HTTP 521 in Cloudflare

Origin server refused the Cloudflare edge connection — often a firewall blocking Cloudflare IPs.

Why Cloudflare returns 521

Origin server refused the Cloudflare edge connection — often a firewall blocking Cloudflare IPs.

How to fix it

Whitelist Cloudflare's IP ranges at the origin firewall (see cloudflare.com/ips).

Where this surfaces in Cloudflare

The Cloudflare edge before your origin ever sees the request. Look at the Ray ID in the response.

Cloudflare — CDN, WAF, and edge network in front of your origin.

What HTTP 521 means generally

The origin web server has refused or is not responding to Cloudflare's connection.

HTTP 521 is a Cloudflare-specific error code indicating that the origin web server refused the connection from Cloudflare. This means Cloudflare can't reach your server at all.

Other common causes of 521

• Origin server is completely down
• Origin firewall blocking Cloudflare IPs
• Origin server crashed
• Port not open on origin server

SEO impact

Critical. Your entire site is unreachable through Cloudflare. Search engines will see an error page.

How to catch it next time

LemWatch monitors through CDN layers like Cloudflare and can distinguish between CDN errors and origin server issues.