The Domain Name System

1
The Domain Name System
2
Hostnames

• IP Addresses are great for computers
• IP address includes information used for routing.
• IP addresses are tough for humans to remember.
• IP addresses are impossible to guess.
• ever guessed at the name of a WWW site?

3
The Domain Name System

• The domain name system is usually used to
  translate a host name into an IP address .
• Domain names comprise a hierarchy so that names
  are unique, yet easy to remember.

4
DNS Hierarchy
edu
com
org
jp
rpi
albany
5
Host name structure

• Each host name is made up of a sequence of labels
  separated by periods.
• Each label can be up to 63 characters
• The total name can be at most 255 characters.
• Examples
• whitehouse.gov
• barney.the.purple.dinosaur.com
• monica.cs.rpi.edu

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Domain Name

• The domain name for a host is the sequence of
  labels that lead from the host (leaf node in the
  naming tree) to the top of the worldwide naming
  tree.
• A domain is a subtree of the worldwide naming
  tree.

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Top level domains

• edu, gov, com, net, org, mil,
• Countries each have a top level domain (2 letter
  domain name).
• New top level domains include
• .aero .biz .coop .info .name .pro

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DNS Organization

• Distributed Database
• The organization that owns a domain name is
  responsible for running a DNS server that can
  provide the mapping between hostnames within the
  domain to IP addresses.
• So - some machine run by RPI is responsible for
  everything within the rpi.edu domain.

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DNS Distributed Database

• There is one primary server for a domain, and
  typically a number of secondary servers
  containing replicated databases.

rpi.edu DNS server
rpi.edu DNS DB
rpi.edu DNS DB
rpi.edu DNS DB
rpi.edu DNS DB
Authoritative
Replicas
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DNS Clients

• A DNS client is called a resolver.
• A call to gethostbyname()is handled by a resolver
  (typically part of the client).
• Most Unix workstations have the file
  /etc/resolv.conf that contains the local domain
  and the addresses of DNS servers for that domain.

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/etc/resolv.conf

• domain rpi.edu
• 128.113.1.5
• 128.113.1.3

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nslookup

• nslookup is an interactive resolver that allows
  the user to communicate directly with a DNS
  server.
• nslookup is usually available on Unix
  workstations.

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DNS Servers

• Servers handle requests for their domain
  directly.
• Servers handle requests for other domains by
  contacting remote DNS server(s).
• Servers cache external mappings.

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Server - Server Communication

• If a server is asked to provide the mapping for a
  host outside its domain (and the mapping is not
  in the server cache)
• The server finds a nameserver for the target
  domain.
• The server asks the nameserver to provide the
  host name to IP translation.
• To find the right nameserver, use DNS!

15
DNS Data

• DNS databases contain more than just
  hostname-to-address records
• Name server records NS
• Hostname aliases CNAME
• Mail Exchangers MX
• Host Information HINFO

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The Root DNS Server

• The root server needs to know the address of 1st
  (and many 2nd) level domain nameservers.

rpi
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Server Operation

• If a server has no clue about where to find the
  address for a hostname, ask the root server.
• The root server will tell you what nameserver to
  contact.
• A request may get forwarded a few times.

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DNS Message Format

• HEADER
• QUERIES
• Response RESOURCE RECORDS
• Response AUTHORITY RECORDS
• Response ADDITIONAL INFORMATION

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Question Format

• Name domain name (or IP address)
• Query type (A, NS, MX, )

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Response Resource Record

• Domain Name
• Response type
• Class (IP)
• Time to live (in seconds)
• Length of resource data
• Resource data

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UDP TCP

• Both UDP and TCP are used
• TCP for transfers of entire database to secondary
  servers (replication).
• UDP for lookups
• If more than 512 bytes in response - requestor
  resubmits request using TCP.

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Name to Address Conversion

• There is a library of functions that act as DNS
  client (resolver).

