DOMAIN NAME SYSTEM

Domain Name System

• Hostname Resolution

• DNS

• Name Lookup with DNS

• Domain Name Servers

• DNS Database

• Reverse Lookups

Hostname Resolution

• TCP/IP uses 32 bit address (dotted decimal)

• Hard for us - we use “ordinary” (canonical) names

• Host Name Resolution - application must find IP address corresponding to name

• Lookup routines provided by resolver library (gethostbyname & gethostbyaddr)

• Solutions:– Local file (/etc/hosts)

• maintenance problem

– Network Information System (NIS)• stores host file info in master database

• OK for small nets (maint & distribution)

– Network Information Center (NIC) HOSTS.TXT• pre-1984

• central site - maint & distribution problems

Domain Name System• Hierarchy of domains

– edu (Mostly US) educational institutions like universities, etc.

– com Commercial organizations, companies.

– org Non-commercial organizations.

– net Gateways and other administrative host on a network

– mil US military institutions

– gov US government institutions.

– Outside USA:• fr - france

• de - Germany

• au - Australia

Name Lookup with DNS

• Giant distributed database

• Name Servers provide info on a domain or set of domains

• Two (or more) name servers per zone (see whois)

Sample Lookup

• Your host needs address of slotnick.psycho.havid.edu• host queries names server at root domain• root name server (rns) - not in my domain - in .edu domain• rns contacts edu zone name server (zns) - list provided to rns• rns queries one of the edu zns • edu zns knows havid.edu runs a root of their own & points rns

to it • rns presents query for slotnik to havid.edu rns & IP address is

returned• info (Ipfor slotnick, havid.edu & .edu) cached for a TTL period

(just in case)

Domain Name Servers

• Authoritative (Master name servers)

• Primary/secondary

DNS Database