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A simple DNS explanation using nslookup

By: Ranjan S. Chari
2003

The Domain Name System (acronymm DNS) is a distributed internet directory service. It's function is to translate a domain into an IP address. DNS is used for everything that's related to communicating with a web address be it email, browsing, FTP etc.

So if DNS fails for some reason all activities on the internet would come to a standstill.

The way domain names and DNS work
A domain name is first purchased thru a Domain Registrar. Ex: Netsol.com, Directnic.com etc. At the time you sign up for the domain, you're asked to submit your personal information, and information on 2 or more Name Servers. This information is stored on a 'root DNS server'.When someone searches for your domain on the web or using any other service that needs to get hold of details on your domain, these root servers are queried. An exception is when you cache the DNS information locally.

Now, the root DNS server returns your domain's Name Server information to the requester. The Name Server in turn provides the requested the requested information, namely:

"A" record: The IP address of the domain

Example: www.myserver.com A 21.22.23.24

"MX" record: The mailserver for the domain

The MX records are used to figure out where to deliver your email.

Example: myserver.com MX 0 mail.myserver.com
Example: myserver.com MX 10 mail1.myserver.com

Above, the example shows us that myserver.com has 2 mail servers which preference 0 & 10. So if the server with preference '0' is inaccessible the next preference server is contacted.

"CNAME" or Canonical Name

This is used to point something like abc.myserver.com to 65.64.66.127 or some IP address.

Many a times you need to make changes or to create a DNS entry. To do this simply, Windows has a program called NSLOOKUP. Using this tool, one can find a lot of information on a domain.

On your Windows PC or your Linux/UNIX box, from the command line (prompt) run 'nslookup'

On the prompt type in the following:

> set querytype=MX
> ibm.com

The first command is to specify that you want to check the MX record.
The second command is to specify the domain for which you want the check to be done.

Other Queries:

> set querytype=CNAME
> set querytype=A
> set querytype=NS
> set querytype=ALL

Sample Output for 'set querytype=MX':

Server: UnKnown
Address: 192.168.0.1

Non-authoritative answer:
ibm.com MX preference = 0, mail exchanger = ns.watson.ibm.com

ibm.com nameserver = ns.watson.ibm.com
ibm.com nameserver = ns.austin.ibm.com
ibm.com nameserver = ns.almaden.ibm.com
ibm.com nameserver = internet-server.zurich.ibm.com
ns.watson.ibm.com internet address = 198.81.209.2
ns.austin.ibm.com internet address = 192.35.232.34
ns.almaden.ibm.com internet address = 198.4.83.35
internet-server.zurich.ibm.com internet address = 195.176.20.204
>


Similarly you can do:

> set querytype=ALL
> ibm.com
Server: UnKnown
Address: 192.168.0.1

Non-authoritative answer:
ibm.com internet address = 129.42.16.99
ibm.com internet address = 129.42.17.99
ibm.com internet address = 129.42.18.99
ibm.com internet address = 129.42.19.99
ibm.com nameserver = ns.watson.ibm.com
ibm.com nameserver = ns.austin.ibm.com
ibm.com nameserver = ns.almaden.ibm.com
ibm.com nameserver = internet-server.zurich.ibm.com
ibm.com
primary name server = ns.watson.ibm.com
responsible mail addr = nrt.watson.ibm.com
serial = 2002112102
refresh = 3600 (1 hour)
retry = 1800 (30 mins)
expire = 604800 (7 days)
default TTL = 600 (10 mins)
ibm.com MX preference = 0, mail exchanger = ns.watson.ibm.com

ibm.com nameserver = ns.watson.ibm.com
ibm.com nameserver = ns.austin.ibm.com
ibm.com nameserver = ns.almaden.ibm.com
ibm.com nameserver = internet-server.zurich.ibm.com
ns.watson.ibm.com internet address = 198.81.209.2
ns.austin.ibm.com internet address = 192.35.232.34
ns.almaden.ibm.com internet address = 198.4.83.35
internet-server.zurich.ibm.com internet address = 195.176.20.204


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