DNS (Domain Naming System)

1. What is DNS?

DNS  stands for Domain  Naming System. The DNStranslates Internet domain and host names            to IP addresses.

        DNS automatically converts the names we type in our Web browser address bar to the IP                      addresses of Web servers hosting those sites.

DNS implements a distributed database to store this name and address information for all public          hosts on the Internet. 

        DNS assumes IP addresses do not change (are statically assigned rather than dynamically                    assigned).

2. What is DNS server and how it works?

In any network, the hosts primarily communicate between each other through IP addresses. For          example, if my computer is doing a google search, my computer is actually communicating                with the IP address of one of the web servers of google.com. However, even if the computer is            efficient with numbers, humans on the other hand work better with names. For this reason, the            TCP/IP protocol includes the Domain Name System (DNS)  to link between IPs and computer            names i.e. hostnames. The DNS is a distributed database of computers that is responsible for                resolving hostnames against IP addresses and vice-versa.

Any DNS query involves two parts.

(i) The Resolver: The resolver forms up or initiates the query. The resolver itself does not run                                          as a program. /etc/resolve.conf is an example of a resolver.

(ii) Name Server: The Name Server is the service running in the server that responds to the                                               DNS query generated by the resolver i.e. answers to the question of the                                                 resolver.

The working  DNS :

(i) The client initiates a query to find a domain  example.com. The client sends the query to                       the DNS server of the ISP. (The DNS Server IP in the client computer is set as the IP                           address of the DNS Server of the ISP)

(ii) The DNS Server of the ISP first checks it's own cache to check whether it already knows                      the answer. But as the answer is not present, it generates another query. As the Top  Level                    Domain of example.com  is  .com, so the DNS server queries the  Internet Registration                        Authority to find who is responsible for example.com.

(iii) The Internet Registration Authority  responds to the ISP by answering the query.

(iv)  Once the ISP DNS Server knows the authoritative name servers, it contacts the                                      authoritative name servers to find out the IP address for www.example.com i.e. the IP                          address of host www in the domain example.com.

        (v) example.com responds to the ISP DNS Server by answering the query and providing the IP                  address of the web server i.e. www

(vi) The ISP DNS Server stores the answer in it's cache for future use and answers to the client                   by sending the IP address of the www server.

(vii)The client may store the answer to the DNS query in it's own cache for future use. Then the                 client communicates directly with the www server of domain example.com using the IP                       address.

(viii) The www server responds by sending the index.html page.

3. What is the format of the domain name?

Like  a physical address,  internet domain names are hierarchical way. If the Fully Qualified                Domain Name is www.google.co.in ,  

        the  www  is the Hostname,  

             google  is the  Domain,             

                    co  is the  Second  Level  

                     in  is the  Top  Level  Domain.

4. What are the files we have to edit to configure the  DNS?

There are  four  files to edit to configure the DNS. They are  /etc/named.conf,                                         /etc/named.rfc1912.zones, Forward  Lookup  Zoneand  Reverse  Lookup  Zone.  DNS                         provides a centralised database for resolution.  Zone is storage databasewhich contains all the             records. 

Forward  Lookup  Zone  is used to resolveHostnames  to  IP addresses.

Reverse  Lookup  Zone  is used to resolve  IP addresses  to  Hostnames.

5. What are the DNS record  and  explain them?

(i) SOA  Record :  (Start  of  Authority)

SOA  contains the general  administration  and  control  information about the domain.

(ii) Host  A  Record :

(a)  It  is nothing but aForward  Lookup  Zone.

(b)  It maps  Hostname  to  IP address.

(iii) PTR :  (Pointer  Record)

(a)  It is nothing but a  Reverse  Lookup  Zone.

(b)  It maps  IP address  to  Hostname.

(iv) NS  Record :  (Name  Server  Record)

It  stores the  DNS  server  IP addresses.

(v) MX  Record :  (Mail  Exchange  Record)

It  stores the records of the  Mail  Server  IP address.

(vi) CNME  Record :

It  is nothing but Host's  Canonical  name allows additional  names  or  aliases to be used                      locate a system.

