Saturday, 30 April 2011

Distributed Information Systems (DIS) Exam questions and Answers. Domain Name Service (DNS)

Clearly describe the purpose and architecture of the Domain Name Service (DNS). Your answer should:

  • Discuss the structure of the name space, and how this is reflected in the organisation of the domain name database.
  • Describe the functionalities of the various types of DNS server, and
  • Illustrate at least one example of an iterative query and at least one example of a recursive query.

The main purpose of the DNS is to resolve human-friendly URLs into IP addresses
(numerical, logical addresses).

Architecture of DNS
- Domain Name Service (DNS) is a hierarchical system of name servers, each authoritive for
one or more domains within the Domain Name Space.
Organisation of name servers, root servers, then those lower down the hierarchy to leaf (zonedomain)
level. Delegation of responsibility resulting in a scalable and easy-to-modify
distributed database

The DNS hierarchical structure above

The following four types of server are used in the DNS:

Primary master retrieves data from the host that it runs on and its data is held in a stored
database. The Secondary master gets its data from another master that authoritative for
the zone (i.e. a Primary Master). It contacts an authoritative name server and
pulls the zone data over greatly reduce administrative load. The Caching name server does
not have a database of mappings between IP addresses and names at start-up. It knows of
Primary and Secondary servers which can supply such information if required. The use of
caching servers are used to reduce the load on Primary and Secondary servers. The slave
name server operates in a similar way to a Caching name server however it is less
sophisticated than the other types of server and cannot not follow redirections.
Query types could be either recursive or iterative. With iterative queries the NS gives the
best answer it already knows. This might result in the name server referring the requester to a
‘closer’ name server that it knows. – There is no additional querying of other name servers.
In contrast with recursive query resolution is managed by a single NS. The NS must return
the final answer. This may involve querying other name servers and following ‘referrals’
received thus resulting in further queries being sent to other name servers.

Recursive query above

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