An HTTP header is a field of an HTTP request or response that passes additional information, altering or precising the semantics of the message or of the body. Headers are case-insensitive, begins at the start of a line and are immediately followed by a
':' and a value depending of the header itself. The value finish at the next CR or at the end of the message.
Traditionally, headers are classed in categories, though this classification is no more part of any specification:
- General header: Headers applying to both requests and responses but with no relation to the data eventually transmitted in the body.
- Request header: Headers containing more information about the resource to be fetched or about the client itself.
- Response header: Headers with additional information about the response, like its location or about the server itself (name, version, …).
- Entity header: Headers containing more information about the body of the entity, like its content length or its MIME-type.
A basic request with one header:
GET /example.http HTTP/1.1 Host: example.com
Redirects have mandatory headers (
302 Found Location: /NewPage.html
A typical set of headers:
304 Not Modified Access-Control-Allow-Origin: * Age: 2318192 Cache-Control: public, max-age=315360000 Connection: keep-alive Date: Mon, 18 Jul 2016 16:06:00 GMT Server: Apache Vary: Accept-Encoding Via: 1.1 3dc30c7222755f86e824b93feb8b5b8c.cloudfront.net (CloudFront) X-Amz-Cf-Id: TOl0FEm6uI4fgLdrKJx0Vao5hpkKGZULYN2TWD2gAWLtr7vlNjTvZw== X-Backend-Server: developer6.webapp.scl3.mozilla.com X-Cache: Hit from cloudfront X-Cache-Info: cached
- Syntax of headers in the HTTP specification.