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?
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