6. What is the profile of the  DNS?

Package         : bind  and  caching-name

Script         : /etc/init.d/named

Configuration  file : /etc/named.conf   and  /etc/named.rfc1912.zones

Client's  configuration  file : /etc/resolve.conf

Document  root : /var/named/

Log  file         : /var/log/messages

Deamon         : named

Port  number         : 53

7. How to configure the  DNS  server?

(i) Install the packages  bind,  caching-name for  RHEL - 6  &bind,  cashing-name  and                            unbound  for  RHEL - 7.

# yum  install  bind*   caching-name*  -y(to install the  DNS  packages  for  RHEL - 6)

# yum  install  bind*   caching-name*  unbound*  -y(to install the DNS packages for RHEL - 7)

(ii) Change the  hostname by adding fully qualified domain name and make it permanent.

# hostname    <fully qualified  domain  name>(to change the hostname in  RHEL - 6)

# hostname  server9.example.com(example for setting  hostname temporarily in  RHEL - 6)

# hostnamectl  set   <fully qualified domain name>(to change the hostname in  RHEL - 7)

# hostnamectl set server9.example.com(example for setting hostname temporarilyin  RHEL - 7)

# vim   /etc/hosts (open this file and go to last line  and  type as below  in  RHEL - 6  only)

<IP address><fully qualified domain name> <hostname> 172.25.9.11 server9.example.com server9 (for example of the above syntax)

# vim /etc/sysconfig/network(open this file and go to last line and type as below in RHE-6 only)

HOSTNAME=<fully qualified domain name>

HOSTNAME= server9.example.com (for example of the above syntax)

(ii) Open the  DNS  main configuration file by  # vim   /etc/named.conf    command.

*   Go to line number  11  and  edit this line as below.

listen-on  port  53  {  127.0.0.1;  <server  IP address>;  };

Example :   listen-on  port  53  {127.0.0.1;  172.25.9.11;  };

*   Go to line number   17 and edit this line as below.

allow-query {  localhost;  <Network  ID>/<netmask>;  };

Example :   allow-query      {localhost;  172.25.9.0/24;  };(save  and  exit  this  file)

(iii) Open the  DNS  zone reference file by  # vim   /etc/named.rfc1912.zones    command

*   Go to line number  19  and  copy  5  lines  and paste them  at last of the file.

zone   "<domain  name>"    IN   {

type-master;

file    "<forward  lookup  zone file name>";

allow-update   {  none;   };

};

Example :   zone    "example.com"    IN   {

   type-master;

   file    "named.forward";

   allow-update   {   none;   };

      };

*   Go to line number  31  and  copy  5  lines  and  paste them at last of the file.

zone   "<Three  octets  of the  DNS  server  IP address> . in . addr . arpa"    IN   {

type-master;

file    "<reverse  lookup  zone file name>";

allow-update   {  none;   };

};

Example :   zone    "9.25.172 . in . addr . arpa"    IN   {

   type-master;

   file    "named.reverse";

   allow-update   {   none;   };

      }; (save  and  exit  this  file)

(iv) Copy  /var/named/named.localhost    file  to  /var/named/named.forward  and  edit as                           follows.

# cp   -p    /var/named/named.localhost     /var/named/named.forward

# vim   /var/named/named.forward

  *   Go to line number 2  and  edit as follows.

  @ IN   SOA    <DNS  server  fully  qualified  domain  name> . com root . <domain  name> .      {

  *   Go to line number   8  and  edit as follows.

NS <DNS  server  fully  qualified  domain  name> .

A <DNS  server  IP address>

<DNS  server  fully  qualified  domain  name> IN      A <DNS  server  IP address>

<Client 1  fully  qualified  domain  name> IN      A <Client 1  IP address>

<Client 2  fully  qualified  domain  name> IN      A <Client 2  IP address>

<Client 3  fully  qualified  domain  name> IN      A <Client 3  IP address>

  www    IN CNAME <DNS  server  fully  qualified  domain  name>

Example :  The line number  2  should be edited  as follows.

  @ IN    SOA    server9.example.com.     root.example.com.     {

  The line number  8  should be edited as follows.

NS server9.example.com.

