Domain Name System(DNS)
Domain Name System (DNS)
--Domain-based Hierarchy
-Alternative: Flat name space
--Basically a naming scheme
--Maps IP addresses to
-Hosts and Domain Names
DNS Name Space
--Many top domains & sub domains
--Each may cover many hosts
--Hierarchy can be represented in a tree structure
--Top Level
-Generic and Countries
--Some countries repeat generic domains,some don’t
Domain Name Space
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Naming Issues
--Each component can be up to 63 chars ong
--Total Length of path < 255 chars
--Naming usually follows organizational boundaries
-Irrespective of physical location
DNS Hierarchy
--Distributed Database
--Database in DNS Servers
--Each Server
-Is authoritative for a domain
-i.e., responsible for a domain
-This domain is then called a “zone”
-If domain divided into subdomains then “zone” and
domain” are different
Domains and Zones
Inverse Domain
--Special Domain
--Normal domains map name to address
--Inverse Domain
-Maps and address to a name
--Uses a Pointer Record (PTR) to handle query
Implementation Issues
--Root Server
-More than 13 root servers around the world
-On installing DNS in NT root servers automatically installed
--Primary DNS server
-Creates, maintains & update zone file
--The zone file for which is an authority
--Secondary DNS Server
-Mirrors the primary DNS server
-Basically is there for redundancy
--Both authoritative in the zone they serve
Implementation Issues
--Database Organization
--Each domain as Resource Records (RR)
--RR:
-Domain-Name
-Time-to-live
-Class (IN: Internet Information)
-Type (of record)
-Value (depends on type of record)
Resource Record
--Domain Name
-Variable-length field, contains Domain Name
--Domain Type
--Domain Classes
--Time-to-Live
-32 bit field
-Defines the number of seconds the answer is valid.
-Receiver can cache this answer for this period of time.
--Value of zero: single transaction, not to be cached
--Resource Data Length
-Size of the resource record
--Resource Data
-Variable length
-Contains answer to query or
-Domain Name of Authoritative server or
-Additional information
Resource Record: Record Data
--Value depends on type field
-A number. In octets
- IPv4 is 4 Octets
- IPv6 is 16 octet
-Domain Name
- Expressed as sequence of labels
- Preceded by a number (No of chars in label)
-Offset Pointer
- Instead of a Domain Name
-Character String
- One-byte length field followed by the number of chars
DNS Resolution
--Process called: name-address resolution
--Process
-Host calls a resolver
--Resolver = DNS Client
-Resolver access DNS Server
-If DNS server has answer then it responds
-If DNS doesn’t have reponse, passes query to NS server
immediately above
--Recursive Resolution
-Query passed to parent DNS until answer is found
--Iterative Resolution
-Queried server responds with address of DNS server that it
thinks may have the answer
Recursive Resolution
Iterative Resolution
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