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DNS: How Domain Name System Works to Resolve Hostnames to IP Addresses
1.
2. PROBLEM FACED…???
Identify human - names & id
Similarly host identified – host name
Host name like www.yahoo.co.in , www.karunya.edu
Fully alphanumeric characters – little information – location of host
Difficult to process by router
Host are identified by IP address
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3. SERVICES PROVIDED BY DNS
Prefer only alphanumeric characters only
Routers Support –IP Address
To solve Translate hostname to IP address
done by DNS
distributed database implemented – hierarchy of DNS Servers
Application layer – host to query the database
It runs UDP Port & Port 53
Commonly used with Http , ftp, Smtp
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4. HOW ACTUALLY WORKS…?
https://www.karunya.edu/results through Brower
First send Web server ,before it have to fetch IP Address of Host
name
Client Side DNS application
For that browser extract host name from URL & Send along with
DNS Application
DNS Application send to hostname to DNS Server & Receive
Reply of IP Address
Browser receives IP Address from DNS ,initiate Connection with
TCP to the HTTP Server -- Send Web server –fetch Web pages
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5. HOST ALIASING
Suppose rest1.relay.west-coast.enter-prise.com –canonical
hostname
Aliases are enterpriase.com , www.enterprise.com
Aliases are easier than original host name
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6. LOAD DISTRIBUTION
Ndtv.com – Breaking News
Replicated of hostname at Different Servers & Different IP Address
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8. To scale - large of no servers in hierarchical & distributed among
world
Single Server Can’t do all mapping technique
Three Classes of DNS Servers
(1) Root DNS Servers
(2) Top level Domain DNS Servers
(3) Authoritative DNS Server
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9. ROOT DNS SERVERS
13 root DNS Servers
Mostly –North America
Security & Reliability purpose act as replicated servers
TOP LEVEL DOMAIN SERVER
.com, .org, .au, .edu, .gov
Country domains - .in, .uk , .us,.pak
Company Network Solution maintain – com top level domain
Company Educause maintains – edu top level domain
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10. AUTHORITATIVE DNS SERVER
Organization host their host name with IP Address – Details are
stored
Each organization can choose their own authoritative server
For storing details – cost based
LOCAL DNS SERVER --- NAME SERVER
Identify near local DNS Server via ISP
ISP host with IP address of local DNS Server
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12. DNS CACHING
Request arrives with Host name for IP address ,Both – Stored in
local memory –DNS Server
Same Request arrives – provides Desired IP Address
Cache in formation –discarded – period of time
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13. RESOURCE RECORD
(Name,Value,Type,TTL)
TTL-time to live of resource record
Type-A , Name-host name, Value – IP Address
Type-NS, Name-Domain,Value –Host name of authoritative
DNS Server,know how to obtain IP Address
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15. 12 bytes in header part followed by four variable length fields
Identification is set by client and returned by server. It match the
request and response
QR is a 1-bit field: 0 means the message is a query, 1 means it's a
response.
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16. opcode is a 4-bit field. The normal value is 0 (a standard query). Other
values are 1 (an inverse query) and 2(server status request).
AA is a 1-bit flag that means "authoritative answer.“
TC is a 1-bit field that means "truncated.“With UDP this means the
total size of the reply exceeded 512 bytes, and only the first 512 bytes
of the reply was returned.
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17. RD is a 1-bit field that means "recursion desired." This bit can be set
in a query and is then returned in the response. This flag tells the
name server to handle the query itself, called a recursive query. If
the bit is not set, and the requested name server doesn't have an
authoritative answer
RA is a 1-bit field that means "recursion available." This bit is set to
1 in the response if the server supports recursion.
There is a 3-bit field that must be 0.
rcode is a 4-bit field with the return code. The common values are 0
(no error) and 3 (name error).
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