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• 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?

• 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

• 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

• 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

• The root of the DNS database is managed by the Internet Network Information Center (www.internic.com)

• Names of top-level domains– Com commercial sites

– Edu educational sites

– Int international organizations

– Net network organiozations

– Org noncommercial organizations

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, microsoft.com is a domain and this domain main be controlled by more than one zone file

Four Types of Name Servers

• Primary

• Secondary

• Master

• Caching-Only

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

• 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

Forwarders

• 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

• 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

• 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

• 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

Three Types of Queries to a DNS Server

• Recursive

• Iterative

• Inverse

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

• 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

• The client provides the IP address and wants to determine the domain name

• A special domain called in-addr.arpa in the DNS namespace was created to help resolve the IP address to the domain name

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