A 172.25.9.11

server9.example.com.    IN     A 172.25.9.11

client9.example.com.    IN     A 172.25.9.10

client10.example.com.    IN     A 172.25.9.12

client11.example.com.    IN     A 172.25.9.13

www    IN CNAME         server9.example.com. (save  and  exit  this  file)

(v) Copy  /var/named/named.empty    file  to  /var/named/named.reverse  and  edit as follows.

# cp   -p    /var/named/named.empty     /var/named/named.reverse

# vim   /var/named/named.reverse

  *   Go to line number  2  and  edit as follows.

  @ IN   SOA    <DNS  server  fully  qualified  domain  name> . com root . <domain  name> .      {

  *   Go to line number   8  and  edit as follows.

NS <DNS  server  fully  qualified  domain  name> .

<Last  octet  of the  DNS  server  IP address> IN      PTR      <DNS  server  fully  qualified  domain  name>

<Last  octet  of the  Client 1  IP address> IN      PTR      <Client 1  fully  qualified  domain  name>

<Last  octet  of the  Client 2  IP address> IN      PTR      <Client 2  fully  qualified  domain  name>

<Last  octet  of the  Client 3  IP address> IN      PTR      <Client 3  fully  qualified  domain  name>

<DNS  server  fully  qualified  domain  name> IN      A <DNS  server  IP address>

  www    IN CNAME <DNS  server  fully  qualified  domain  name>

Example :  The line number  2  should be edited  as follows.

  @ IN    SOA    server9.example.com.     root.example.com.     {

  The line number  8  should be edited as follows.

NS server9.example.com.

11    IN     PTR server9.example.com.

10    IN     PTR client9.example.com.

12    IN     PTR client10.example.com.

13    IN     PTR client11.example.com.

server9.example.com. IN     A         172.25.9.11

www    IN CNAME         server9.example.com. (save  and  exit  this  file)

(vi) Check the  DNS configuration files for syntax  errors.

# named-checkconf    /etc/named.conf

# named-checkconf    /etc/named.rfc1912.zones

# name-checkzone     <domain  name><forward  lookup  zone>

Example :   # named-checkzone     example.com    /var/named/named.forward

# named-checkzone     <domain  name><reverse  lookup  zone>

Example :   # named-checkzone     example.com   /var/named/named.reverse

(vii) Give full permissions  to the forward  and  reverse  lookup zones.

# chmod    777   /var/named/named.forward

# chmod    777   /var/named/named.reverse

(viii) Open   /etc/sysconfig/network-scripts/ifcfg-eth0   and  enter the  DNS  domain details  if                     not present.

# vim   /etc/sysconfig/network-scripts/ifcfg-eth0 (go to last line and type as follows)

    DNS 1=example.com (save  and  exit  this  file)

(ix) Add the  DNS  server  IP address  in  /etc/resolve.conf

# vim   /etc/resolve.conf

  search <domain  name>

  namesever <IP address  of  the  DNS  server>

Example :

  search example.com

  nameserver 172.25.9.11 (save  and  exit  this  file)

(x) Restart the  DNS  server deamons.

# service  named  restart (to restart the  deamon  in  RHEL - 6)

# chkconfig  named  on (to enable the  deamon  at next boot time in  RHEL - 6)

# systemctl  restart  named   unbound (to restart  the  deamons  in  RHEL - 7)

# systemctl enable named unbound (to enable the  deamons  at next boot time in  RHEL -7)

(xi) Add the  DNS  service to the  IP tables   and  Firewall.

# setup (then select the  Firewall  configuration  option  and  add  DNS  in  RHEL - 6)

# service  iptables  restart (to restart the IP tables  in  RHEL - 6)

# service  iptables  save (to save the IP tables  in  RHEL - 6)

# firewall-cmd --permanent --add-service=dns(to add the DNS service to firewall in RHEL - 7)

# firewall-cmd     --complete-reload (to  reload the firewall  in  RHEL - 7)

(xi) Check  whether the  DNS  is resolving  or  not.

