A Cloudflare Error 521 indicates that your website’s origin server is refusing connections from Cloudflare. This error generally occurs when the origin server is unreachable or actively denying Cloudflare’s requests. Below are the common causes and troubleshooting steps:
1. Firewall or Security Software Blocking Cloudflare
The origin server’s firewall or security software might block Cloudflare’s IPs.
Solution:
- Ensure Cloudflare’s IP ranges are whitelisted in your firewall/security software. Cloudflare IP Ranges
2. Web Server Offline or Misconfigured
The origin server may be down or misconfigured, preventing connections.
Solution:
- Verify that the server is online and operational.
- Check that your web server software (e.g., Apache, Nginx) is running correctly.
3. Server Overload
High traffic or resource-intensive operations may overload the server, causing it to drop connections.
Solution:
- Monitor server performance and optimize resource usage.
- Increase server capacity if necessary.
4. Incorrect SSL/TLS Configuration
If the SSL/TLS settings on the origin server are incompatible with Cloudflare, connections may fail.
Solution:
- Verify your SSL/TLS settings in Cloudflare’s dashboard (e.g., Full, Flexible, or Full (Strict)).
- Ensure the origin server has a valid SSL certificate.
5. IP Misconfiguration
If Cloudflare connects to the wrong IP, the origin server might not respond.
Solution:
- Confirm the correct origin IP is set in your Cloudflare DNS records.
How to Fix Error 521
1. Check Server Logs
- Inspect your origin server’s logs for connection attempts from Cloudflare’s IPs to identify any blocks or errors.
2. Whitelist Cloudflare's IPs
- Add Cloudflare's IPs to the server's whitelist in the firewall and security configurations.
3. Test Server Availability
- Directly access your server’s IP in a browser or via curl to confirm it’s accessible without Cloudflare.
Example:
bash curl -I http://your-server-ip
4. Review Hosting Provider Settings
- If your server is hosted with a provider, ensure their settings allow connections from Cloudflare.
5. Cloudflare Configuration
- Verify your DNS settings in the Cloudflare dashboard.
- Use the appropriate SSL/TLS mode based on your server’s SSL certificate.
6. Contact Support
- If you’re unable to resolve the issue, contact your hosting provider or Cloudflare support for assistance.