Using the Domain Name Service

In TCP/IP networks, each network interface (for example, an Ethernet card on a host computer) is identified by an IP address. Because IP addresses are hard to remember, an association is made between an easy-to-remember name and the IP address —much like the association between a name and a telephone number. For example, instead of having to remember that the IP address of Red Hat's Web server is 209.132.177.50, you can simply refer to that host by its name, www.redhat.com. When you type www.redhat.com as the URL in a Web browser, the name www.redhat.com has to be translated into its corresponding IP address. This is where the concept of DNS comes in.

DNS is a distributed, hierarchical database that holds information about computers on the Internet. The information includes the host name, the IP address, and mail routing information. This information resides in many DNS hosts on the Internet; that's why the DNS database is called a distributed database. The primary job of DNS is to associate host names to IP addresses and vice versa.

In ARPANET—the precursor to today's Internet, the association between host names and IP addresses was maintained in a text file named HOSTS.TXT, which was managed centrally and distributed to each host. As the number of hosts grew, it became clear that a static host table was unreasonable. DNS was proposed by Paul Mockapetris to alleviate the problems associated with using a static host table. As formally documented in RFCs 882 and 883 (November 1983), DNS introduced two key concepts:

• Use of hierarchical domain names, such as www.ee.umd.edu and www.redhat .com

• Distributed responsibility for managing the host database by using DNS servers throughout the Internet

DNS, as we know it today, is an Internet standard documented in RFCs 1034 and 1035. The standard has been updated and extended by several other RFCs: 1101, 1183, 1348, 1886, 1995, 1996, 2136, 2181, 2308, 2845, 2930, 2931, 3007, 3110, 3226, 3403, 3596, 3597, 3645, 3646, 4025, 4033, 4034, and 4035. The earlier updates define data encoding, whereas later ones focus on improving DNS security. To read these and other RFCs online, visit theRFC Index at www.faqs.org/rfcs/rfc-index.html.

DNS defines the following:

♦ A hierarchical domain-naming system for hosts

♦ A distributed database that associates a name with an IP address

♦ Library routines that network applications can use to query the distributed DNS database (this library is called the resolver library)

♦ A protocol for DNS clients and servers to exchange information about names and IP addresses

Nowadays, all hosts on the Internet rely on DNS to access various Internet services on remote hosts. As you may know from personal experience, when you obtain Internet access from an Internet service provider (ISP), the ISP provides you with the IP addresses of name servers. These are the DNS servers your system accesses whenever host names have to be mapped to IP addresses.

If you have a small LAN, you may decide to run a DNS server on one of the hosts or to use the name servers provided by the ISP. For medium-sized networks with several subnets, you can run a DNS server on each subnet to provide efficient DNS lookups. On a large corporate network, the corporate domain (such as microsoft.com) is further subdivided into a hierarchy of subdomains, and several DNS servers may be used in each subdomain.

The following sections provide an overview of the hierarchical domain-naming convention, and describe BIND—the implementation of DNS used on most UNIX systems, including Fedora Linux.

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