Domain Name System(DNS)

Introduction

• DNS stands for “Domain Name System”.
• All the websites on the Internet have unique names associated with them called domain name. Thus, to launch a website, we must have a unique domain name. In order to avoid any conflict within the names of the websites the concept of domain names was developed.
• Way back in the ARPANET era, there was simply a file, called hosts.txt that listed all the hosts and their IP addresses. Every night, all the hosts would fetch it from the site at which it was maintained. For a network of a few hundred large timesharing machines, this approach worked reasonably well. However, when thousands of minicomputers and PCs were connected to the net all the time, then the size of the files would become too large and even more important, host name conflicts would occur constantly unless names were centrally managed, something unthinkable in a huge international network due to the load and latency. Managing a large and constantly changing set of domain names is a nontrivial problem. To solve these problems, DNS (the Domain Name System) was invented.

Definition

• DNS is a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme. It is primarily used for mapping host or domain names and e-mail destinations to its given related IP addresses.

Features

• DNS is one of the unique service that uses both TCP and UDP in their processing i.e. TCP is used when zone exchanged and UDP is used by DNS when resolving a hostname.
• The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative name server for each domain to keep track of its own changes, avoiding the need for a central register to be continually consulted and updated.
• In a DNS, each domain is named by the path upward from it to the (unnamed) root. The components are separated by periods (pronounced ”dot”). Domain names can be either absolute or relative. An absolute domain name always ends with a period (e.g., eng.sun.com.), whereas a relative domain name does not. Relative names have to be interpreted in some context to uniquely determine their true meaning. In both cases, a named domain refers to a specific node in the tree and all the nodes under it.
• Domain names are case insensitive, so edu, Edu, and EDU mean the same thing. Component names can be up to 63 characters long, and full path names must not exceed 255 characters.

Types/Category of Domain Name

• A domain name can be categorised into – (A)Generic Domain Name and (B)Country Domain Name (C)Hybrid/Mix Domain Name 
  

  • Generic Domain Name :

Some common Generic domain names are – 

• • • .gov – represents government agencies
    • .net – represents network organisations
    • .org – represents non-profit organisations
    • .edu – represents educational organisations
    • .com – represents commercial organisations
    • .mil – represents military or defence agencies
    • .info – represents Information Service Provider agencies
    • .arpa – represents ARPA agency

• • Country Domain Name :

Some common Country domain names are – 

• • • .in – represents India country
    • .au – represents Austrailiya country
    • .us – represents America country
    • .uk – represents Great Britain country
    • .ca – represents Canada country

• • Hybrid/Mix Domain Name :

Structure

• The DNS has following major components :

  • Resolver : 

  • Domain Name Space :

    • Conceptually, the Internet is divided into over 200 top-level domains, where each domain covers many hosts. Each domain is partitioned into sub domains, and these are further partitioned into sub-sub domain, and so on. All these domains can be represented by a tree structure. The leaves of the tree represent domains that have no sub domains. A leaf domain may contain a single host, or it may represent a company and contain thousands of hosts.
    • The top-level domains come in two flavors :
      • Generic Domain Name :
        • The original generic domains were .com (commercial), .edu (educational institutions), .gov (the Government), .int (certain international organizations), .mil (the U.S. armed forces), .net (network providers), and .org (nonprofit organizations).
        • In November 2000, ICANN approved four new, general-purpose, top-level domains, namely, .biz (for businesses), .info (for information), .name (for people’s names), and .pro (for professions, such as doctors and lawyers). In addition, three more specialized top-level domains were introduced at the request of certain industries. These are .aero (for aerospace industry), .coop (for co-operatives) and .museum (for museums). Other top-level
          domains will be added in the future.
      • Country Domain Name :
        • The country domains include one entry for every country, as defined in ISO 3166.
        • This domain name is denoted by two letters of country name such as .in (for India), .us (for America), .au (for Austrailiya), .jp(for Japan), .nl(for Netherland) etc.

  • Name Servers :

Working Mechanism

• To map a domain name onto an IP address, an application program calls a library procedure called the RESOLVER, passing it the name as a parameter. The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolver, which then returns it to the caller. Armed with the IP address, the program can then establish a TCP connection with the destination or send it UDP packets.

Functions

• The most basic task of DNS is to translate host or domain names to its given respective IP addresses.
• DNS makes it possible to assign Internet names to organizations (or concerns they represent) independent of the physical routing hierarchy represented by the numerical IP address.