HTTP 501 Not Implemented vs 521 Web Server Is Down
Both HTTP 501 (Not Implemented) and 521 (Web Server Is Down) belong to the 5xx Server Error category. 501 indicates that the server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it. Meanwhile, 521 means that cloudflare-specific. The origin server refused the connection.
Description
The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
When You See It
When using an HTTP method (like PATCH) that the server doesn't support.
How to Fix
Use a different HTTP method that the server supports. Check server documentation.
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
HTTP 501: The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
HTTP 521: Cloudflare-specific. The origin server refused the connection.
You encounter 501 when when using an HTTP method (like PATCH) that the server doesn't support.
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
When to Use Which
For 501 (Not Implemented): Use a different HTTP method that the server supports. Check server documentation. For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs.