What are HTTP Headers?

HTTP headers are key-value pairs that are included in both HTTP requests and HTTP responses. They are sent before the actual data (the body) and are separated from the body by a blank line. Headers are text-based, making them relatively easy to read and understand.

Here's a simplified example of what some HTTP headers might look like in a request:

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64)
Accept: text/html,application/xhtml+xml

And here's an example of some headers in a response:

HTTP/1.1 200 OK
Date: Tue, 26 Sep 2023 12:00:00 GMT
Server: Apache/2.4.41 (Unix)
Content-Type: text/html; charset=UTF-8
Content-Length: 1234

Common HTTP Header Fields

There are many different HTTP headers, each serving a specific purpose. They can be broadly categorized into:

• Request Headers: Sent by the client to provide information about the request and the client itself.
• Response Headers: Sent by the server to provide information about the response and the server.
• General Headers: Can be used in both requests and responses.

Here are some of the most common and important HTTP headers:

Request Headers

• Host:: Specifies the domain name of the server (e.g., www.example.com). This is required in HTTP/1.1 requests.
• User-Agent:: Identifies the client making the request (e.g., the browser type and version).
• Accept:: Specifies the types of content the client is willing to accept (e.g., text/html, image/jpeg, application/json).
• Accept-Encoding:: Specifies the encoding methods the client can handle (e.g., gzip, deflate for compression).
• Accept-Language:: Specifies the preferred languages of the client (e.g., en-US, fr-CA).
• Cookie:: Sends cookies (small pieces of data stored by the browser) back to the server.
• Authorization:: Provides credentials for authenticating with the server (e.g., for accessing protected resources).
• Referer: [sic]: Indicates the URL of the page that linked to the requested resource.
• If-Modified-Since:: Makes the request conditional. The server will only send the resource if it has been modified since the specified date.

Response Headers

• Content-Type:: Specifies the type of content in the response body (e.g., text/html; charset=UTF-8, image/jpeg, application/json). This tells the browser how to interpret the data.
• Content-Length:: Specifies the size of the response body in bytes.
• Server:: Identifies the web server software being used (e.g., Apache/2.4.41, nginx/1.18.0).
• Date:: Indicates the date and time the response was generated.
• Cache-Control:: Provides directives for caching the response (e.g., public, private, max-age).
• Expires:: Specifies a date and time after which the response should be considered stale.
• Last-Modified:: Indicates the date and time the resource was last modified.
• Set-Cookie:: Sends cookies from the server to the client.
• Location:: Used in redirection responses (3xx status codes) to specify the new URL.
• ETag: An identifier for a version of the resource.