Explore Error Code 521 and how to fix

Error Code 521: Mystery Behind Website Downtime

Acquiring resources on the Internet generally includes making requests to a specific host in a server where the resources are kept. Any device, like a computer or mobile phone, can do it with internet access. In this case, the device that requests the resource is known as the client, and the server from which all resources are requested is known as an HTTP request.

The exchange of information over the Internet is controlled by HTTP protocol. When a server receives a request for resources from the client, it is said that an HTTP request is made. While the server issues resources to the client, it also issues a three-digit status code that defines how the request was received and how it was handled.

Status codes have different categories, which can be recognized from the first digit of the code. The first digit 1 status code indicates that the client’s request is under process, and the status code starting with 2 defines that the client’s request is received and accepted.

Codes starting with 3 denote redirection, 4 denotes error from the client side, and a status code with 5 indicates the server error. HTTP assigned official status codes and one unofficial status code is 521, which indicates that the web server is down.

In this article,  we will learn what the 521 error code is. What are the causes of this code, and how can we fix them? So, let’s start exploring.

What is the 521 Error Code?

The error 521 happens when the server tries to acquire resources with Cloudflare’s proxy by using refuses connections. Cloudflare tries to establish a connection with port server 80 or 443.

But receives a refusal error while connecting. When Cloudflare encounters this refusal, it shows an error message with error code 521.

This error mainly occurs because of security issues or by firewall software and also if the original server denies Cloudflare’s request.

Causes of Error Code 521

Whenever we see error code 521, we perceive that the web server is down, but it’s not true. Many other reasons cause this error.

The following are the other causes; let’s go through them one by one:

1. Web Server is Denying the Request

Cloudflare’s reverse proxy server receives the client’s requests and directs them to the servers with the required resources. As a result, the origin server or Firewall security finds that Cloudflare is requesting more than enough resources and declares these requests as attacks. 

As a result, the origin server blocks Cloudflare’s IP addresses, and it becomes impossible for Cloudflare to connect with the origin server. This all becomes a cause of error 521, although the server is working perfectly.

2. Misconfiguration From Server

CDN is Cloudflare’s Content Delivery Network; it is a reverse proxy server. While working with a CDN, all the servers need to be arranged appropriately. If misarrangements happen while setting a CDN, clients will receive a message of Error 521 when they request the origin server.

3. Issues with Cloudflare’s SSL

Cloudflare aids in the encryption of traffic among a server and a client utilizing a Secure Socket Layer(SSL) Certificate. SSL Certificates are utilized to certify the identification of websites and establish an encoded connection.

The origin server will deny the connection request if there is a problem with the web SSL Certificate used by Cloudflare. As a result, the Error code 521 occurs.

4. Web Server Cloud is Sown

If your origin web server is offline from which you are requesting a resource, Cloudflare reverse proxy server fails to establish a secure connection with it. Then, it results in a 521 error code.

It also happens if the processes of the web server are not working correctly, making it problematic for Cloudflare to connect with it.

Different Ways to Fix Error Eode 521

There are different ways to solve the issues regarding the 521 error code. These ways include:

i. Inspect the Origin Server

Inspect the Origin Server

When the origin server is down or offline, this error can happen. To inspect if the origin server is up and working properly, first open a terminal window and carry out a Ping Command combined with the website you want to examine if its origin server is up and smoothly working.

If the server is working smoothly, you will see the following responses from the server, as shown in the image below:

ii. Whitelist all the Cloudflare’s IP Addresses

If your origin server is online, but you are still receiving the message of error code 521, the next step you need to do is whitelist all Cloudflare’s IP addresses. Sometimes, because of too many requests origin server blocks the requests. The easiest way to make sure that these requests are not blocked anymore is to:

  • Make certain that you are not denying the Cloudflare IP in iptables, in your firewall, or .htaccess.
  • Ensure that your hosting service provider is not rate-restricted. Also, inspect to see if your IP requests from Cloudflare IP are not denied. If your hosting service is ding them, ask them to whitelist all the IP addresses.
  • An incorrect firewall arrangement can cause errors and result in 521 errors.

iii. Inspect the SSL Certificates

Cloudflare will operate differently with the SSL certificate; it depends on which encryption mode the SSL Certificate uses, whether it is a strict, full, or flexible encryption mode. Cloudflare encryption mode is beneficial for maintaining a connection between the Cloudflare proxy service and the origin server.

While using the strict encryption mode of an SSL Certificate, a problem can arise with the website, and a 521 error code will occur. To eliminate this issue, use an SSL Certificate from a trusted authority. Also, a Full Encryption mode can help overcome this error issue.

iv. Other Technical Issues to Be Fixed

After trying all the above-mentioned solutions, if you still face 521 errors. Then, you need to solve the following technical issues.

  • If you are a newcomer to the Cloudflare HTTP, the origin web server may use the wrong arrangements. Ensure that the server has updated the configurations. But still, if the problem is not solved, you need to use the flexible encryption mode of the SSL Certificate instead of the full mode.
  • Also, you need to update the Bad Behavior and mod_security regularly. It will also help to see if the mod_security rules are not denying the Cloudflare requests.
  • If you receive an error notification, “railgun.wan_error: connection failed”. It means that your railgun configuration is incorrect. You should disable it and reload your website again.

Final Remarks - Error Code 521

If you are browsing your website and suddenly you see a message of 521 error, it means that your web server is offline or down. In other words, it means that your origin server is not responding to Cloudflare’s requests.

It sometimes happens because of security issues or a server’s Firewall; these two block Cloudflare’s requests.

You need to follow the above-mentioned solutions to fix these issues, and you will get rid of this error 521.

FAQs - Error Code 521

By examining firewall setups, DNS settings, and server availability, you can resolve Error Code 521. The article includes comprehensive instructions.

No, your internet connection is not the cause of Error Code 521. It’s a server-side problem that is frequently related to Cloudflare.

Data loss is not a result of Error Code 521 alone. However, persistent server downtime might affect the accessibility of data.

Error code 521 is frequently a server-side error rather than a user-side one. It signifies that the server cannot fulfill the user’s request.

No, a 404 error and a 521 error are not the same. A 404 error means the requested page could not be located, whereas a 521 error means a server connectivity problem.

Error number 521 must be fixed immediately to prevent significant alterations to your website’s operation. A bad user experience might result from delayed resolution.

Yes, a trustworthy hosting company may make a significant difference in avoiding error code 521. They can provide server resources, uptime warranties, and support to keep your site operating effectively.

Using internet tools and services, you can check your website for error code 521 and other problems. Services for website monitoring may be used to find errors proactively.


Zayne Z

Meet Zayne VoIP Blog's dedicated Content Researcher with 5+ Years Experience. Passionate about VoIP technology, he delivers accurate, engaging articles, keeping readers informed on industry trends with excellence.