1. Origin Server Is Offline
The most straightforward cause is that the web server is completely down.
This can happen due to:
Server crash
Power outage at the data center
Hosting provider downtime
Accidental server shutdown
Kernel panic or OS failure
Operating Systems
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.
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:
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.
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