HTTP 226 IM Used vs 521 Web Server Is Down
HTTP 226 (IM Used) is a 2xx Success response, while 521 (Web Server Is Down) is a 5xx Server Error response. 226 indicates that the server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations. In contrast, 521 means that cloudflare-specific. The origin server refused the connection.
Description
The server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations.
When You See It
When using delta encoding to reduce bandwidth.
How to Fix
Rarely encountered. The response uses Instance Manipulations (IM) as specified.
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
226 is a 2xx Success response, while 521 is a 5xx Server Error response.
HTTP 226: The server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations.
HTTP 521: Cloudflare-specific. The origin server refused the connection.
You encounter 226 when when using delta encoding to reduce bandwidth.
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
When to Use Which
For 226 (IM Used): Rarely encountered. The response uses Instance Manipulations (IM) as specified. For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs.