Overview#HTTP Warn Codes are defined in RFC 7234 section-5.5 The "Warning" HTTP Header Field is used to carry additional information about the status or transformation of a message that might not be reflected in the HTTP Status Code.
HTTP Warn Codes information is typically used to warn about possible incorrectness introduced by caching operations or transformations applied to the payload of the message.
Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error HTTP Status Code, distinguishes these responses from true failures. Warning header fields can in general be applied to any message, however some HTTP Warn Codes are specific to Cache-Control and can only be applied to HTTP Response messages.
|110||Warning||Response is Stale||A cache SHOULD generate this whenever the sent response is stale.||RFC 7234|
|111||Warning||Revalidation Failed||A cache SHOULD generate this when sending a stale response because an attempt to validate the response failed, due to an inability to reach the server.||RFC 7234|
|112||Warning||Disconnected Operation||A cache SHOULD generate this if it is intentionally disconnected from the rest of the network for a period of time.||RFC 7234|
|113||Warning||Heuristic Expiration||A cache SHOULD generate this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.||RFC 7234|
|199||Warning||Miscellaneous Warning||The warning text can include arbitrary information to be presented to a human user or logged. A system receiving this warning MUST NOT take any automated action, besides presenting the warning to the user.||RFC 7234|
|214||Warning||Transformation Applied||This Warning code MUST be added by a proxy if it applies any transformation to the representation, such as changing the content-coding, media-type, or modifying the representation data, unless this HTTP Warn Codes already appears in the response.||RFC 7234|
|299||Warning||The warning text can include arbitrary information to be presented to a Human user or logged. A system receiving this warning MUST NOT take any automated action.||RFC 7234|