HTTP: The Definitive Guide

Name

Pragma

Synopsis

The Pragma header is used to pass directions along with the message. These directions could be almost anything, but often they are used to control caching behavior. Proxies and gateways must not remove the Pragma header, because it could be intended for all applications that receive the message.

The most common form of Pragma, Pragma: no-cache, is a request header that forces caches to request or revalidate the document from the origin server even when a fresh copy is available in the cache. It is sent by browsers when users click on the Reload/Refresh button. Many servers send Pragma: no-cache as a response header (as an equivalent to Cache-Control: no-cache), but despite its common use, this behavior is technically undefinded. Not all applications support Pragma response headers.

Chapter 7 discusses the Pragma header and how it is used by HTTP/1.0 applications to control caches.

Type

Request header

Basic Syntax

Pragma: 1# pragma-directive^[7]

Example

Pragma: no-cache

^[7] The only specification-defined Pragma directive is “no-cache”; however, you may run into other Pragma headers that have been defined as extensions to the specification.