Ans :
The concept of mapping a domain name to an IP address and vice-versa is known as
resolution process. The resolution process is basically a client server platform.
Whenever a user needs to map an address to a domain or vice-versa, the DNS calls a
client program called resolver. The resolver subsequently contacts the nearest DNS
server (name server) with a request. In case the server has the desired information, it
replies back with the results. Otherwise it suggests the resolver to other domain
servers or asks other servers to provide the desired information. The resolver, after
receiving the results asserts the information and thereafter delivers the desired
information to the specific host process. The steps followed in the resolution are
below
1) The user program issues a request such as the gethostbyname( ) system call. (This
particular call is used to ask for the IP address of a host by passing the host
name as a parameter).
2) The resolver formulates a query to the name server.
3) The name server checks to see if the answer is in its local authoritative database
or cache, and if so, returns it to the client. Otherwise, it will query other available
name server(s), starting down from the root of the DNS tree or as high up the tree
as possible.
4) The user program will finally be given a corresponding IP address (or host name,
depending on the query) or an error if the query could not be answered.
As the resolution process is carried out with the help of queries, these domain name
request queries can be one of two types: recursive or iterative. A flag bit in the
domain name query specifies whether the client desires a recursive query, and a flag
bit in the response specifies whether the server supports recursive queries. A
recursive query requests that the server should itself issue a query to determine the
requested information and return the complete answer to the client. However, an
iterative query means that the name server should return what information it has
available and also a list of additional servers for the client to contact to complete the
query.
The concept of caching is also utilized in DNS. Whenever a name-server receives a
request from a client, the name server subsequently sends the query to other servers.
Later on, when it receives the response, it first stores this information in its own cache
memory before sending the desired information to the client. Now onwards, if any
client desires the same information, first the name-server can simply check its cache
memory and resolve the query. However, such a response is designated as nonauthoritative. The domain name responses from the name server can be one of two
types: authoritative and non-authoritative.
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