A Guide to HTTP Headers

HTTP headers are the unsung heroes of web communication, providing the essential details that help browsers and servers talk to each other effectively. These small but mighty pieces of metadata play a crucial role in how data is transferred over the web, influencing everything from content delivery to security measures. In this guide, we'll explore what HTTP headers are, their structure, their significance, and a detailed look at 50 of the most commonly used HTTP headers.

What Are HTTP Headers?

HTTP headers are key-value pairs included in HTTP requests and responses. They carry important information about the request being made or the response being sent back. Think of headers as the notes that accompany a package, detailing what's inside and how it should be handled.

Categories of HTTP Headers

HTTP headers can be grouped into several categories based on their function:

1. General Headers: These apply to both requests and responses but don’t relate to the content itself.
2. Request Headers: Specific to requests, these provide details about the client and the resource being requested.
3. Response Headers: Specific to responses, these convey information about the server’s reply.
4. Entity Headers: These offer details about the resource itself, such as its size and type.

Structure of HTTP Headers

Each header consists of a name and a value, separated by a colon. For instance:

You can include multiple headers in a single request or response, with each one on a new line. While not every header is required, they can significantly enhance communication.

Why Are HTTP Headers Important?

HTTP headers are vital for a variety of reasons:

• Content Negotiation: They help clients and servers agree on the type of content being exchanged (like HTML or JSON).
• Caching Control: Headers dictate how responses are cached, which can improve loading times and reduce server load.
• Security: They can enhance security protocols, manage sessions, and handle authentication.
• Cross-Origin Resource Sharing (CORS): Headers control how resources are shared across different domains.

The 50 Most Commonly Used HTTP Headers

Let’s take a closer look at 50 of the most commonly used HTTP headers, broken down into their respective categories for clarity.

General Headers

1. Date:

   • Example: Date: Wed, 31 Oct 2024 14:12:00 GMT
   • Description: Tells when the message was sent.

2. Cache-Control:

   • Example: Cache-Control: no-cache
   • Description: Instructs how caching should be handled.

3. Connection:

   • Example: Connection: keep-alive
   • Description: Indicates whether to keep the connection open.

4. Pragma:

   • Example: Pragma: no-cache
   • Description: Used for backward compatibility with older caches.

5. Trailer:

   • Example: Trailer: Expires
   • Description: Signals that more fields will be present in the trailer of a message.

Request Headers

6. Accept:

   • Example: Accept: text/html, application/json
   • Description: Lists content types the client can process.

7. Accept-Encoding:

   • Example: Accept-Encoding: gzip, deflate
   • Description: Specifies supported content encodings.

8. Accept-Language:

   • Example: Accept-Language: en-US,en;q=0.5
   • Description: Indicates preferred languages for the response.

9. Authorization:

   • Example: Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ=
   • Description: Contains credentials for authenticating the client.

10. Cookie:

    • Example: Cookie: sessionId=abc123
    • Description: Includes cookies previously set by the server.

11. Host:

    • Example: Host: www.example.com
    • Description: Specifies the server's domain name.

12. If-Modified-Since:

    • Example: If-Modified-Since: Sat, 29 Oct 2022 19:43:31 GMT
    • Description: Requests the resource only if it has changed since a certain date.

13. If-None-Match:

    • Example: If-None-Match: "abc123"
    • Description: Requests the resource only if it doesn’t match the given entity tag (ETag).

14. Origin:

    • Example: Origin: https://www.example.com
    • Description: Indicates the origin of the request for CORS.

15. User-Agent:

    • Example: User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3
    • Description: Provides details about the client software.

16. Referer:

    • Example: Referer: https://www.google.com/
    • Description: Tells where the request originated from.

17. Range:

    • Example: Range: bytes=0-499
    • Description: Requests a specific part of a document.

18. TE:

    • Example: TE: trailers, deflate;q=0.5
    • Description: Indicates the transfer encodings the client can accept.

19. Upgrade:

    • Example: Upgrade: websocket
    • Description: Signals a request to change the protocol being used.

20. Max-Forwards:

    • Example: Max-Forwards: 10
    • Description: Limits the number of times the request can be forwarded.

Response Headers

21. Access-Control-Allow-Origin:

    • Example: Access-Control-Allow-Origin: *
    • Description: Specifies which origins can access the resource (CORS).

22. Content-Type:

    • Example: Content-Type: text/html; charset=UTF-8
    • Description: Indicates the media type of the resource.

