Warning header is soon to be deprecated; see Warning (https://github.com/httpwg/http-core/issues/139) and Warning: header & stale-while-revalidate (https://github.com/whatwg/fetch/issues/913) for more details.
Warning general HTTP header contains information about possible problems with the status of the message. More than one
Warning header may appear in a response.
Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only be applied to response messages.
Warning: <warn-code> <warn-agent> <warn-text> [<warn-date>]
Warningis required to be deleted from a stored response after validation.
1xxwarn-codes describe the freshness or validation status of the response and will be deleted by a cache after deletion.
2xx warn-codes describe some aspect of the representation that is not rectified by a validation and won't be deleted by a cache after validation unless a full response is sent.
The name or pseudonym of the server or software adding the
Warning header (might be "-" when the agent is unknown).
Warningheader is sent, include a date that matches the
The HTTP Warn Codes registry at iana.org defines the namespace for warn codes.
|110||Response is Stale||A response provided by a cache is stale (the expiration time set for it has passed).|
|111||Revalidation Failed||An attempt to validate the response failed, due to an inability to reach the server.|
|112||Disconnected Operation||The cache is disconnected from the rest of the network.|
|113||Heuristic Expiration||Sent If a cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.|
|199||Miscellaneous Warning||Arbitrary, non-specific warning|
|214||Transformation Applied||Added by a proxy if it applies any transformation to the representation, such as changing the content-coding, media-type or the like.|
|299||Miscellaneous Persistent Warning||Same as 199, but indicating a persistent warning|
Warning: 110 anderson/1.3.37 "Response is stale" Date: Wed, 21 Oct 2015 07:28:00 GMT Warning: 112 - "cache down" "Wed, 21 Oct 2015 07:28:00 GMT"
|RFC 7234, section 5.5: Warning||Hypertext Transfer Protocol (HTTP/1.1): Caching|
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