Apache: The Definitive Guide, 3rd Edition

What Does a Web Server Do?

The whole business of a web server is to translate a URL either into a filename, and then send that file back over the Internet, or into a program name, and then run that program and send its output back. That is the meat of what it does: all the rest is trimming.

When you fire up your browser and connect to the URL of someone’s home page — say the notional http://www.butterthlies.com/ we shall meet later on — you send a message across the Internet to the machine at that address. That machine, you hope, is up and running; its Internet connection is working; and it is ready to receive and act on your message.

URL stands for Uniform Resource Locator. A URL such as http://www.butterthlies.com/ comes in three parts:

<scheme>://<host>/<path>

So, in our example, < scheme> is http, meaning that the browser should use HTTP (Hypertext Transfer Protocol); <host> is www.butterthlies.com ; and <path> is /, traditionally meaning the top page of the host.^[1] The <host> may contain either an IP address or a name, which the browser will then convert to an IP address. Using HTTP 1.1, your browser might send the following request to the computer at that IP address:

GET / HTTP/1.1
Host: www.butterthlies.com

The request arrives at port 80 (the default HTTP port) on the host www.butterthlies.com. The message is again in four parts: a method (an HTTP method, not a URL method), that in this case is GET, but could equally be PUT, POST, DELETE, or CONNECT; the Uniform Resource ...