Domain Name System (DNS) Network Information Center (NIC) : HOSTS.TXT

DNS -- Hierarchical Namespace (Root) edumilgovcom Pentagonemichumich White house ibmyahoo cislab

Fully Qualified Domain Names & Domains Ends with a “DOT” cispdc.cislab.emich.edu. mail.emich.edu Generic Domain See right side >> Domain NameMeaning comCommerical eduEducational govGovernment intInternational milU.S Military netMajor Network orgNon-profit Country codeISO 2-letter

Mapping Domain Names to IP Address Name Server: Holds name-to-address database or point to other who does Name Resolver: Answer requests from the client Mapping IP Address to Domain Names Pointer Query: A domain name query to find the host name associated with an IP addresss  Using reverse lookup  = in-addr.arpa

Distributed name space Zones of Authority (Zones) Group of Symbolic Domain Name In each of these zones, on or more hosts maintains a database of names and IP mapping table These local name servers are interconnected into a hierarchical tree of domain Each zone contains a part or sub-tree of the hierarchical tree of the domain Authority of a zone: Name server in the zone Delegate authority: Superior domain name server

Resolution Process User issues gethostbyname() / gethostbyaddr() request Resolver formulates a query to NS NS check local authoritative DB or cache first and return. Otherwise, check one of 13 root Name Servers or as high as possible Return to user program with corresponding IP address, host name, or error message

Resolver Operation Two types of queries Recursive: find the answer all the way through all the name server Non-Recursive: list name server references Two types of responses Authoritative: request for a domain in a zone over its authority Non-authoritative:

Domain Name Server Operation Primary NS: Loads a zone’s information from disk, and has authority over the zone Secondary NS: Has authority over the zone but obtains the zone information from primary NS using a process call ZONE TRANSFER Caching-only: No authority for any zone but obtain all info. and cache it in memory

DNS Distributed database record format DNS Resource Record (RR): it provides a mapping between domain names and host IP addresses General Format Name: Domain Name to be defined TTL: Time-To-Live in second Class: protocol family usually IN (internet) Type: Type of resource (see next page) Rdata: Depends on the type

DNS Distributed database record format Resource Record Type and Rdata Start of Authority (SOA) = full NS name Name Server (NS) = Host name Domain Name Pointer (PTR) = Domain name Canonical Name (CNAME) = Domain name Mail Exchange (MX) = value and host name Host Address (A) = 32 bit IP address

BIND DNS Server named.conf file zone " in-addr.arpa" { ;loopback zone type master; file "pz/ "; }; zone "linux.net" {;forward Zone type master; file "pz/linux.net"; }; zone " in-addr.arpa" {;Reverse Zone type master; file "pz/ "; };

pz/ File $TTL IN SOA ns.linux.net. ( 1 ; Serial 8H ; Refresh 2H ; Retry 4W ; Expire 1D) ; Minimum TTL NS ns.linux.net. 1 PTR localhost.

pz/linux.net file $TTL IN SOA ns.linux.net. ( ; 8H ; 2H; 4W; 1D ) NS ns; name server MX 10 mail; Primary Mail localhost A ns A mail A wwwA webCNAMEwww

pz/ file $TTL IN SOA ns.linux.net.( ; 8H ;2H ;4W ;1D) ; NS ns.linux.net. 2PTR ns.linux.net. 4 PTR mail.linux.net. 5 PTR