dns explained

8/3/2019 DNS Explained

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DNS Domain NameSystem

By : Jihadah Binti AhmadST 214219


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Domain names and IP addressesThe domain name system (DNS) is an Internet-wide distributeddatabase that translates between domain names and IP addressesPeople prefer to use easy-to-remember names instead of IPaddresses when using the InternetThe Internet Corporation for Assigned Names and Numbers ( ICANN)manages the domain name system and allocation of IP addresses .

Domain names are alphanumeric names for IP addresses e.g128.143.22.36 --> www.virginia.edu(IP Address) (domain name)

H istory- Before DNS1970s ARPANET, the name-to-IP address was done by downloading asingle file (hosts.txt) from a central server with FTP.

Problemstraffic and loadName collisionsConsistency

DNS is created in 1983 by Paul Mockapetris


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DNS Components

Comprised of three components A name spaceName server : Servers making that name space availableResolvers (clients) which query the servers about the namespace


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DNS Components- Name SpaceTh e Name Space

The name space is the structure of the DNS database An inverted tree with the root node at the top

Each node has a labelThe root node has a null label, written as

The naming system for DNS is a hierarchical and logical tree structure

called the domain namespaceB elow top-level domain, administration of name space is delegated toorganizationsEach organization can delegate further

third-level node

second-level node second-level node

top-level node

third-level node third-level node

second-level node

top-level node

second-level node second-level node

top-level node

The root node""


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foo foo

top-1

foo at&t

top-2

bar baz

top-3

""

DNS Components- Name SpaceNaming syntax

Each node in the tree musthave a label

A string of up to 63 8 bit bytesLegal characters for

hostnames and domainnames ; A-Z, 0-9, and - only with a-zand A-Z treated as the same(NOT Case sensitive)

Sibling nodes must haveunique labelsThe null label is reserved for the root node


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DNS Components- Name Space

D omain name system-exampleH osts and DNS domains are named basedon their position in the domain treeEvery node in the DNS domain tree can beidentified by a unique Fully Qualified DomainName (FQDN). The FQDN gives the positionin the DNS tree.

A FQDN consists of labels(cs,virginia,edu) separated by a period(.).There can be a period (.) at the end.

cs.virginia.edu

Each node in the DNS tree represents a DNSnameEach branch below a node is a DNS domain.

DNS domain can contain hosts or other domains ( subdomains )

Example:DNS domains are., edu, virginia.edu, cs.virginia.edu


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DNS Components- Name Space

T op-level domains

Three types of top-level domains:Generic Top Level Domains (gTLD): 3-character codeindicates the function of the organization

Examples: gov, mil, edu, org, com, netCountry Code Top Level Domain (ccTLD): 2-character country or region code

Examples: us, va, jp, de,myReverse domains: A special domain (in-addr.arpa)used for IP address-to-name mapping

There are more than 200 top-level domains.


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DNS Components- Name Server Name Servers

Name servers store informationabout the name space in units calledzones

The name servers that load a

complete zone are said to haveauthority for or be authoritative forthe zone

DNS server answers queries abouthosts in its zoneUsually, more than one name server are authoritative for the same zone

This ensures redundancy andspreads the load

Also, a single name server may beauthoritative for many zones


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DNS Components- Name Server D NS domain and zones

Each zone is anchored at aspecific domain node, butzones are not domains.

A D NS domain is a branch of the namespace

A zone is a portion of the DNSnamespace generally stored ina file (It could consists of multiple nodes)

A server can divide part of itszone and delegate it to other servers


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DNS Components- Name Server

Primary and secondary name servers

For each zone, there must be a primary name server and asecondary name server

The primary server (master server) maintains a zone file whichhas information about the zone. Updates are made to theprimary server The secondary server copies data stored at the primary server.

Adding a host:W hen a new host is added (gold.cs.virginia.edu) to a zone, theadministrator adds the IP information on the host (IP address andname) to a configuration file on the primary server


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DNS Components- Name Server R oot name servers

The root name servers know how to find the authoritative nameservers for all top-level zones.There are only 13 root name servers

Root servers are critical for the proper functioning of name resolution

Nameserver Operated by:

A Verisign (US East Coast)

B University of S. California Information Sciences Institute (US W est Coast)

C PSI (US East Coast)

D University of Maryland (US East Coast)

E NASA (Ames) (US W est Coast)

F Internet Software Consortium (US W est Coast)

G U. S. Dept. of Defense (ARL) (US East Coast)

H U. S. Dept. of Defense (DISA) (US East Coast)

I KTH (SE)

J Verisign (US East Coast)

K RIPE-NCC (UK)

L ICANN (US W est Coast)

M W IDE (JP)


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DNS Components- Resolver R esolver and name server 1. An application program on a

host accesses the domainsystem through a DNS client,called the resolver

2. Resolver contacts DNS server,called name server

3. DNS server returns IP addressto resolver which passes theIP address to application

4. Reverse lookups are alsopossible, i.e., find thehostname given an IP address

5. If DNS server do not has theanswer, it will it will queryother name servers, starting atthe root tree

CLIENT


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DNS Components- Resolver

R ecursive and Iterative Queries

There are two types of queries:Recursive queriesIterative (non-recursive) queries

The type of query is determined by a bit in the DNS query

Recursive query: W hen the name server of a host cannot resolve aquery, the server issues a query to resolve the query

Iterative queries: W hen the name server of a host cannot resolve aquery, it sends a referral to another server to the resolver


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Recursive queriesIn a recursive query, theresolver expects the responsefrom the name server

If the server cannot supply theanswer, it will send the queryto the closest knownauthoritative name server (here: In the worst case, theclosest known server is theroot server)

The root sever sends a referralto the edu server. Queryingthis server yields a referral tothe server of virginia.edu and so on


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Iterative queries

In an iterative query, the nameserver sends a closest knownauthoritative name server the areferral to the root server.

This involves more work for theresolver


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Caching

To reduce DNS traffic, name servers caches informationon domain name/IP address mappings

W hen an entry for a query is in the cache, the server does not contact other serversNote: If an entry is sent from a cache, the reply from theserver is marked as unauthoritative

dns explained

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