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.

DNS

Internet Naming Hierarchy

The silent dot at the end of all addresses .com

.net

.org

.in

.sidbi

.ac

.iitk

www www .co

DNS Setup

DNS Operation

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 Setup

DNS Configuration

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/

Reverse Zone File:

/var/named/chroot/var/named/

DNS Setup

Sample Master named.conf

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;};

DNS Setup

Sample Forward Zone File

$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 mail proxy IN IN IN A A CNAME 203.200.95.142

203.200.95.144

mail

DNS Setup

Sample Reverse Zone File

$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.

DNS Setup

Configuring Local Resolver

/etc/resolv.conf

server 127.0.0.1

DNS Setup

Test DNS

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

DNS Setup

Advanced BIND Features

Access List Round Robin Load Sharing Dynamic DNS Update

DNS Setup

Access Control List

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; };

DNS Setup

Dynamic DNS Update

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; };

DNS Setup

Round Robin Load Sharing

Load Balancing can be achieved by use of multiple A records for the same name:

www www www IN IN IN A A A 203.200.95.140

203.200.95.141

203.200.95.142