# dig  <DNS server fully qualified name> (to check the resolving from hostname to IP address)

# dig  -x <DNS  server  IP address>(to check the resolving from  IP address  to  hostname)

Example :   # dig    server9.example.com

           # dig    -x    172.25.9.11

(xii) Check the resolution with ping test.

      # ping  -c3  <DNS  server  fully  qualified  domain  name>(to check the ping test with hostname)

     # ping    -c3    <IP address of the DNS server> (to check the ping test with IP address)

Example :

# ping   -c3    server9.example.com

# ping   -c3    172.25.9.11

(xiii) Check the resolution with host command.

   # host    <hostname>  (to check the resolution with hostname)

   # host    <IP address>  (to check the resolution with IP address)

Example :

   # host     server9.example.com

   # host     172.25.9.11

(xiv) Check the resolution with  nslookup  command.

   # nslookup     <hostname> (to check the resolution with hostname)

   # nslookup     <IP address> (to check the resolution with IP address)

Example :

   # nslookup server9.example.com

   # nslookup 172.25.9.11

8. How to configure the  DNS  client?

(i) First  assign the static  IP address to the client.

(ii) Set the hostname to the client.

(iii) Restart the network service by   #service  network  restart   command.

(iv) Open   /etc/resolve.conf   file  and  edit as below.

# vim   /etc/resolve.conf

  search    <domain  name>

  nameserver     <DNS  server  IP address>

Example :

  search     example.com

  nameserver     172.25.9.11 (save  and  exit  this  file)

(v) Check  whether the  DNS  is resolving  or  not.

# dig   <DNS server fully qualified  name>(to check the resolving from hostname to IP address)

# dig    -x    <DNS  server  IP address>    (to check the resolving from  IP address  to  hostname)

Example :   # dig    server9.example.com

           # dig    -x    172.25.9.11

           # dig    client9.example.com

          # dig    -x    172.25.9.10

(vi) Check the resolution with ping test.

# ping    -c3   <DNS  client fully qualified domain  name>(to check the ping test with hostname)

# ping    -c3    <IP address of the DNS server>     (to check the ping test with IP address)

Example :

  # ping   -c3    client9.example.com

  # ping   -c3    172.25.9.10

  # ping   -c3    server9.example.com

  # ping   -c3    172.25.9.11

(vii) Check the resolution with host command.

   # host    <hostname> (to check the resolution with hostname)

   # host    <IP address>         (to check the resolution with IP address)

Example :

   # host     server9.example.com

   # host     172.25.9.11

   # host     client9.example.com

   # host     172.25.9.10

(viii) Check the resolution with  nslookup  command.

   # nslookup     <hostname> (to check the resolution with hostname)

   # nslookup     <IP address>         (to check the resolution with IP address)

Example :

   # nslookup server9.example.com

   # nslookup 172.25.9.11

   # nslookup client9.example.com

   # nslookup 172.25.9.10

9. How to configure the  Secondary  DNS  server?

(i) Install the packages  bind,  caching-name  for  RHEL - 6  &bind,  cashing-name  and                            unbound  for  RHEL - 7.

# yum  install  bind*   caching-name*    -y (to install the  DNS  packages  for  RHEL - 6)

# yum  install  bind* caching-name*  unbound* -y(to install the DNS packages for RHEL- 7)

(ii) Change the  hostname by adding fully qualified domain name and make it permanent.

# hostname    <fully qualified  domain  name> (to change the hostname in  RHEL - 6)

# hostname   server6.example.com (example for setting  hostname temporarily in  RHEL - 6)

# hostnamectl  set   <fully qualified domain name>(to change the hostname in  RHEL - 7)

# hostnamectl  set  server6.example.com(example for setting hostname temporarily in RHEL- 7)

# vim   /etc/hosts (open this file and go to last line  and  type as below  in  RHEL - 6  only) 

<IP address> <fully qualified domain name> <hostname>

172.25.6.11 server6.example.com server6 (for example of the above syntax)

# vim   /etc/sysconfig/network(open this file and go to last line and type as below in RHEL-6)  o

HOSTNAME=<fully qualified domain name>

HOSTNAME=server6.example.com (for example of the above syntax)

(ii) Open the  DNS  main configuration file by  # vim   /etc/named.conf    command.

