HTTP 412 Precondition Failed vs 520 Web Server Returned an Unknown Error
HTTP 412 (Precondition Failed) is a 4xx Client Error response, while 520 (Web Server Returned an Unknown Error) is a 5xx Server Error response. 412 indicates that one or more conditions in the request headers (If-Match, If-Unmodified-Since) evaluated to false. In contrast, 520 means that cloudflare-specific. The origin server returned an unexpected response that Cloudflare could not process.
Description
One or more conditions in the request headers (If-Match, If-Unmodified-Since) evaluated to false.
When You See It
When using conditional requests with ETags and the resource has changed.
How to Fix
Refresh the ETag/Last-Modified value and retry with updated preconditions.
Description
Cloudflare-specific. The origin server returned an unexpected response that Cloudflare could not process.
When You See It
When Cloudflare receives an empty, unknown, or malformed response from the origin.
How to Fix
Check origin server logs. Ensure responses include proper headers. Verify SSL configuration.
Key Differences
412 is a 4xx Client Error response, while 520 is a 5xx Server Error response.
HTTP 412: One or more conditions in the request headers (If-Match, If-Unmodified-Since) evaluated to false.
HTTP 520: Cloudflare-specific. The origin server returned an unexpected response that Cloudflare could not process.
You encounter 412 when when using conditional requests with ETags and the resource has changed.
You encounter 520 when when Cloudflare receives an empty, unknown, or malformed response from the origin.
When to Use Which
For 412 (Precondition Failed): Refresh the ETag/Last-Modified value and retry with updated preconditions. For 520 (Web Server Returned an Unknown Error): Check origin server logs. Ensure responses include proper headers. Verify SSL configuration.