Warning

Note: The Warning header has been 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.

The Warning 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.

Header type	Request header, Response header
Forbidden header name	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 deletion.
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>

A date. This is optional. If more than one Warning header is sent, include a date that matches the 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"

Specifications

Specification
Hypertext Transfer Protocol (HTTP/1.1): Caching (HTTP/1.1) # header.warning

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`Warning`	Yes	12	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes

Warning

Warning

Syntax

Directives

Warning codes

Examples

Specifications

Browser compatibility

See also