*   Go to line number  11  and  edit this line as below.

listen-on  port  53  {  127.0.0.1;  <server  IP address>;  };

Example :   listen-on  port  53  {127.0.0.1;  172.25.6.11;  };

*   Go to line number   17 and edit this line as below.

allow-query {  localhost;  <Network  ID>/<netmask>;  };

Example :   allow-query      {localhost;  172.25.6.0/24;  }; (save  and  exit  this  file)

(iii) Open the  DNS  zone reference file by  # vim   /etc/named.rfc1912.zones    command

*   Go to line number  19  and  copy  5  lines  and paste them  at last of the file.

zone   "<domain  name>"    IN   {

type-slave;

file    "slaves/<forward  lookup  zone file name>";

master   {  <Primary  DNS  server  IP address;   };

};

Example :   zone    "example.com"    IN   {

   type-slave;

   file    "slaves/named.forward";

master   {   172.25.9.11;   };

      };

*   Go to line number  31  and  copy  5  lines  and  paste them at last of the file.

zone   "<Three  octets  of the  DNS  server  IP address> . in . addr . arpa"    IN   {

type-slave;

file    "slaves/<reverse  lookup  zone file name>";

master   {  <Primary  DNS  server  IP address;   };

};

Example :   zone    "9.25.172 . in . addr . arpa"    IN   {

   type-slave;

   file    "slaves/named.reverse";

   master   {   172.25.9.11;   };

      }; (save  and  exit  this  file)

(iv) Copy  /var/named/slaves/named.localhost to /var/named/slaves/named.forward  and  edit as                 follows.

# mkdir    /var/named/slaves

# cp   -p    /var/named/slaves/named.localhost     /var/named/slaves/named.forward

# vim   /var/named/slaves/named.forward

  *   Go to line number  2  and  edit as follows.

  @ IN   SOA    <secondary  DNS  server  fully  qualified  domain  name> . com root . <domain  name> . {

  *   Go to line number   8  and  edit as follows.

NS <DNS  server  fully  qualified  domain  name> .

A <DNS  server  IP address>

<secondary  DNS  server  fully  qualified  domain  name>    IN      A      <secondary  DNS  server  IP address>

<DNS  server  fully  qualified  domain  name> IN      A <DNS  server  IP address>

<Client 1  fully  qualified  domain  name> IN      A <Client 1  IP address>

<Client 2  fully  qualified  domain  name> IN      A <Client 2  IP address>

<Client 3  fully  qualified  domain  name> IN      A <Client 3  IP address>

  www    IN CNAME <DNS  server  fully  qualified  domain  name>

Example :  The line number  2  should be edited  as follows.

  @ IN    SOA    server6.example.com.     root.example.com.     {

  The line number  8  should be edited as follows.

NS server6.example.com.

A 172.25.6.11

server6.example.com.    IN     A 172.25.6.11

server9.example.com.    IN     A 172.25.9.11

client9.example.com.    IN     A 172.25.9.10

client10.example.com.    IN     A 172.25.9.12

client11.example.com.    IN     A 172.25.9.13

www    IN CNAME         server6.example.com. (save  and  exit  this  file)

(v) Copy  /var/named/slaves/named.empty    file  to  /var/named/slaves/named.reverse  and  edit                 as follows.

# cp   -p    /var/named/slaves/named.empty     /var/named/slaves/named.reverse

# vim   /var/named/slaves/named.reverse

  *   Go to line number  2  and  edit as follows.

  @ IN   SOA    <secondary  DNS  server  fully  qualified  domain  name> . com root . <domain  name> . {

  *   Go to line number   8  and  edit as follows.

NS <secondary  DNS  server  fully  qualified  domain  name> .

<Last octet of the secondary DNS server IP address>IN PTR <secondary DNS server fully  qualified domain  name>

<Last  octet  of the  DNS  server  IP address> IN PTR <DNS  server fully qualified domain  name>

<Last  octet  of the  Client 1  IP address>IN   PTR   <Client 1  fully  qualified  domain  name>

<Last  octet  of the  Client 2  IP address>IN   PTR   <Client 2  fully  qualified  domain  name>

<Last  octet  of the  Client 3  IP address>IN  PTR    <Client 3  fully  qualified  domain  name>

<secondary  DNS  server  fully  qualified  domain  name> IN A <secondary DNS  server IP address>

  www    IN CNAME <secondary  DNS  server  fully  qualified  domain  name>

Example :  The line number  2  should be edited  as follows.

