HTTP - Dleifnesor/NET-215 GitHub Wiki

Introduction to HTTP

Foundations of the Web

The web operates on three foundational technologies:

  1. HTML (HyperText Markup Language)
    Provides consistent formatting of resources/documents.

  2. URL (Uniform Resource Locator)
    Provides an addressing scheme to uniquely locate any resource.

  3. HTTP (HyperText Transfer Protocol)
    Defines the protocol for exchanging resources/documents between systems.

Web Resources

A web resource is any content served by a web server. It may be:

Static Resources

Stored as files on the server:

  • .html, .jpg, .pdf, .mp4, etc.
  • Located directly in the web server's filesystem.

Dynamic Resources

Generated in real time by server-side applications:

  • Vary based on user input, session state, time, database queries, etc.
  • Examples: real estate search, live camera feeds, dynamic dashboards.

URIs and URLs

  • A URI (Uniform Resource Identifier) is a unique name for a web resource.
  • A URL is the most common type of URI and points to the location of a resource.

Example URL:

http://www.joes-hardware.com/specials/saw-blade.gif

Absolute vs. Relative URLs

Type Definition Example
Absolute Full path including domain http://example.com/images/pic.jpg
Relative Path relative to current site images/pic.jpg, ../about/page.html

Rules:

  • Use relative paths for internal links.
  • Use absolute paths for external links.

Understanding Web Roots

Web paths begin from the web root, not the server's full filesystem.

  • Apache (Linux): /var/www/html
  • IIS (Windows): C:\InetPub

Example:

URL: http://test.com/cncs/cyber.html

  • Apache: /var/www/html/cncs/cyber.html
  • IIS: C:\InetPub\cncs\cyber.html

http://test.com/var/www/html/cncs/cyber.html won't work.

HTTP and TCP/IP

HTTP relies on TCP/IP for network communications.

Concepts:

  • Requires IP address + Port
  • Port 80 = default HTTP
  • Port 443 = default HTTPS

Example URLs:

http://207.200.83.29:80/index.html
http://www.netscape.com:80/index.html
http://www.netscape.com/index.html

DNS resolves hostnames to IPs.

HTTP Privacy

HTTP is not secure:

  • Anyone can read data (forms, cookies, headers).
  • Use HTTPS (TLS encryption) to protect data.

Forms

HTML forms collect and submit user input:

Example Form:

<form action="welcome_get.php" method="get">
  Name: <input type="text" name="name"><br>
  E-mail: <input type="text" name="email"><br>
  <input type="submit">
</form>

Example GET Request:

GET /welcome_get.php?name=adam&[email protected] HTTP/1.1

* Types of HTTP Methods

HTTP defines a number of request methods that indicate the desired action to be performed on a resource.

Method Description
GET Retrieves data from the server. Should not have any side effects.
POST Sends data to the server to create or process a resource. Often used with forms.
PUT Replaces or creates a resource at a specific URI. Idempotent.
DELETE Removes the specified resource from the server.
HEAD Same as GET, but returns only headers (no response body). Useful for testing links or caching.
OPTIONS Describes the communication options for the target resource. Often used in CORS (Cross-Origin Resource Sharing).
PATCH Applies partial updates to a resource.
TRACE Echoes the received request so the client can see what (if any) changes or additions have been made by intermediate servers.

Notes on Method Behavior

  • Safe methods: GET, HEAD, OPTIONS, TRACE — do not modify data.
  • Idempotent methods: GET, PUT, DELETE, HEAD, OPTIONS — can be repeated without causing additional effects.
  • Non-idempotent methods: POST, PATCH — may result in changes each time they're repeated.

Example PHP Script:

<html>
<body>
Welcome <?php echo $_GET["name"]; ?><br>
Your email address is: <?php echo $_GET["email"]; ?>
</body>
</html>

🚦 GET vs POST

Feature GET POST
Data sent in URL Body of request
Cacheable Yes No
Browser history Yes No
Bookmarkable Yes No
Length restrictions Yes No
Use with sensitive data No Yes

HTTP Messages

Two types:

  • Request (from client)
  • Response (from server)

Format:

Start-Line
Header: Value
Header: Value

[Optional Body]

Example HTTP Request

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Referer: https://google.com
If-Modified-Since: Sat, 28 Nov 2009 03:50:37 GMT

Example HTTP Response

HTTP/1.1 200 OK
Date: Sat, 28 Nov 2009 04:36:25 GMT
Server: LiteSpeed
Content-Type: text/html; charset=UTF-8
Content-Encoding: gzip

<!DOCTYPE html>
<html>...</html>

Important Headers

Request Headers:

  • Host – Domain to request
  • User-Agent – Info about browser & OS
  • Referer – Previous page
  • If-Modified-Since – For caching

Response Headers:

  • Content-Type – MIME type of response
  • Last-Modified – For client-side caching
  • Cache-Control – How client should cache data

HTTP Status Codes

Code Description
200 OK – Success
301 Moved Permanently (Redirect)
304 Not Modified (Use cached version)
400 Bad Request
401 Unauthorized
403 Forbidden
404 Not Found
500 Internal Server Error
418 I'm a teapot (nerds)
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