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
Warningis required to be deleted from a stored response after validation.1xxwarn-codes describe the freshness or validation status of the response and will be deleted by a cache after deletion.-
2xxwarn-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
Warningheader (might be "-" when the agent is unknown). - <warn-text>
- Advisory text describing the error.
- <warn-date>
- Optional. If more than one
Warningheader is sent, include a date that matches theDateheader.
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 | Firefox | Edge | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|---|
| Basic Support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
| Feature | Android | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
|---|---|---|---|---|---|---|---|
| Basic Support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |