DNS

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DNS
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Introduction

• What is DNS?
• Hierarchy or Tree
• Dot used as a separator

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Top-level domain

• Domain Name Meaning
• COM Commercial organizations
• EDU Educational institutions
• GOV Government institutions
• MIL Military groups
• NET Major network support
• ORG Organizations other than those above
• ARPA Temporary ARPANET domain
• INT International organizations
• country codeEach country (geographical scheme)

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zone

• Domain Delegation
• Primary (Master) Name Server
• Contains the writable authoritative copy for the
  zones
• Secondary (Slave) Name Server
• Contains mirror copy of the data from a primary
  nameserver

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Introduction to DNS

• DNS Clients
• /etc/resolv.conf
• Authoritative DNS Servers
• provide the information for your DNS domain
• Caching DNS Server
• Most servers dont ask authoritative servers for
  DNS directly
• ask a caching DNS server

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How Does It Work?

• Listens at UDP and TCP port 53
• UDP for routine queries
• TCP used for zone transfers

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Basic DNS Testing of DNS Resolution

• The Host Command
• host www.google.com
• The nslookup Command
• nslookup www.google.com
• nslookup

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The /etc/resolv.conf File

• Nameserver
• IP address of your DNS nameserver
• Search
• Domain
• localdomain use by default
• search my-site.com my-site.net my-site.org
• nameserver 192.168.1.100
• nameserver 192.168.1.102

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BIND

• Downloading and Installing the BIND

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How To Get BIND Started

• service named start
• service named stop
• service named restart

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DNS File Locations

• named.conf
• /etc/named.conf
• zone files
• /var/named

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Named.conf

• This file usually has two zone areas
• Forward zone file
• Reverse zone file

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Forward zone file

• zone "my-site.com"
• type master
• file "my-site.zone"

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Reverse zone file

• zone "1.168.192.in-addr.arpa"
• type master
• notify no
• file "192-168-1.zone"

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Configuring The Zone Files

• By default located in the directory /var/named
• Each zone file contains a variety of records
  (SOA, NS, MX, A, and CNAME)

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Time to Live Value

• D signifies days
• W signifies weeks
• H signifies hours
• TTL 3D

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Standard Resource Records

• SOA
• Start of Authority, Marks the begining of a
  zone's data
• NS
• IP address or CNAME of the name server
• A
• IP address of server

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• MX
• Mail server DNS name
• CNAME
• Server name alias
• PTR
• Last octet of server's IP address

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The SOA Record

• Format
• Name Class Type Name-Server Email-Address
  Serial-No Refresh Retry Expiry Minimum-TTL

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• _at_ IN SOA ns1.my-site.com. hostmaster.my-site.com
  . ( 2004100801   serial
• 4H   refresh
• 1H   retry
• 1W   expiry
• 1D )   minimum

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the general format

• Name Class Type Data

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• eeunix IN A 130.111.148.45
• IN MX 0 eeunix
• IN MX 1 maine.maine.edu.

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Trouble Shoot

• Domain cmu.ac.th
• ns.cmu.ac.th
• Domain Delegation
• eng.cmu.ac.th
• ???/????? work ?
• Transfer zone ?