Warning

The Warning header, as the name implies, is intended to relay additional information about potential problems related to the response. In practice, it is normally used to alert the Web client to problems in a caching system.

The basic format of a Warning header is as follows:

Warning: 110 proxya.localdomain “Response is stale” 

Optionally, a date can also be included after the error description. If included, the date should be in a proper HTTP format and enclosed with quotation marks, although many implementations fail to include the quotation marks.

The following warning codes are defined in the HTTP specification.

110 Response is stale

This simply indicates that the resource being returned is a stale copy from the caching system ...

Get HTTP Developer’s Handbook now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.