HTTP 411 Length Required vs 521 Web Server Is Down
HTTP 411 (Length Required) is a 4xx Client Error response, while 521 (Web Server Is Down) is a 5xx Server Error response. 411 indicates that the server refuses to accept the request without a Content-Length header. In contrast, 521 means that cloudflare-specific. The origin server refused the connection.
Description
The server refuses to accept the request without a Content-Length header.
When You See It
When sending a request body without specifying Content-Length.
How to Fix
Include a Content-Length header in your request.
Description
Cloudflare-specific. The origin server refused the connection.
When You See It
When Cloudflare can't establish a TCP connection to the origin server.
How to Fix
Check that the origin server is running and the firewall allows Cloudflare IPs.
Key Differences
411 is a 4xx Client Error response, while 521 is a 5xx Server Error response.
HTTP 411: The server refuses to accept the request without a Content-Length header.
HTTP 521: Cloudflare-specific. The origin server refused the connection.
You encounter 411 when when sending a request body without specifying Content-Length.
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
When to Use Which
For 411 (Length Required): Include a Content-Length header in your request. For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs.