requestheader

A request header is part of an HTTP request that carries metadata about the client, the request, and the preferred form of the response. Along with the request line, which contains the method, target URL, and HTTP version, the header fields provide information used by servers to process the request.

Each header field consists of a name and a value, separated by a colon. Field names are case-insensitive. Common headers include Host, User-Agent, Accept, Accept-Language, Accept-Encoding, Cookie, Authorization, Content-Type, Content-Length, and Connection.

Servers use request headers to route requests, apply content negotiation, manage caching, and enforce access control. For example, Host determines the virtual host, Accept and Accept-Language guide content negotiation, and Authorization transmits credentials. The headers also influence how responses are formatted, compressed, or cached on the client side.

Security and privacy considerations include validating and sanitizing header values that are echoed in responses, guarding against header injection, and imposing size limits to prevent abuse. Client and server implementations may differ in how headers are parsed and exposed, and developers should treat headers as potentially untrusted input.

The HTTP specifications, notably RFC 7230 through RFC 7235, define header semantics and syntax. Related concepts include cookies (RFC 6265) and content negotiation mechanisms. In practice, applications access and manipulate request headers through server-side frameworks or client-side APIs to implement authentication, localization, and interoperability with diverse clients.