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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 theDate
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
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | Servo |
---|---|---|---|---|---|---|---|
Basic Support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Edge Mobile | Firefox for Android | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|---|
Basic Support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |