HTTP 521: Web Server Is Down
HTTP 521 Web Server Is Down is a Cloudflare-specific status code indicating that the origin web server refused Cloudflare's connection. This typically means the origin server is down, crashed, or is actively refusing connections on the expected port. Cloudflare acts as a reverse proxy, and when it cannot establish a TCP connection to the origin, it returns 521 to the visitor.
Status
521 Web Server Is DownExample
GET / HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/htmlHTTP/1.1 521 Web Server Is Down
Content-Type: text/html; charset=utf-8
Server: cloudflare
CF-Ray: 8a1b2c3d4e5f6789-IAD
Content-Length: 316
<!doctype html>
<html lang="en">
<head>
<title>Error 521: Web server is down</title>
</head>
<body>
<h1>Error 521: Web server is down</h1>
<p>Cloudflare could not connect to the origin web server.</p>
<p>Please try again in a moment. If the problem persists, contact the site owner.</p>
</body>
</html>How to Troubleshoot
Verify the origin server is running
SSH into your origin server and check that the web server process is running and listening on the correct port.
sudo systemctl status nginx && ss -tlnp | grep ':80\|:443'
Whitelist Cloudflare IP ranges
Ensure your firewall allows connections from all Cloudflare IP ranges. Cloudflare publishes their IP ranges at cloudflare.com/ips.
Check port connectivity from outside
Test whether the origin server's port is reachable from the internet, bypassing Cloudflare.
Scan PortsCommon Causes
Origin server is down or crashed
highThe web server process (Nginx, Apache, Node.js) on the origin server has stopped running or crashed.
Firewall blocking Cloudflare IPs
highThe origin server's firewall is blocking connections from Cloudflare's IP ranges, preventing the reverse proxy connection.
Origin server listening on wrong port
mediumThe web server is running but listening on a different port than what Cloudflare is configured to connect to (typically 80 or 443).
Quick Diagnostics
Related Status Codes
Connection Timed Out
Cloudflare could not negotiate a TCP handshake with the origin server (Cloudflare).
A Timeout Occurred
Cloudflare established a TCP connection but the origin server did not respond with an HTTP response in time (Cloudflare).
Bad Gateway
The server received an invalid response from the upstream server.
Frequently Asked Questions
This reference was compiled from official RFCs, protocol specifications, and hands-on troubleshooting experience. AI tools were used primarily for formatting and organizing the content on the page.