DNS

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DNS

• DNS

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DNS
DNS

• DNS is a network service that enables clients to
  resolve names to IP address and vice-versa.
• Allows machines to be logically grouped by domain
  names.
• Provides email routing information.

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Internet Naming Hierarchy
DNS
The silent dot at the end of all addresses
.org
.in
.com
.net
.sidbi
.co
.ac
.iitk
www
www
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DNS Operation
DNS Setup

• A DNS server maintains the name to IP address
  mapping of the domain for which it is the name
  server.
• The DNS server for a domain is registered with
  the domain registrar and the entry is maintained
  by the Internet Root-Servers (13) or Country
  Level Root-Servers.
• Whenever a server is queried, if doesnt have the
  answer, the root servers are contacted.
• The root servers refer to the DNS server for that
  domain (in case the domain is a top level domain)
  or the Country Root Server (in case the domain is
  country level domain).

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DNS Configuration
DNS Setup

• named daemon is used
• A DNS Server may be caching/master/slave server
• The named.ca file has information of all Root
  Servers.
• There is a Forward Zone file and a Reverse Zone
  file for every domain.
• Configuration file
• /var/named/chroot/etc/named.conf
• Forward Zone File
• /var/named/chroot/var/named/ltforward_zone_filegt
• Reverse Zone File
• /var/named/chroot/var/named/ltreverse_zone_filegt

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Sample Master named.conf
DNS Setup

• zone "."
• type hint
• file "named.ca"
• zone "0.0.127.in-addr.arpa"
• type master
• file "named.local"
• allow-query any
• zone "iitk.ac.in"
• type master
• file "hosts.db"
• allow-query any
• zone "95.200.203.IN-ADDR.ARPA"
• type master
• file "hosts.rev.203.200.95"
• allow-query any

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Sample Forward Zone File
DNS Setup

• TTL 86400
• _at_ IN SOA ns.iitk.ac.in.
  root.ns.iitk.ac.in. (
• 200605091 Serial
• 10800 Refresh - 3
  hours
• 3600 Retry - 1 hour
• 1209600 Expire - 1 week
• 43200 ) Minimum TTL for
  negative answers - 12 hours
• IN NS ns.iitk.ac.in.
• IN NS proxy.iitk.ac.in.
• IN MX 5 mail0.iitk.ac.in.
• IN MX 10 mail1.iitk.ac.in.
• IN MX 20 mail2.iitk.ac.in.
• ORIGIN iitk.ac.in.
• ns IN A 203.200.95.142
• mail IN A 203.200.95.144
• proxy IN CNAME mail

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Sample Reverse Zone File
DNS Setup
TTL 86400 ORIGIN 200.203.in-addr.arpa. 95
IN SOA ns1.iitk.ac.in.
root.ns1.iitk.ac.in. (
200605091 Serial 10800
Refresh - 5 minutes
3600 Retry - 1 minute
1209600 Expire - 1 weeks
43200 ) Minimum TTL for negative
answers - 12 hours IN NS
ns.iitk.ac.in. IN NS
proxy.iitk.ac.in. ORIGIN 95.200.203.in-addr.arpa
. 142 IN PTR ns.iitk.ac.in. 144
IN PTR mail.iitk.ac.in.
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Configuring Local Resolver
DNS Setup

• /etc/resolv.conf
• server 127.0.0.1

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Test DNS
DNS Setup

• nslookup
• host
• dig
• Test your DNS with the following DNS diagnostics
  web site dnsstuff.com

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Advanced BIND Features
DNS Setup

• Access List
• Round Robin Load Sharing
• Dynamic DNS Update

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Access Control List
DNS Setup

• ACL is a list of semi-colon separated IP
  addresses or networks.
• Create ACLs and apply directives on specified
  ACLs
• acl mynetwork 172.31.0.0/16 172.30.1.1
• allow-query mynetwork
• allow-transfer mynetwork
• allow-update mynetwork

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Dynamic DNS Update
DNS Setup

• Allows the DNS server to update the Name and IP
  address of hosts on the network.
• To disable DDNS, use the directive
• allow-update none

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Round Robin Load Sharing
DNS Setup

• Load Balancing can be achieved by use of multiple
  A records for the same name
• www IN A 203.200.95.140
• www IN A 203.200.95.141
• www IN A 203.200.95.142