Apache: The Definitive Guide, 3rd Edition

Apache and Domain Name Servers

In one way the Web is like the telephone system: each site has a number that uniquely identifies it — for instance, 192.168.123.5. In another way it is not: since these numbers are hard to remember, they are automatically linked to domain names — www.amazon.com, for instance, or www.butterthlies.com, which we shall meet later in examples in this book.

When you surf to http://www.amazon.com, your browser actually goes first to a specialist server called a Domain Name Server (DNS), which knows (how it knows doesn’t concern us here) that this name translates into 208.202.218.15.It then asks the Web to connect it to that IP number. When you get an error message saying something like “DNS not found,” it means that this process has broken down. Maybe you typed the URL incorrectly, or the server is down, or the person who set it up made a mistake — perhaps because he didn’t read this book.

A DNS error impacts Apache in various ways, but one that often catches the beginner is this: if Apache is presented with a URL that corresponds to a directory, but does not have a / at the end of it, then Apache will send a redirect to the same URL with the trailing / added. In order to do this, Apache needs to know its own hostname, which it will attempt to determine from DNS (unless it has been configured with the ServerName directive, covered in Chapter 2. Often when beginners are experimenting with Apache, their DNS is incorrectly set up, and great confusion can result. ...