23. Content-Length:

    • Example: Content-Length: 348
    • Description: Size of the response body in bytes.

24. Content-Encoding:

    • Example: Content-Encoding: gzip
    • Description: Specifies how the content is encoded.

25. Content-Language:

    • Example: Content-Language: en-US
    • Description: Indicates the natural language of the content.

26. ETag:

    • Example: ETag: "abc123"
    • Description: Provides a unique identifier for the specific version of a resource.

27. Last-Modified:

    • Example: Last-Modified: Sat, 29 Oct 2022 19:43:31 GMT
    • Description: Indicates when the resource was last modified.

28. Location:

    • Example: Location: https://www.example.com/new-location
    • Description: Used in redirections to specify a new URL.

29. Server:

    • Example: Server: Apache/2.4.1 (Unix)
    • Description: Provides information about the server software.

30. Set-Cookie:

    • Example: Set-Cookie: sessionId=abc123; HttpOnly
    • Description: Sends cookies from the server to the client.

31. WWW-Authenticate:

    • Example: WWW-Authenticate: Basic realm="User Visible Realm"
    • Description: Indicates the authentication method that should be used.

32. Refresh:

    • Example: Refresh: 5; URL=https://www.example.com
    • Description: Tells the browser to refresh the page after a certain time.

33. X-Content-Type-Options:

    • Example: X-Content-Type-Options: nosniff
    • Description: Prevents browsers from MIME-sniffing a response.

34. X-Frame-Options:

    • Example: X-Frame-Options: DENY
    • Description: Protects against clickjacking by controlling frame display.

35. X-XSS-Protection:

    • Example: X-XSS-Protection: 1; mode=block
    • Description: Enables XSS filters in browsers.

36. Link:

    • Example: Link: <https://www.example.com>; rel="canonical"
    • Description: Provides information about related resources.

37. Content-Disposition:

    • Example: Content-Disposition: attachment; filename="filename.jpg"
    • Description: Indicates whether content should be displayed inline or treated as an attachment.

38. Expires:

    • Example: Expires: Wed, 31 Oct 2024 14:12:00 GMT
    • Description: Specifies when the response content should be considered stale.

39. Age:

    • Example: Age: 600
    • Description: Indicates the age of the object in a proxy cache.

40. Retry-After:

    • Example: Retry-After: 120
    • Description: Informs how long to wait before making a follow-up request.

Entity Headers

41. Allow:

    • Example: Allow: GET, POST, HEAD
    • Description: Lists HTTP methods allowed for a specific resource.

42. Content-MD5:

    • Example: Content-MD5: 1B2M2Y8AsgTpgAmY7PhCfg==
    • Description: Provides an MD5 checksum of the content for integrity verification.

43. Content-Range:

    • Example: Content-Range: bytes 0-499/1234
    • Description: Indicates the part of a document being sent.

44. Accept-Ranges:

    • Example: Accept-Ranges: bytes
    • Description: Indicates that the server supports range requests.

45. Warning:

    • Example: Warning: 199 Miscellaneous warning
    • Description: Provides additional information about the message.

46. Expires:

    • Example: Expires: Wed, 31 Oct 2024 14:12:00 GMT
    • Description: Specifies when the response content should be considered stale.

47. Link:

    • Example: Link: <https://www.example.com>; rel="canonical"
    • Description: Provides information about related resources.

48. Content-Security-Policy:

    • Example: Content-Security-Policy: default-src 'self';
    • Description: Defines security policies to mitigate XSS and data injection attacks.

49. X-Permitted-Cross-Domain-Policies:

    • Example: X-Permitted-Cross-Domain-Policies: master-only
    • Description: Controls whether Flash and Adobe Acrobat can load content from the site.

50. Content-Security-Policy-Report-Only:

    • Example: Content-Security-Policy-Report-Only: default-src 'self'; report-uri /csp-violation-report-endpoint/
    • Description: Allows observation of potential policy violations without enforcing them.

Conclusion

HTTP headers may seem like small details, but they are essential for effective web communication. Understanding these headers and their functions can empower you to optimize web applications, enhance security, and manage data more effectively. By familiarizing yourself with the 50 headers we've discussed, you’ll gain valuable insights into how data is transmitted over the web, how to troubleshoot issues, and how to implement best practices in web development and security. Whether you're a developer, a security professional, or just a curious web user, knowing your HTTP headers can make a big difference!