Domain Name System (DNS)

1
Domain Name System (DNS)

• Network Information Center (NIC) HOSTS.TXT

2
DNS -- Hierarchical Namespace
(Root)
edu
mil
gov
com
Pentagon
emich
umich
Whitehouse
ibm
yahoo
cislab
3
Fully Qualified Domain Names Domains
Domain Name Meaning
com Commerical
edu Educational
gov Government
int International
mil U.S Military
net Major Network
org Non-profit
Country code ISO 2-letter

• Ends with a DOT
• cispdc.cislab.emich.edu.
• www.yahoo.com
• mail.emich.edu
• Generic Domain
• See right side gtgt

4

• 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
• 129.34.139.30 139.34.129.in-addr.arpa

5
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

6
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

7
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

8
Domain Name Server Operation

• Primary NS
• Loads a zones 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

9
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

10
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

11
BIND DNS Server named.conf file

• zone "0.0.127.in-addr.arpa" loopback zone
• type master
• file "pz/127.0.0"
• zone "linux.net" forward Zone
• type master
• file "pz/linux.net"
• zone "196.168.192.in-addr.arpa" Reverse Zone
• type master
• file "pz/192.168.196"

12
pz/127.0.0 File

• TTL 3D
• _at_ IN SOA ns.linux.net. (
• 1 Serial
• 8H Refresh
• 2H Retry
• 4W Expire
• 1D) Minimum TTL
• NS ns.linux.net.
• 1 PTR localhost.

13
pz/linux.net file

• TTL 3D
• _at_ IN SOA ns.linux.net. (
• 199802151 8H 2H 4W 1D )
• NS ns name server
• MX 10 mail Primary Mail
• localhost A 127.0.0.1
• ns A 192.168.196.2
• mail A 192.168.196.4
• www A 192.168.196.5
• web CNAME www

14
pz/192.168.196 file

• TTL 3D
• _at_ IN SOA ns.linux.net.(
• 199802151 8H 2H 4W 1D)
• NS ns.linux.net.
• 2 PTR ns.linux.net.
• 4 PTR mail.linux.net.
• 5 PTR www.linux.net.