HTTP working, methods, virtual hosting

How HTTP works

HyperText Transfer Protocol (HTTP) is the application layer protocol of the Web. It uses a client/server request-response model over TCP (port 80; HTTPS 443). It is stateless: the server keeps no memory of earlier requests.

1. The user types a URL, e.g. http://www.example.com/index.html.
2. The browser asks DNS for the IP address of www.example.com.
3. The browser opens a TCP connection to port 80 of the server.
4. It sends an HTTP request message (request line, headers, blank line, optional body).
5. The server finds the object and sends a response (status line such as 200 OK, headers, body).
6. The browser parses the HTML; for each embedded object (image, CSS) it sends further requests.
7. The connection is closed (HTTP/1.0) or kept alive (HTTP/1.1 persistent).

 Browser                        Web server
   |--- TCP connect ----------->|
   |--- GET /index.html ------->|
   |<-- 200 OK + HTML ----------|
   |--- GET /logo.png --------->|
   |<-- 200 OK + image ---------|

HTTP methods

• GET: retrieve a resource.
• POST: send form data in the body for processing.
• HEAD: headers only, no body.
• PUT: upload or replace a resource.
• DELETE: remove a resource.
• OPTIONS and TRACE: query the server's capabilities, and echo the request back for testing.

Virtual hosting

Virtual hosting runs several web sites on one server machine, saving hardware and IP addresses. Types: IP-based (a different IP per site), port-based (a different port per site) and name-based (same IP; the Host: header selects the site). Example: www.a.com and www.b.com both at 192.168.1.10, each with its own <VirtualHost> and DocumentRoot in Apache.