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In HTTP/1.1, whenever a cache returns a response that is neither first-hand or "fresh enough", it attaches a warning to that effect, using a Warning general header field. The warning allows clients to take appropriate action. Warnings may be used for other purposes, both cache-related and otherwise. The use of warning, rather than an error status code, distinguish these responses from true failures. For more information about warnings, refer to the HTTP/1.1 draft specification (RFC 2616, Sections 13.1.2, 14.46).
Copyright © 2003, John Yannakopoulos <email@example.com>