Add Cloudflare IPs to firewall allowlists
Disable rules that block proxy traffic
Update CSF or UFW rules
Adjust hosting provider security settings
Failure to whitelist Cloudflare IPs is one of the most frequent causes of Error 521.
Step 4: Check Firewall and Security Software
Review:
Server firewall rules
Proxying & Filtering
Fail2Ban settings
ModSecurity rules
DDoS protection systems
Make sure:
Ports 80 and 443 are open
Cloudflare IPs are not rate-limited
No rules are blocking repeated proxy requests
Step 5: Check Server Resource Usage
Monitor:
CPU usage
RAM usage
Disk I/O
Active connections
Software
If resources are maxed out:
Optimize your website
Upgrade your hosting plan
Add caching
Use a load balancer
An overloaded server may appear “down” to Cloudflare even if it’s technically running.
Network Security
Step 6: Review Web Server Configuration
Check for:
Correct listening ports
Proper virtual host setup
Correct SSL certificates
No binding to 127.0.0.1 only
Open Source
Make sure the web server is listening on public interfaces, not just localhost.
Step 7: Temporarily Pause Cloudflare (Testing Only)
To confirm whether the issue is Cloudflare-related:
Pause Cloudflare
Switch DNS to “DNS only”
Computer Servers
Access the site directly
If the site works without Cloudflare:
The problem is almost certainly firewall or IP blocking related
Error 521 vs Similar Cloudflare Errors
Understanding related errors can help with diagnosis.
Error 520
Unknown origin server error
Server returned an unexpected response
Error 522
Connection timed out
Server didn’t respond in time
Proxying & Filtering
Error 523
Origin server unreachable
DNS or routing issue
Error 521
Connection refused
Server actively rejected Cloudflare
How Error 521 Affects SEO and User Experience
Repeated Error 521 incidents can negatively impact your website in several ways:
SEO Impact
Search engines may reduce crawl frequency
Prolonged downtime can hurt rankings
Pages may be temporarily deindexed
User Experience
Visitors lose trust
Higher bounce rates
Reduced conversions
Business Impact
Lost sales
Missed leads
Damaged brand reputation
Best Practices to Prevent Error 521
Prevention is better than cure. Here are best practices to minimize the risk of Error 521.
1. Properly Configure Firewalls
Always:
Whitelist Cloudflare IP ranges
Network Security
Regularly update firewall rules
Avoid aggressive rate limiting on trusted proxies
2. Monitor Server Health
Use monitoring tools to track:
Uptime
Resource usage
Connection limits
Early detection helps prevent downtime.
3. Scale Your Infrastructure
If your site is growing:
Upgrade hosting resources
Use load balancing
Proxying & Filtering
Implement caching
Optimize databases
4. Keep Software Updated
Regularly update:
Operating system
Web server software
CMS and plugins
Outdated software is more prone to crashes and security blocks.
5. Review Hosting Provider Policies
Ensure your host:
Supports Cloudflare
Allows proxy traffic
Computer Servers
Does not block Cloudflare IPs by default
When to Contact Support
If Error 521 persists after troubleshooting:
Contact your hosting provider
Contact Cloudflare support
Provide logs and timestamps
Ask for IP blocking verification
Having detailed logs greatly speeds up resolution.
Conclusion
Error Code 521 – “Web Server Is Down” is not always as alarming as it sounds. In most cases, the web server is actually running but refusing Cloudflare’s connection due to firewall rules, resource limits, or configuration issues.
Software
By understanding:
How Cloudflare communicates with origin servers
The common causes of Error 521
Proper troubleshooting and prevention techniques
you can quickly diagnose and fix the problem—and prevent it from happening again.
A well-configured server, properly whitelisted Cloudflare IPs, and proactive monitoring are the keys to keeping your website online, fast, and reliable.
Error 521 is a Cloudflare-specific error that occurs when Cloudflare cannot establish a connection with the website’s origin server. In simple terms:
Cloudflare is working, but the web server hosting the website is refusing or failing to respond to Cloudflare’s requests.
When this happens, Cloudflare displays the message:
“Web server is down (Error code 521)”
This error indicates that Cloudflare attempted to connect to the origin server (for example, an Apache, Nginx, or LiteSpeed server), but the server either:
Refused the connection, or
Open Source
Did not respond at all
How Cloudflare Works (Simplified Explanation)
To understand Error 521, it helps to know how Cloudflare operates.
When Cloudflare is enabled for a website:
A visitor requests your website.
The request first goes to Cloudflare’s servers.
Cloudflare forwards the request to your origin web server.
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