The DNS hierarchy can be compared to the hierarchy of the Linux filesystem. Hostnames in individual domains parallel filenames in individual directories, and, like the root directory of the filesystem, DNS has a root domain.
In both the filesystem and the DNS system, the names of objects reveal the rooted hierarchical structure. Filenames move from the most general, the root (/), to the most specific, the individual file. Domain names start with the most specific, the host, and move to the most general, the root (.). A domain name that starts with a host and goes all the way to the root is called a fully qualified domain name (FQDN). For example, wren.foobirds.org is the FQDN of one of the systems on our imaginary network.
The top-level domains (TLDs)—such as org, edu, jp, and com—are serviced by the root servers. The second-level domain, foobirds in the example, is the domain that has been officially assigned to our imaginary organization. When you're officially assigned a domain by your parent domain, a pointer is placed in the parent domain that points to your server as the server responsible for your domain. It is this delegation of authority that makes your domain part of the overall domain system. How to delegate authority for subdomains is covered later in this chapter.
Note This book assumes that you already have an official domain name and IP address.
If you don't, and you need information on how to obtain a domain name or IP address, see TCP/IP Network Administration, Third Edition, by Craig Hunt (O'Reilly, 2002).
The analogy to the filesystem goes beyond just the structure of names. Files are found by following a path from the root directory through subordinate directories to the target directory. DNS information is located in a similar manner. Linux learns the location of the root filesystem during the boot process from the grub.conf file or the lilo.conf file. Similarly, your DNS server locates the root servers during startup by reading a file, called the hints file, which contains the names and addresses of the root servers. (You will create that file later in this chapter.) Via queries, the server can find any host in the domain system by starting at the root and following pointers through the domains until it reaches the target domain.
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