Warning

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

BCD tables only load in the browser

See also