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Some information about dns (domain name system)
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DNS (Domain Name System)
Presented By: Satya Prakash SharmaMCA: III rd sem.
Your Comput
er
Website to
access
bla bla bla bla bla
We are here
We want to reach
here
Main DNS part
Domain Name SystemDNS translates the host names to
an IP Address.Its like a directory lookup service
that provides a mapping between the name of a host on the internet and its numerical address.
Total Four elements comprise the DNS and they are:
Domain Name Space
DNS Database
Name Server
Resolvers
Domain Name Space
DNS Database
Name Server
Resolvers
Domain Name SpaceA (32bit) IP address helps in uniquely
recognizing a particular website.Remembering names rather than IP
address.◦google.com◦yahoo.com◦ in.com◦mit.edu◦bbc.co.uk
continued…Here is a portion of the domain
naming tree...root
ukauusorgnetgovedumilcom
acm
ieee
shore
treas
mitappl
eibm
lcsaiinforaleigh
itso
..
continued…When we move down the internet domain
tree, we will get a leaf node that identify specific hosts on the internet.
These hosts are assigned internet addresses.e.g., the main host at MIT, with a domain
name of mit.edu, has four IP address: 18.7.21.77, 18.7.21.69, 18.7.21.70, 18.7.21.110
The subordinate domain lcs.mit.edu had the IP address 18.26.0.36.
Domain Name Space
DNS Database
Name Server
Resolvers
DNS Database DNS is based on a hierarchical database containing
resource records (RRs) that include the name, IP address, and other information about hosts.
Format for DNS Resource Record is as below:
Domain Name
Type
Time to Live
Rdata
Rdata field length
Class
Bit 0 1632
continued… Domain Name: The domain name in a RR must correspond to
the human readable form. Type: Identifies the type of resource in this RR.
◦ A: a host address.
◦ CNAME: Canonical name. Specifies an alias name for a host and maps this to the canonical (true) name.
◦ MINFO: Mailbox or mail list information. Maps a mailbox or mail list name to a host name.
Class: Identifies the protocol family. Time to Live: This field specifies the time interval that the
resource record may be cached before the source of the information should again be consulted.
Rdata Field Length: Length of the Rdata field in octets. Rdata: The Rdata is a 32-bit IP address, and for the CNAME type,
the Rdata is a domain name.
continued…
UserResponse
User Program
Name Resolver
Name server
Cache
Database
Foreign name server
Cache
Database
User query
User System
Cache
Internet
Query
Que
ry
resp
onse
response
A user program requests an IP address for a domain name.
A resolver module in the local host or local ISP formulates a query for a local name server in the same domain as the resolver.
The local name server checks to see if the name is in its local database or cache.
When a response is received at the local name server, it stores the name/address to its local cache and will maintain this entry for the amount of time specified in time to live field.
The user program will get either the IP address or error message.
Domain Name Space
DNS Database
Name Server
Resolvers
Name ServerWhy it is not
centralized ?o Single point of failure
o Traffic volume
o Maintenance
o No server has all name-to-IP address mappings
Local name servers:• Each ISP, company has local name
server.• Host DNS query first goes to local
name server.
Authoritative name server:
Can perform name/address translation for that host’s name.
DNS: Root name server Contacted by local name server that cannot resolve
name Root name server:
◦ Contacts authoritative name server if name mapping is not known
◦ Gets mapping◦ Returns mapping to local name server
nslookupnslookup is an interactive
resolver that allows the user to communicate directly with a DNS server.
nslookup is available on both Unix and windows workstation.
Domain Name Space
DNS Database
Name Server
Resolvers
Resolver:The client called the resolver is
transparent to the user and is called by an application to resolve names into real IP addresses or vice-versa.
A full resolver a program distinct from the user program which forwards all queries to a name server for processing.
DNS Messages:0 8 16 21 24 28
31Identifier QR opcode AA TC RD RA reserved RCODE
o Identifier: Assigned by the program that generates any kind of query. The same identifier is used in any response, enabling the sender to match queries and responses.
o QR: Indicates whether this message is a query of response. If query = 0, response = 1.o Opcode: Indicates whether this is a standard query , an inverse query (address to name).o AA(authoritative answer): valid in a response, and indicates whether the responding
name server is an authority for the domain name.o TC(Truncated): indicates whether the response message was truncated due to length
greater then permitted on the transmission channel, then the response switches from UDP to TCP.
o RD(Recursion Desired): directs the server to pursue the query recursively.o RA(Recursion Available): set or cleared in a response to denote whether recursive
query support is available in the name server.o RCODE(Response Code): no error, format error(server unable to interpret query), server
failure, name error(domain name does not exist).
THANKS