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DNS Data Flow
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Example suranga.com
This is named.conf in /etc/ folder According to
the following configuration I have created a zone
called suranga.com and in that zone im3 is a
host. Therefore the full name of that host would
be im3.suranga.com. like that you can have any
amount of PCs in your domain. This will
definitely works if you type following according
to your network.
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Example Network
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This is named.conf in /etc/ folder

• options
• directory "/var/named"
• // query-source address port 53
• zone "." IN
• type hint
• file "named.ca"
• zone "localhost" IN
• type master
• file "localhost.zone"
• allow-update none
• zone "0.0.127.in-addr.arpa" IN
• type master
• file "named.local"
• allow-update none
• zone "suranga.com" IN

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BIND Files

• Following files should be created in /var/named/
  folder we do not need to customize the default
  file named.ca in the /var/named/ folder.
• 1. 0.0.127.in-addr.arpa.zone
• 2. 200.168.192.in-addr.arpa.zone
• 3. suranga.com.zone
• 4. localhost.zone
• 5. named.local

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The contents of 0.0.127.in-addr.arpa.zone as
follows

• TTL 86400
• _at_ IN SOA localhost.
  root.localhost (
• 1 serial
• 28800 refresh
• 7200 retry
• 604800 expire
• 86400 ttk
• )
• _at_ IN NS localhost.
• 1 IN PTR localhost.

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200.168.192.in-addr.arpa.zone file

• TTL 86400
• _at_ IN SOA 192.168.200.8.
  root.localhost (
• 1 serial
• 28800 refresh
• 7200 retry
• 604800 expire
• 86400 ttk
• )
• _at_ IN NS localhost.
• 1 IN PTR localhost.
• 2 IN PTR im3.

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suranga.com.zone file

• TTL 86400
• _at_ IN SOA _at_ root.suranga.com (
• 1 serial
• 28800 refresh
• 7200 retry
• 604800 expire
• 86400 ttl
• )
• IN NS suranga.com.
• _at_ IN A 192.168.200.8
• im3 IN A 192.168.200.250

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localhost.zone file

• TTL 86400
• _at_ IN SOA _at_ root.localhost (
• 1 serial
• 28800 refresh
• 7200 retry
• 604800 expire
• 86400 ttl
• )
• IN NS localhost.
• _at_ IN A 127.0.0.1

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named.localhost file

• TTL 86400
• _at_ IN SOA localhost.
  root.localhost. (
• 1997022700
  Serial
• 28800
  Refresh
• 14400
  Retry
• 3600000
  Expire
• 86400 )
  Minimum
• IN NS localhost.
• 1 IN PTR localhost.

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Results

• Now go to the prompt and type service named start
  and type nslookup commands to verify your domain
  as follows.
• nslookup gtsuranga.com
• Server 192.168.200.8Address 192.168.200.853N
  ame suranga.comAddress 192.168.200.8
• gtim3.suranga.com
• Server 192.168.200.8Address 192.168.200.853N
  ame im3.suranga.comAddress 192.168.200.250

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Configuration Files

• named configuration file (/etc/named.conf)
• It basically defines the parameters that point to
  the sources of domain database information, which
  can be local files or on remote servers.
• Hint file (cache file)(/var/named/named.ca)
• It actually provides the name of root server
  which gets activated in case the machine name,
  which is to be searched, is not there in user
  defined zone.
• localhost file (/var/named.local)
• All configuration have a local domain Database
  for resolving address to the host name localhost.
  
• Zone
• Basically a zone that keeps the information about
  the domain database.

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More on zone files

• _at_ It means from the origin to the lastname
  object that is suranga.com.
• IN This stands for Internet servers
• SOA This stands for Start Of Authority. It
  marks the beginning of a zones data and defines
  the parameter that affects the entire zone.
  Followed by the current machine name where the
  DNS server is maintained.
• 20000011301serial This is the serial number--a
  numeric value that tells or notifies the slave
  server, that the database has been updated. So
  slave server should also update it.
• 3600refresh This is the refresh cycle in
  seconds. In every refresh cycle the slave server
  comes to master server and checks for the updated
  database.

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More on zone files

• 1800retry This particular line refers to the
  retry cycle which in turn means that the slave
  server should wait before asking the master
  server again in case master server doesnt
  respond.
• 1209600expire This is the time for slave server
  to respond to queries of client for the
  expiration time if master server fails and has to
  be up and not getting up. After this period slave
  server also fails to solve the queries of clients
  and sits idle.
• 432100default_ttl This refers to the default
  time to leave, for this domain to work for, when
  named is once started. Remember the user doesnt
  have to play with this unless he wants that the
  query time from the slave server should be
  somewhat less or more. In case we want to change,
  we should change only the refresh time in both
  master and slave. The best way is to make it 2,
  which means after each 2 seconds slave server
  will query to master server.