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 its 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.

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:

Redirects have mandatory headers (Location):

A typical set of headers: