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HTTP Header

An HTTP header is a field of an HTTP request or response that passes additional context and metadata about the request or response. For example, a request message can use headers to indicate it's preferred media formats, while a response can use header to indicate the media format of the returned body. Headers are case-insensitive, begin at the start of a line and are immediately followed by a ':' and a header-dependent value. The value finishes at the next CRLF or at the end of the message.

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HTTP header

The HTTP and Fetch specifications refer to a number of header categories, including:

  • 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, …).
  • Representation header : metadata about the resource in the message body (e.g. encoding, media type, etc.).
  • Fetch metadata request header : Headers with metadata about the resource in the message body (e.g. encoding, media type, etc.).

A basic request with one header:

http 
                                    
                                        
                                            GET
                                            
                                                
                                                    /
                                                    example.html
                                                
                                            
                                            HTTP/1.1
                                        
                                        
                                            Host
                                            :
                                            example.com

Redirects have mandatory headers ( Location ):

http 
                                    
                                        302 Found

                                        
                                            Location
                                            :
                                            /NewPage.html

A typical set of headers:

http 
                                    
                                        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

Note: Older versions of the specification referred to:

  • General header : Headers applying to both requests and responses but with no relation to the data eventually transmitted in the body.
  • Entity header : Headers containing more information about the body of the entity, like its content length or its MIME-type (this is a superset of what are now referred to as the Representation metadata headers)

See also

Updated on April 20, 2024 by Datarist.