If the server is offline, Cloudflare has nothing to connect to, resulting in Error 521.
2. Web Server Is Overloaded
When a server runs out of resources, it may stop accepting new connections.
Common overload causes include:
Traffic spikes
DDoS attacks
Poorly optimized applications
Memory leaks
Insufficient CPU or RAM
When the server reaches its connection limit, it may actively refuse Cloudflare’s requests, triggering Error 521.
3. Firewall Blocking Cloudflare IP Addresses
One of the most common causes of Error 521 is a misconfigured firewall.
Computer Servers
If your server firewall (such as:
iptables
UFW
CSF
Windows Firewall
Hosting provider security rules
is blocking Cloudflare’s IP ranges, your server will refuse Cloudflare’s connection attempts.
To Cloudflare, this looks like the web server is down—even if it’s actually running.
4. Web Server Software Is Not Running
Sometimes the server itself is online, but the web service is not.
For example:
Apache stopped
Network Security
Nginx crashed
LiteSpeed failed to start
In this case:
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