How to Fix Cloudflare "Error 521: Web Server is Down": Complete Resolution Guide
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Publisher ID: ca-pub-XXXXXXXXXXXXXXXX (Configured in config.php)Cloudflare Error 521 occurs when Cloudflare edge proxies attempt to connect to your origin server on port 80 or 443, but the origin server actively refuses the connection (TCP connection refused or dropped).
1) Verify your web server (Apache/LiteSpeed) is actually running. 2) Ensure your hosting firewall (iptables/UFW/ModSecurity) has not blocked Cloudflare's IP ranges. 3) Test connecting directly to your origin server IP using cURL to confirm it responds.
📋 Guide Table of Contents
1. Problem Diagnosis & Symptoms
Visitors see the branded Cloudflare error page with "Host Error: Error 521 Web server is down".
2. Primary Root Causes
- Cause: Web server crashed on origin.
- Cause: Origin security firewall blacklisting Cloudflare proxy IPs.
3. Step-by-Step Diagnostic Fixes
Whitelist Cloudflare IPs in Server Firewall
Ensure firewall allows incoming connections from Cloudflare official IP blocks (cloudflare.com/ips).
4. How to Verify the Resolution
- Test direct IP connection: curl -k -I https://ORIGIN_IP
5. Diagnostic Troubleshooting Matrix
| Symptom Observed | Root Cause | Recommended Action |
|---|---|---|
| Error 521 occurs only on HTTPS pages | Origin server does not have an active SSL certificate configured on port 443 | Install an SSL certificate or Cloudflare Origin CA certificate on origin |
6. Critical Mistakes to Avoid
- ❌ Assuming Cloudflare is down when the origin server is stopped.
7. Frequently Asked Questions
❓ Where can I find Cloudflare's official IP list?
At https://www.cloudflare.com/ips.
Official Technical References
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