Published May 25, 2023
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HTTP headers let the client and the server pass additional information with an
HTTP request or response. An HTTP header consists of its case-insensitive name
followed by a colon (:), then by its value. Whitespace before the value is
ignored.
Custom proprietary headers have historically been used with an X- prefix, but
this convention was deprecated in June 2012 because of the inconveniences it
caused when nonstandard fields became standard in RFC 6648; others are listed
in an IANA registry, whose original content was defined in RFC 4229. IANA also
maintains a registry of proposed new HTTP headers.
Headers can be grouped according to their contexts:
Request headers contain more information about the resource to be fetched, or about the client requesting the resource.
Response headers hold additional information about the response, like its location or about the server providing it.
Representation headers contain information about the body of the resource, like its MIME type, or encoding/compression applied.
Payload headers contain representation-independent information about payload data, including content length and the encoding used for transport.