DNS

DNS

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.

Internet Naming Hierarchy

DNS

The silent dot at theend of all addresses

.com .net .org .in

.sidbi

www

.ac .co

.iitk

www

.yahoo

www

DNS OperationDNS 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 doesn’t 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).

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/<forward_zone_file>

Reverse Zone File:

/var/named/chroot/var/named/<reverse_zone_file>

Sample Master named.confDNS 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;};

};

zone "iitk.ernet.in" {

type slave;

file "hosts.iitk.ernet.in";

masters { 202.141.40.10; };

allow-query {any;};

Sample Forward Zone File

DNS Setup

$TTL 86400

@ 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

Sample Reverse Zone FileDNS 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.

Configuring Local ResolverDNS Setup

/etc/resolv.conf

server 127.0.0.1

Test DNSDNS Setup

nslookup

host

dig

Test your DNS with the following DNS diagnostics web site: dnsstuff.com

Advanced BIND FeaturesDNS Setup

Access List

Round Robin Load Sharing

Dynamic DNS Update

Access Control ListDNS 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; };

Dynamic DNS UpdateDNS 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; };

Round Robin Load SharingDNS 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