The server responds to ping
SSH works
But ports 80 or 443 are closed
Cloudflare cannot connect, leading to Error 521.
5. Incorrect Server Configuration
Misconfigured server settings can also cause Error 521, including:
Incorrect port configuration
Binding the web server to localhost only
SSL/TLS misconfigurations
Broken virtual host settings
Software
Even small configuration mistakes can cause the server to refuse external connections.
6. Hosting Provider Blocking Cloudflare
Some hosting providers:
Block Cloudflare IPs by default
Require manual whitelisting
Limit connections from proxy services
If Cloudflare is not allowed to access the server, Error 521 will occur.
7. TCP/IP Connection Refused
At a lower level, Error 521 may occur when:
The server sends a TCP “RST” (reset) packet
Connection limits are exceeded
Web Services
SYN floods are detected and blocked
This usually points to a network or firewall-level issue.
How to Fix Error Code 521 (Step-by-Step)
Now let’s move on to solutions. The correct fix depends on the root cause.
Step 1: Check If the Origin Server Is Online
Start with the basics:
Can you access the website directly via IP address?
Can you SSH into the server?
Is the hosting control panel accessible?
If the server is offline:
Restart it
Open Source
Contact your hosting provider
Check server status pages
Step 2: Restart the Web Server
Restart your web server software:
Apache
Nginx
LiteSpeed
This often resolves temporary crashes or memory issues.
Also check:
Error logs
Access logs
System logs
Computer Servers
Look for crashes, segmentation faults, or fatal errors.
Step 3: Whitelist Cloudflare IP Addresses
Cloudflare publishes a list of IP ranges used by its network. These must be allowed through your firewall.
Actions to take:
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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