HTTPheader

An HTTP header is a component of an HTTP message that carries metadata about the request or response. Headers convey information such as content type, length, encoding, caching policies, authentication credentials, and client capabilities. HTTP messages consist of a start line (request line or status line), a header section, and an optional body. The header section contains one or more header fields, each in the form Field-Name: value. Field names are case-insensitive and headers are terminated by a blank line.

Header fields can be categorized as general headers, request headers, response headers, and entity headers. Some headers may appear multiple times or hold comma-separated lists. The precise interpretation of a header is defined by the HTTP specification and may depend on the context of a request or response.

Common headers include Host, User-Agent, Accept, Accept-Language, Content-Type, Content-Length, Content-Encoding, Authorization, Cache-Control, Expires, Referer, and Set-Cookie. They are used for routing, negotiation, content handling, caching, security, and session management. Examples: Content-Type: text/html; charset=UTF-8 and Cache-Control: no-store.

In HTTP/1.1 the header section is transmitted as plain text with CRLF separators. HTTP/2 and later introduce a binary framing and header compression, but the concept of header fields remains. Pseudo-headers such as :method and :path are used in HTTP/2 to convey the same information.

Best practices include avoiding duplicate or conflicting headers, normalizing header names, and not placing sensitive data in logs or referers. Headers play a central role in content negotiation, caching, authentication, and content encoding across web communications.