Warning header

[!NOTE] The header was deprecated because it is not widely generated or surfaced to users (see RFC9111). Some of the information can be inferred from other headers such as {{httpheader("Age")}}.

The HTTP Warning {{glossary("request header", "request")}} and {{glossary("response 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.

Header type	{{Glossary("Request header")}}, {{Glossary("Response header")}}
{{Glossary("Forbidden request header")}}	No

Syntax

Warning: <warn-code> <warn-agent> <warn-text> [<warn-date>]

Directives

<warn-code>
- : A three-digit warning number. The first digit indicates whether the Warning is required to be deleted from a stored response after validation.
  - 1xx warn-codes describe the freshness or validation status of the response and will be deleted by a cache after successful validation.
  - 2xx warn-codes describe some aspect of the representation that is not rectified by a validation and will not be deleted by a cache after validation unless a full response is sent.
<warn-agent>
- : The name or pseudonym of the server or software adding the Warning header (might be "-" when the agent is unknown).
<warn-text>
- : An advisory text describing the error.
<warn-date> {{optional_inline}}
- : A date. If more than one Warning header is sent, include a date that matches the {{HTTPHeader("Date")}} header.

Warning codes

The HTTP Warn Codes registry at iana.org defines the namespace for warning codes.

Code	Text	Description
110	Response is Stale	The response provided by a cache is stale (the expiration time set for the response has passed).
111	Revalidation Failed	An attempt to validate the stale response failed due to an inability to reach the server.
112	Disconnected Operation	The cache is intentionally disconnected from the rest of the network.
113	Heuristic Expiration	A cache heuristically chose a freshness lifetime greater than 24 hours and the age of the response is greater than 24 hours.
199	Miscellaneous Warning	Arbitrary information that should be presented to a user or logged.
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	Arbitrary information that should be presented to a user or logged. This warn-code is similar to the warn-code 199 and additionally indicates a persistent warning.

Examples

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"

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