How to Fix Cloudflare Error 521: Fix Web Server Configuration Error

Cloudflare as the world's leading content delivery network (CDN) and security provider, has become the tool of choice for many organizations to protect and accelerate their websites. When migrating websites from cloud servers to Cloudflare for acceleration, some users may encounter 521 Errornamely Web server not available The error.

In this article, we will take an in-depth look at 521 Error and provides webmasters using cloud servers and Cloudflare with effective methods on how to avoid and resolve this issue.

What is Cloudflare error 521?

Cloudflare error 521 indicate web (loanword) server (computer)unavailable. When Cloudflare tries to establish a connection with the source server, the connection fails for some reason, preventing the site from loading. A common prompt for this error is:"Web server is down", i.e. Cloudflare cannot access the source server.

This issue usually occurs when Cloudflare is used and the source server is not responding correctly to the requests made by Cloudflare. Visitors are then presented with an error page served by Cloudflare and are unable to access the site content.

How to avoid Cloudflare error 521?

1. Ensure that the source server is up and running

First the source server has to be in working condition. If the source server is down or undergoing maintenance, Cloudflare will not be able to connect to it. Therefore, check your server regularly to keep it running stable. If it goes down, restart it promptly or contact your hosting provider to fix the problem.

2. Adjusting server firewall settings

Check your cloud server's firewall or security group settings to put the Cloudflare's IP address Whitelisted to allow connections from Cloudflare, which provides a complete list of IP addresses that should not be blocked by firewalls or other security software.

You can find more information on the Cloudflare website Get its IP address range:Cloudflare IP List

3. Open HTTP and HTTPS ports

Cloudflare via port 80 (HTTP) respond in singing 443 (HTTPS) communicate with the source server. Let the server firewall be to allow communication on these two ports. If you are using a cloud server (e.g. AWS, Google Cloud, DigitalOcean, etc.), check and make sure these ports are open.

4. Verify the SSL configuration of the source server

If you're using SSL (HTTPS), the source server configuration must be valid SSL CertificateCloudflare's Full Strict A valid SSL certificate is required for the source server, otherwise Cloudflare cannot establish a secure connection to the server.

The source server does not have a valid certificate, you can use Cloudflare's Flexible mode (Flexible)As a temporary solution, but in the long run, it is better to install valid certificates for the source server.

You can check and configure the SSL certificate in the following ways:

• utilization SSL Labs tool to check the server's SSL configuration.

• Ensure that the certificate is issued by a trusted Certificate Authority (CA) and has not expired.

• If you are using a self-signed certificate, consider upgrading to a certificate certified by a CA.

5. Checking DNS settings

Check that Cloudflare's DNS records are correct and point to the correct source server IP address. Log in to the Cloudflare backend and check your site's DNS SettingsThe first step is to make all of the A Records respond in singing CNAME records Correct. Any DNS errors may cause Cloudflare to be unable to connect to the origin server.

6. Restarting the Web Server

If you try the above and the problem persists, try restarting the web server (e.g. Apache(e.g., Nginx, etc.). Sometimes a server may lose connection due to high load or temporary failure, and a reboot can help restore normal service.

sudo service apache2 restart
# or
sudo service nginx restart

7. Contact a Cloud Service Provider

If you are using a cloud service (e.g. AWS, Google Cloud, DigitalOcean, etc.) and are still experiencing issues, it is recommended to contact your cloud service provider's support team to check for network configuration issues or server problems.

reach a verdict

Cloudflare Error 521 appear, usually related to the configuration of the source server or thefirewallsSetup issues are relevant. By checking that the source server is working properly, adjusting firewall rules, opening ports, and SSL configuration correctly, you can effectively avoid and fix this error. For cloud server users, it is also important that the DNS settings are correct and that the firewall allows access from Cloudflare's IP address.

By taking these steps to resolve the 521 error, you will also be able to keep your website running efficiently and stably while using the acceleration and security features provided by Cloudflare.

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