Server Error

HTTP 521 Web Server Is Down

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

Check a URL for HTTP 521

Paste any URL to see whether it currently returns 521. Runs a live HTTP probe and follows redirects.

What does HTTP 521 mean?

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.

Common Causes

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

How to Fix It

1.Check if your origin server is running
2.Whitelist Cloudflare IP ranges in your firewall
3.Verify the correct origin IP in Cloudflare settings
4.Check that your web server is listening on the correct port

SEO Impact

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

Monitor HTTP 521 with LemWatch

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

Related Status Codes

502

Bad Gateway

523

Origin Is Unreachable

A Timeout Occurred

Insufficient Storage

Loop Detected

Network Authentication Required

521 Web Server Is Down on your platform

Same status code, different fix depending on where you host. Jump to a platform-specific deep dive:

521 in Cloudflare

Free Website Tools

Check your site for common issues that cause HTTP errors:

View all HTTP Status Codes