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/HTTP

Warning

The 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.

Header type	General 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 won't 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>

Advisory text describing the error.

<warn-date>

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 warn codes.

Code	Text	Description
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 Warning	Same as 199, but indicating 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	Title
RFC 7234, section 5.5: Warning	Hypertext Transfer Protocol (HTTP/1.1): Caching

Browser compatibility

Feature	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari
Basic support	Yes	Yes	Yes	Yes	Yes	Yes

Feature	Android webview	Chrome for Android	Edge mobile	Firefox for Android	IE mobile	Opera Android	iOS Safari
Basic support	Yes	Yes	Yes	Yes	Yes	Yes	Yes