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:
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