Domain Name System

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Revision as of 22:07, 10 April 2009

DNS is part of the Application Layer of the TCP/IP Internet Protocol Suite.^[2]

DNS is a hierarchical naming system used to map human-readable machine names into Internet Protocol addresses (IP addresses) for the Internet.^[1] Although IP addresses provide a compact representation for identifying machines, users prefer to assign machines with pronounceable, meaningful, and easy-to-remember names.^[1] DNS can be thought of as the Internet’s Phone Book. Instead of looking up a phone number using the person’s name or business’s name, the IP address can be looked up by using the domain name. For example, DNS translates the domain name www.mcmaster.ca into 130.113.64.65. DNS is also able to translate from IP addresses to domain names, which is known as Reverse Mapping or Inverse Mapping.^[1]

DNS is part of the Application Layer of the TCP/IP Internet Protocol Suite. It has two conceptually independent aspects. First, it specifies the name syntax and rules for delegating authority over names.^[1] Secondly, it specifies the implementation of a system that efficiently maps names to addresses.^[1]

History of DNS

In the late 1960s, the U.S. Department of Defense Advanced Research Projects Agency (ARPA, later known as DARPA) funded and began working toward an internet technology called the ARPANET.^[1] Each computer on the network was given an address for interconnecting with other computers on the network. In addition, this information was stored on a central site, called the Network Information Center (NIC), in a single file named HOSTS.TXT.^[3,4] Moreover, the HOSTS.TXT file contains information regarding the name-to-address mapping of all computers connected to the ARPANET.^[3] Each computer on the network needed to retrieve this file in order to communication with other computers on the network.^[3] With only a small amount of computers interconnected at the start, HOSTS.TXT worked well. However, there are limitations with this scheme. If there is a change in the address in one of the computers on the network, every computer that want to communication with that machine would need to update its host file. Also, as the network traffic increases, there is a need to overcome problems such as overlapping names. As a result, DNS was invented to solve these limitations.

The Domain Name System was invented by Jon Postel, Paul Mockapetris and Craig Partrige in 1983.^[4] In addition, they published the original specification, RFC 882, for the DNS.^[4] Over the years, several more RFCs have been published to update the DNS specification, making the RFC 882 obsolete.

Name Syntax And Rules Of DNS

Delegation Of Authority For Names

Delegation of authority for names in the Domain Name System.

The DNS uses a hierarchical naming scheme. A hierarchical naming scheme operates similar to the management structure of a business organization. At the top level, the namespace is divided and each division is delegated with authority.^[1] In addition, if there is a change in a division, the top level would not be affected.^[1] The syntax of the assigned names often reflects the hierarchical authority delegated.^[1] Using the following namespace as an example:

local.site

site is the name of the site authorized by the central authority; local is the part that is controlled by the site. The period (“.”) is a delimiter to separate the names. Such hierarchical naming scheme is known as domain names. Domain name is a name consisting of a sequence of labels separated by periods (dots).^[1] Each suffix of a domain name is also a domain name. In addition, a domain name denotes a set of one or more hosts. In the case of a domain name denoting a single host (known as a host name), the domain name looks no different from a domain name denoting a set of hosts.

Subset Authority

In the hierarchical naming scheme, authority can be subdivided at each level. The idea is to subdivide the namespace until the subdivision is small enough to be manageable.^[1] When subdividing namespace, another partition of the name is introduced. The following example shows the syntax of a group subdivision being added to names already partitioned:^[1]

local.group.site

In DNS, the individual sections of names that represent sites and groups are called labels.^[1] As a result, the domain name:

cas.mcmaster.ca

contains three labels: cas, mcmaster, and ca. Any suffix of a label in the domain name is called a domain. The lowest level domain in the above example is cas.mcmaster.ca (the domain name for the Computer and Software Department at McMaster University); the second level domain is mcmaster.ca (the domain name for McMaster University); and the top-level domain is ca (the domain name for Canada).

Top-Level Domains

The following table is a list of top-level domain names and their meanings:

Domain Name	Meaning
aero	Air transport industry
arpa	Intrastructure domain
biz	Businesses
com	Commerical organization
coop	Cooperative associations
edu	Educational institution (4-year)
gov	United States government
info	Information
int	International treaty organizations
mil	United States military
museum	Museums
name	Individuals
net	Major network support centers
org	Organizations other than those above
pro	Credentialed professionals
country code	Each country (geographic scheme)

Name Syntax And Type

Implementation of DNS

Mapping Domain Names To Addresses

Domain Name Resolution

Caching