|
A
simple DNS explanation and using nslookup
Author: Ranjan Chari, 2003
Back
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 querytpe=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
|
