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 routing information.

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

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).

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/ Reverse Zone File: /var/named/chroot/var/named/

Sample Master named.conf DNS Setup zone "." { type hint; file "named.ca"; }; zone " in-addr.arpa" { type master; file "named.local"; allow-query {any;}; }; zone "iitk.ac.in" { type master; file "hosts.db"; allow-query {any;}; }; zone " IN-ADDR.ARPA" { type master; file "hosts.rev "; allow-query {any;}; }; zone "iitk.ernet.in" { type slave; file "hosts.iitk.ernet.in"; masters { ; }; allow-query {any;};

Sample Forward Zone File DNS Setup $TTL IN SOA ns.iitk.ac.in. root.ns.iitk.ac.in. ( ; Serial ; Refresh - 3 hours 3600 ; Retry - 1 hour ;Expire - 1 week ) ; 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 mail IN A proxy IN CNAME mail

Sample Reverse Zone File DNS Setup $TTL $ORIGIN in-addr.arpa. 95 IN SOA ns1.iitk.ac.in. root.ns1.iitk.ac.in. ( ; Serial ; Refresh - 5 minutes 3600 ; Retry - 1 minute ; Expire - 1 weeks ) ; Minimum TTL for negative answers - 12 hours IN NS ns.iitk.ac.in. IN NS proxy.iitk.ac.in. $ORIGIN in-addr.arpa. ; 142 IN PTR ns.iitk.ac.in. 144 IN PTR mail.iitk.ac.in.

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

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

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

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” { /16; ; }; allow-query {mynetwork; }; allow-transfer {mynetwork; }; allow-update {mynetwork; };

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; };

Round Robin Load Sharing DNS Setup Load Balancing can be achieved by use of multiple A records for the same name: wwwINA wwwINA wwwINA