Warning
Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
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 Age.
The HTTP Warning request and 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 | 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
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 successful validation.2xxwarn-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
Warningheader (might be "-" when the agent is unknown). <warn-text>-
An advisory text describing the error.
<warn-date>Optional-
A date. 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 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 |
|---|
| HTTP Caching # field.warning |
Browser compatibility
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