DOMAIN NAME SYSTEM Domain Name System Hostname Resolution

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 queries one of the 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