Overview of HTTP headers

HTTP Headers are the core components of the HTTP protocol, and they play a vital role in the communication between the client and the server.Each HTTP request or response message can contain one or more header fields, which provide additional information about the request or response to help the receiver better understand and process the message.

HTTP headers are defined as fields used for HTTP requests or responses, which convey additional context and metadata about the request or response. For example, a request message can use a header to indicate its preferred media format, while a response can use a header to indicate the media format of the return body.

The role of HTTP headers is mainly reflected in the following aspects:

Specify the requested server domain name and port number through the Host header to help the server determine the requested target resource. The User-Agent header contains the information of the requesting user agent, such as browser type, version and operating system, according to which the server can return the response content suitable for the client. The Accept, Accept-Language and Accept-Encoding headers tell the server what types of response media the client can handle, the preferred language and the accepted compression format. Content-Type and Content-Length headers are used in POST requests to tell the server the media type and length of the request body.

The Authorization header is used for authentication information, which is usually included in the request, while the Strict-Transport-Security header tells the browser to connect to the server only through HTTPS to enhance communication security.Cache-Control header specifies the cache instructions that requests and responses follow to help clients and servers manage cache resources. The Location header is usually used with 3xx's response to specify the redirected URL;The WWW-Authenticate header is used for HTTP authentication, while the X-Frame-Options and X-XSS-Protection headers are used to prevent clickjacking attacks and enable reflective cross-site scripting (XSS) filtering of browsers.

HTTP headers can be classified into the following categories according to their purpose and location:

Common header: It is applicable to both request and response messages, but it has nothing to do with the data transmitted by the final message. Such as Date, Server and Via.

Request header: contains more information about the resource to be obtained or the client itself.Such as Accept, User-Agent, and Authorization.

Response header: Contains supplementary information about the response, such as its location or the server itself (name and version, etc.). Such as Server, Set-Cookie and WWW-Authenticate.

Entity Header: Contains more information about entity body, such as its content length or its MIME type. Such as Content-Type, Content-Length and Last-Modified.

In addition, headers can be divided into end-to-end headers and hop-by-hop headers according to the way agents handle them.End-to-end headers must be transmitted to the final message receiver, and intermediate agents must retransmit these unmodified headers and cache them. Hop-by-hop headers are meaningful only for single transport connections and must not be retransmitted or cached by the proxy.

Understanding the classification of HTTP headers is helpful for developers and network administrators to better design and optimize the HTTP communication process and ensure the effective transmission and processing of data.