Page 115 - DCAP602_NETWORK_OPERATING_SYSTEMS_I

Unit 6: Domain Name System

Finding a single server out of all of the servers on the Internet is like annoying to find a single notes
file on drive with thousands of files. In both cases it helps to have some hierarchy built into the
directory to logically group things. The DNS “namespace” is hierarchical in the same type of
upside-down tree structure seen with file systems. Just as you have the root of a partition or
drive, the DNS namespace has a root which is signified by a period.

When specifying the absolute path to a file in a file system you start at the root and go to the
file:

/etc/bind/named.conf
When specifying the absolute path to a server in the DNS namespace you start at the server and
go to the root:
www.aboutdebian.com.
Period after the ‘com’ as it’s significant. It’s how you specify the root of the namespace. An
absolute path in the DNS namespace is called a FQDN (Fully Qualified Domain Name). The use
of FQDNs are prevalent in DNS configuration files and it’s important that you always use that
trailing period.

Internet resources are typically specified by a domain name and a server hostname. The www
part of a URL is often the hostname of the Web server (or it could be an alias to a server with
a different host name). DNS is basically just a database with records for these hostnames. The
directory for the entire telephone system is not stored in one huge phone book. Rather, it is broken
up into many pieces with each city having, and maintaining, its piece of the entire directory in its
phone book. By the same token, pieces of the DNS directory database (the “zones”) are stored,
and maintained, on many different DNS servers located around the Internet. If you want to find
the telephone number for a person in Poughkeepsie, you’d have to look in the Poughkeepsie
telephone book. If you want to find the IP address of the www server in the some-domain.com
domain, you’d have to query the DNS server that stores the DNS records for that domain.
The entries in the database map a host/domain name to an IP address. Here is a naive logical
view of the type of information that is stored (we’ll get to the A, CNAME, and MX designations
in a bit).
A www.their-domain.com 172.29.183.103
MX mail.their-domain.com 172.29.183.217

A debian.your-domain.com 10.177.8.3
CNAME www.your-domain.com 10.177.8.3
MX debian.your-domain.com 10.177.8.3
This is why a real Internet server needs a static (unchanging) IP address. The IP address of the
server’s NIC connected to the Internet has to match whatsoever address is in the DNS database.
Dynamic DNS does provide a way around this for home servers however, which we’ll see later.

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