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Chapter 16 – DNS - PowerPoint PPT Presentation

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Chapter 16 – DNS. DNS. Domain Name Service This service allows client machines to resolve computer names (domain names) to IP addresses DNS works at the Application Layer of the OSI model

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  • Domain Name Service
  • This service allows client machines to resolve computer names (domain names) to IP addresses
  • DNS works at the Application Layer of the OSI model
  • DNS allows users to utilize user-friendly names when referencing other host computers instead of referencing their IP address
  • This system is used extensively on the Internet
  • Consists of a static database – has to manually be updated
why use domain name instead of ip address
Why Use Domain Name Instead of IP Address?
  • Easier for us to remember
  • Name is more reliable
  • IP address could very easily change, but the name will likely stay the same
how dns works
How DNS Works
  • Must translate computer names to IP addresses
  • A hierarchical client/server distributed database system is used
  • The clients are called “resolvers” and the servers are called “name servers”
  • Resolvers send UDP (economy version of TCP) queries to name server to resolve a host name
  • The name server then returns the corresponding IP address
  • If name server is not able to resolve the request, it forwards the request to another name server
dns structure
DNS Structure
  • Composed of a distributed database of names that establish a logical tree structure called the “domain name space”
  • Each node , or domain, in that space is named and can contain subdomains
  • A domain includes all nodes below the specified node
  • Domains and subdomains are grouped into zones to allow for distributed administration of the name space
  • A domain name identifies the computers position within the logical DNS hierarchy
top level domains
Top Level Domains
  • The root of the DNS database is managed by the Internet Network Information Center (
  • Names of top-level domains
    • Com commercial sites
    • Edu educational sites
    • Int international organizations
    • Net network organiozations
    • Org noncommercial organizations
dns zones
DNS Zones
  • A zone is a portion of the DNS namespace that is managed in a particular zone file
  • Each zone is anchored at a particular domain- referred to as the zone’s root domain
  • For example, is a domain and this domain main be controlled by more than one zone file
four types of name servers
Four Types of Name Servers
  • Primary
  • Secondary
  • Master
  • Caching-Only
primary and secondary name servers
Primary and Secondary Name Servers
  • DNS servers store information about the domain namespace
  • These servers generally have one or more zones for which they are responsible
  • The name server is said to have “authority” for those zones
  • Primary name server - gets the data from its zones. All changes in the zone are registered with the primary name server
  • Secondary name server – gets its data from another name server
reasons to have secondary name servers
Reasons to have Secondary Name Servers
  • Redundancy – Use at least two DNS name servers per zone – a primary and a secondary for fault tolerance
  • Remote Locations – Use a secondary name server in remote location that has a large number of clients. This prevents a large amount of communication over a slow link for name resolution
  • Reduce Load on Primary
  • The DNS name server uses its own database first to try to resolve the host name
  • If this fails, it must then must communicate with other DNS name servers to resolve the request
  • This typically requires interaction with DNS name servers located outside of the company on the public Internet
  • It’s a good idea to selectively enable specific DNS name servers within your company for this ytpe of wide-area communication
  • A DNS server designated for this operation is denoted as a “forwarder”
forwarder continued
Forwarder – Continued
  • When a DNS server can’t resolve a name request, it passes the request along to one of the designated forwarder
  • The forwarder carries out whateven communication is needed to resolve the request
  • The result is returned to the original DNS server, which, in turn, sends back the results to the original requester
master name server
Master Name Server
  • When you define a secondary name server for a specific zone, you must also designate another name server from which to obtain the zone information
  • The DNS server that provides this information is referred to as the Master Name Server
  • The master name server can be either a primary or a secondary name server for this zone
  • When a secondary name server starts up, it contacts its master name server and initiates a zone transfer
caching only servers
Caching-Only Servers
  • These are special DNS servers that only perform queries, cache the answers, and return the results
  • When they start up initially, they have to build their cach store over time as service is requested
  • This would be useful for a remote site with a slow link as much less traffic would be sent across the link because these servers don’t perform zone transfers
recursive query
Recursive Query
  • The DNS name server is asked to respond with a requested data or with an error stating that the domain name specified does not exist
  • This is the typical type of query used between a DNS client (resolver) and a DNS server
  • The name server cannot just refer the querier to a different name server
iterative query
Iterative Query
  • The queried name server gives back the best answer it currently has to the query
  • This type of query is typically sent by a DNS server to other DNS servers
  • See handout on example of recursive and iterative queries
inverse query
Inverse Query
  • The client provides the IP address and wants to determine the domain name
  • A special domain called in the DNS namespace was created to help resolve the IP address to the domain name
caching and time to live
Caching and Time to Live
  • When a name server is processing a recursive query, it may be requied to send out several queries to find the domain name
  • The name server caches all the received information during this process for a specified period of time know as the Time to Live (TTL)
  • The TTL is set by the administrator of the zone where the data comes from
  • Volatile network – use small TTL
  • However, small TTLs increase load on your name server
  • TTL value is decreased and entry is removed from cache when counter gets to zero
  • Client resolvers also have data caches and honor the TTL value