HTTP Entity Headers
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Last but not least, we come to the fourth group of HTTP headers: entity headers. These headers provide information about the resource carried in the body of an HTTP message, called an entity in the HTTP standards. They serve the overall purpose of conveying to the recipient of a message the information it needs to properly process and display the entity, such as its type and encoding method.
The most common type of entity is a file or other set of information that has been requested by a client, and for this reason, entity headers most often appear in HTTP responses. However, they can also appear in HTTP requests, especially those using the PUT and POST methods, which are the ones that transfer data from a client to a server.
At least one entity header should appear in any HTTP message that carries an entity. However, they may also be present in certain responses that do not have an actual entity in them. Most notably, a response to a HEAD request will contain all the entity headers associated with the resource specified in the request; these are the same headers that would have been included with the entity, had the GET method been used instead of HEAD on the same resource. Entity headers may also be present in certain error responses, to provide information to help the client make a successful follow-up request.
The following are the entity headers defined in HTTP/1.1:
Lists all the methods that are supported for a particular resource. This header may be provided in a server response as a guide to the client regarding what methods it may use on the resource in the future. The header must be included when a server returns a 405 (Method Not Allowed) response to a request containing an unsupported method.
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