  @ IN    SOA    server6.example.com.     root.example.com.     {

  The line number  8  should be edited as follows.

NS server6.example.com.

11    IN     PTR server6.example.com.

11    IN     PTR server9.example.com.

10    IN     PTR client9.example.com.

12    IN     PTR client10.example.com.

13    IN     PTR client11.example.com.

server6.example.com. IN     A         172.25.6.11

www    IN CNAME         server6.example.com.(save  and  exit  this  file)

(vi) Check the  DNS configuration files for syntax  errors.

# named-checkconf    /etc/named.conf

# named-checkconf    /etc/named.rfc1912.zones

# name-checkzone     <domain  name><forward  lookup  zone>

Example :   # named-checkzone     example.com    /var/named/slaves/named.forward

# named-checkzone     <domain  name><reverse  lookup  zone>

Example :   # named-checkzone     example.com   /var/named/slaves/named.reverse

(vii) Give full permissions  to the forward  and  reverse  lookup zones.

# chmod    777   /var/named/slaves/named.forward

# chmod    777   /var/named/slaves/named.reverse

(viii) Open  /etc/sysconfig/network-scripts/ifcfg-eth0 and enter the DNS domain details  if not                     present.

  # vim   /etc/sysconfig/network-scripts/ifcfg-eth0  (go to last line and type as follows)

    DNS 1=example.com (save  and  exit  this  file)

(ix) Add the  DNS  server  IP address  in  /etc/resolve.conf

# vim   /etc/resolve.conf

  search <domain  name>

  namesever <IP address  of  the  DNS  server>

  namesever <IP address  of  the  secondary  DNS  server>

Example :

  search example.com

  nameserver 172.25.9.11

  nameserver 172.25.6.11 (save  and  exit  this  file)

(x) Restart the  DNS  server deamons.

# service  named  restart     (to restart the  deamon  in  RHEL - 6)

# chkconfig  named  on (to enable the  deamon  at next boot time in  RHEL - 6)

# systemctl  restart  named   unbound (to restart  the  deamons  in  RHEL - 7)

# systemctl  enable  named unbound (to enable the deamons at next boot time in RHEL -7)

(xi) Add the  DNS  service to the  IP tables   and  Firewall.

# setup (then select the  Firewall  configuration  option  and  add  DNS  in  RHEL - 6)

# service  iptables  restart (to restart the IP tables  in  RHEL - 6)

# service  iptables  save (to save the IP tables  in  RHEL - 6)

# firewall-cmd --permanent --add-service=dns(to add the DNS service to firewall in RHEL - 7)

# firewall-cmd     --complete-reload (to  reload the firewall  in  RHEL - 7)

(xi) Check  whether the  DNS  is resolving  or  not.

# dig  <DNS server fully qualified name>(to check the resolving from hostname to IP address)

# dig    -x    <DNS  server  IP address>    (to check the resolving from  IP address  to  hostname)

Example :   # dig    server6.example.com

      # dig    -x    172.25.6.11

(xii) Check the resolution with ping test.

 #ping -c <secondary DNS server fully qualified domain name>(to check the ping test with hostname)

# ping -c3 <IP address of the  secondary  DNS server> (to check the ping test with IP address)

Example :

  # ping   -c3    server6.example.com

  # ping   -c3    172.25.6.11

(xiii) Check the resolution with host command.

   # host    <hostname> (to check the resolution with hostname)

   # host    <IP address>       (to check the resolution with IP address)

Example :

   # host     server6.example.com

   # host     172.25.6.11

(xiv) Check the resolution with  nslookup  command.

   # nslookup     <hostname>  (to check the resolution with hostname)

   # nslookup     <IP address>   (to check the resolution with IP address)

Example :

   # nslookup server6.example.com

   # nslookup 172.25.6.11