An HTTP request header is a field sent with an HTTP request to provide information about the request, the client, or its context. Headers can express preferences, carry credentials, guide caching, or provide conditional-request details. Each field has its own defined meaning and value syntax.
What an HTTP request header contains
HTTP fields are name/value pairs. The field name identifies what the value means, while the field’s specification defines the value’s syntax. Field names are case-insensitive, so Accept and accept refer to the same field. See RFC 9110, Section 5.
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In everyday usage, “request header” means a field carried in a request. More precisely, HTTP field categories can describe a field’s role, not only whether it appears in a request or response. For example, Content-Type may appear in a request but is classified as a representation header. See MDN’s request header glossary and HTTP header glossary.
Example: fields in an HTTP request
This schematic example shows the familiar textual form of an HTTP/1.1 request:
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GET /products HTTP/1.1
Host: example.com
Accept: application/json
Authorization: Bearer <token>Host, Accept, and Authorization are field names; the text after each colon is its value. Here, Accept states a preference for JSON, while Authorization carries credentials in an application-defined or standardized format. The placeholder token is illustrative, not a usable credential.
What request headers do
The field name determines the purpose; “header” does not imply one universal behavior. Common purposes include:
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- Preferences:
Acceptcan indicate which media types a client can process. - Credentials:
Authorizationcan carry authentication information. - Caching: fields such as
Cache-Controlcommunicate caching directives. - Conditional requests: fields can let a client make a request conditional on information about a previously received representation.
These are examples, not interchangeable instructions: consult the definition of a specific field to understand its valid values and effect. MDN’s request header glossary gives further examples, including Accept-Language, User-Agent, and Referer.
How headers relate to the HTTP message
RFC 9110 defines the header section as a sequence of header field lines. It describes the shared semantics of HTTP fields; the protocol version’s message specification defines how messages are represented and framed. See RFC 9110, Section 6.3.
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In HTTP/1.x, headers are commonly shown as text with a field name, colon, and value. That display should not be mistaken for the on-wire form of HTTP/2 and later. Developer tools may show lowercase field names, and HTTP/2 uses special pseudo-header fields whose names begin with a colon. For reference, see MDN’s HTTP headers documentation.
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