Domain Name System

DNS is used to provide electronic mail routing information. The DNS provides the protocol that permits clients and servers to interact with each other. The DNS name space is stored in hierarchical manner. It is exactly same as UNIX file system.

Domain names are case insensitive. Each node contains a label of 63 characters long. The root of the tree is treated as special node with a null label. In the tree every node must contain a unique domain name, but the same label can be made use for different points in the tree. A domain name that ends with a period is known as absolute domain name or a fully qualified domain name.

The Internet uses several hundred top-level domains, where each domain has many hosts. Each domain is divided into sub domains and these are further divided and so on….
The top-level domains are into 3 areas:
1) A special domain Arpa is used for address-to-name mappings.
2) Generic domains are 3 character domains.
3) The 2 character domains depend upon the country codes. These are known as country domains.

Following are list of generic domain:
1) com: Commercial applications.
2) edu: Educational institutions.
3) gov: U. S. governmental organizations.
4) mil: U. S. military.
5) net: Networks.
6) int: International organization.

The generic domain .GOV and .MIL are limited to the United States. Each domain is defined by path upward from it to the root. Periods are used to separate the components. In a zone whenever a new system is installed, the DNS administrator for the zone assigns name and an IP address for the new server’s database. A zone is a sub-tree of the DNS tree that is managed separately. Caching is fundamental property of the DNS. Two letter domain names are:
1) ge= Germany.
2) us= United States.
3) nz=New Zealand.
4) in=India.

Related with cash institution is a host that executes an application protocol or process as the domain name server. Associated with this is a database is called as directory information base (DIB), which includes entire directory related information for that institution. When a new host is to be registered, the manager interactively enters the name and IP address and each other associated information. This information has been assigned to that host into DIB of the local domain name server. A user can then begins transactions that involve the Internet.

Resource records:
Different types of resource records are provided in DNS. An IP address contains a type A and PTR means pointer query. There are approximately 20 different types of resource records available. Some resource records are as follow.
1) A= It defines an IP address. It is stored as a 32-bit binary value.
2) CNAME ="Canonical name". It is represented as a domain name.
3) HINFO = Host information, two arbitrary character strings specifying the CPU and operating system.
4) MX = Mail Exchange records. It provides domain willing to accept email.
5) PTR=Pointer record used for pointer queries. The IP address is represented as a domain name in the in-addr.arpa domain.
6) NS= Name server Record. These specify the authoritative name server for a domain. They are represented as domain names.

Configuration of DNS
DNS server can be configured manually by updating files in the default NINNT installation path \% SYSTEM ROOT %\ SYSTEM 32\DNS. Administration is identical to administration in traditional DNS. These files can be customized using a text editor. The DNS service must then be stooped and restarted.

Components of DNS

DNS include the following components:
1) Domain
2) Domain name
3) Name server
4) Name resolver
5)Name cache
6) Zone

1) Domain: .com, .net are domain
2) Domain name:
Domain name is defined by the DNS as being the sequence of names and domain.
3) Name server:
Name server is the software that maps names to addresses. it does this by mapping domain names to IP addresses.
4) Name resolver:
It is the software that functions as client interacting with a name server.
5) Name cache:
It is the storage used by name resolver to store information frequently used.
6) Zone:
It is a continuous part of domain.

The way DNS is used as follows. An application program calls a library routine called the resolver by passing it the name as a parameter to map a name onto an IP address. The resolver delivers 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 caller. Equipped with the IP address, the program can then establish a TCP connection with the destination or send it UDP packets.