When visiting the website the "Error 521 - Web server is down" Hint, this usually means Cloudflare The connection to the source server fails. While this may appear to be a Cloudflare problem from the user's perspective, the root cause of the error often lies in the source server's own misconfiguration.

This article will provide insight into the emergence of Error 521 of common configuration errors to help webmasters quickly locate and resolve problems.

What is Error 521?

Error 521 is a specific error code returned by Cloudflare that means the client successfully connected to Cloudflare, but Cloudflare was unable to establish a TCP connection with the source server.

This usually occurs in the following two situations:

• Source server down or crashed
• Firewalls or security policies are blocking IP requests from Cloudflare

I. The source server is not started or the port is not listened to

The most common question is The source web service (e.g., Nginx, Apache) is not running properly. Or it is not listening on a port that Cloudflare needs access to (e.g. 80/443).

Exhaustion Recommendations:

• Log in to the server and use the systemctl status nginx maybe systemctl status apache2 Checking Service Status

• Confirm the server listening port (netstat-tuln maybe ss-tuln)
• Check if the server's CPU and memory are full, causing the service to hang.

Second, the firewall is blocking the Cloudflare IP segment.

If the server has enabled thefirewalls(e.g. iptables, firewalld, UFW) without whitelisting Cloudflare's IP segments, Cloudflare's requests will be blocked, resulting in connection failure.

Solution:

• Add Cloudflare's IP range to the firewall whitelist (Cloudflare IP List Official Address)

• Example (using iptables as an example): iptables -I INPUT -p tcp -m multiport --dports 80,443 -s 173.245.48.0/20 -j ACCEPT

ModSecurity or WAF is enabled on the source site, blocking Cloudflare requests.

Some servers turn on extrasafety protectionModules such as ModSecurity, Fail2Ban or Imunify360The Cloudflare system may block the connection by misinterpreting Cloudflare's request as an attack.

Recommended Operation:

• Check firewall logs /var/log/messages,/var/log/modsec_audit.log Availability of interception records

• Temporarily disable ModSecurity and see if it returns to normal.
• Configure whitelist rules to allow Cloudflare IP

IV. Conflicting SSL/TLS settings

If Cloudflare and the source server are using the "Full (Strict)" modeand the source station is not properly installed SSL CertificateThis will cause Cloudflare to fail to complete the handshake.

Inspection is recommended:

• Whether the source server is configured with a valid SSL certificate (Let's Encrypt is recommended)

• Cloudflare backend setting to match or not to match the source (Full, Flexible, Strict)

Fifth, the server response time is too long

A 521 error is also returned when the source server response time exceeds the connection timeout threshold set by Cloudflare (about 100 seconds by default).

Optimization Recommendations:

• Optimize PHPDatabase performance
• Configure caching mechanisms (e.g. Redis, WP Rocket)
• Avoid blocking HTTP requests directly by performing tasks for long periods of time

Cloudflare is misconfigured or DNS points incorrectly.

Sometimes errors can appear on the Cloudflare side, for example:

• DNS A record points to the wrong IP address
• Agent is not enabled for this record in the Cloudflare panel (gray cloud state)

Inspection Recommendations:

• recognize Cloudflare DNS The record IP is correct and the cloud is orange.

• utilization ping (computing) maybe nslookup Tool verifies that parsing is working

VII. Ports not supported by Cloudflare

Cloudflare only supports certain ports (e.g. 80, 443, 8080, 8443, etc.). If the source is listening on an unsupported port, the connection will fail.

Solution:

• Migrating services to supported ports
• Or bypass Cloudflare and use the gray cloud model directly

Recommendations for troubleshooting steps

1. Check that the source station service is up
2. Checking server port listening status
3. Check the firewall orSecurity Plug-insWhether the request is intercepted
4. Verify that the SSL configuration matches the Cloudflare settings
5. Checking DNS Configuration and the Cloudflare Control Panel
6. Check to see if the server is running out of resources or responding with a timeout

Most 521 errors can be quickly located and fixed by systematically troubleshooting the above configuration points. If the error persists, it is recommended that you contactServer operation and maintenanceor hosting service provider to further analyze network access issues.

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