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.

1. Linux System Administration
DNS
DNS

2. 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.

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

4. 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 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).

5. 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>

6. 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;};
};
zone "iitk.ernet.in" {
type slave;
file "hosts.iitk.ernet.in";
masters { 202.141.40.10; };
allow-query {any;};

7. 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

8. 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.

9. Configuring Local Resolver
DNS Setup
/etc/resolv.conf
server 127.0.0.1

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

11. Advanced BIND Features
DNS Setup
Access List
Round Robin Load Sharing
Dynamic DNS Update

12. 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; };

13. 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; };

14. 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

15. Thank you
Sreenatha Reddy K R
https://in.linkedin.com/in/sreenathaa
https://twitter.com/SreenathReddyK
https://www.fb.com/sreenathareddy.kr