521 Solutions

Contact your site administrator or hosting provider to eliminate these common causes:

• Check that your primary web server is responsive
• Examine the webserver error logs for any breakdowns or outages in the web server application.
• Make sure Cloudflare IP addresses aren't blocked or throttled.
• In your origin web server's firewall or other security software, allow all Cloudflare IP ranges.
• Confirm that you have installed a Cloudflare Origin Certificate if your SSL/TLS mode is Full or Full (Strict).
• On the Cloudflare Community, you may find more troubleshooting information.

Conclusion

You most commonly encounter Error 521 when you first include Cloudflare in your content distribution. Cloudflare speeds up websites, defends them from threats, keeps them online at all times, and makes it simple to deploy web apps with a single click. Cloudflare boosts websites without adding hardware, installing software, or changing a line of code, regardless of their size or platform.