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NSlookup Tutorial

What is nslookup ?

Nslookup is a program to query Internet domain name servers. Nslookup has two modes: interactive and non-interactive. Interactive mode allows the user to query name servers for information about various hosts and
domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or do­
main.

DNS Records
Name Value Description
A 1 IPv4 Address (32 bits)
NS 2 Name Server
CNAME 5 Canonical Name
PTR 12 Pointer Record
HINFO 13 Host Information
MX 15 Mail Exchange Record
TXT 16 Text String
AAAA 28 IPv6 Address (128 bits)
AXFR 252 Request for Zone Transfer
ANY 255 Request for All Records

Using nslookup from commnd line

In windows just open the command prompt and type nslookup

In *nix bsed systems open the terminal window and type nslookup

bash$ /usr/etc/nslookup

Default Server: scitsc.ser.ac.uk
Address: 134.220.4.1

> set q=A
> ccub.ser.ac.uk.
Server: scitsc.ser.ac.uk
Address: 134.220.4.1

Name: ccub.ser.ac.uk
Address: 134.220.1.20

> set q=CNAME
> www.ser.ac.uk.
Server: scitsc.ser.ac.uk
Address: 134.220.4.1

www.ser.ac.uk canonical name = ccuf.ser.ac.uk
> set q=MX
> ser.ac.uk.
Server: scitsc.ser.ac.uk
Address: 134.220.4.1

ser.ac.uk preference = 1, mail exchanger = ser.ac.uk
ser.ac.uk inet address = 134.220.1.12
> set q=HINFO
> ccub.ser.ac.uk.
Server: scitsc.ser.ac.uk
Address: 134.220.4.1

ccub.ser.ac.uk CPU=SUN 690MP OS=Solaris 2.4
> set q=PTR
> 12.1.220.134.in-addr.arpa
Server: scitsc.ser.ac.uk
Address: 134.220.4.1

12.1.220.134.in-addr.arpa host name = ccug.ser.ac.uk

DNS Records Explanation

Type A

A simple query for the IP address corresponding to ccub.ser.ac.uk.

Type CNAME

A given host can have several DNS names. One of these is the canonical or reference name. This query reveals that www.ser.ac.uk is really ccuf.ser.ac.uk.

Type MX

A mail exchanger query, this time for the domain ser.ac.uk which reveals that mail sent to user@ser.ac.uk is actually sent to 134.220.1.12 which is really ccug.ser.ac.uk. (see below).

Type HINFO

An HINFO query. This is only useful if the DNS domain administrator has bothered to create the relevant records and keep them up to date.

Type PTR

A PTR query. This shows "inverse" or "reverse" resolution. Notice the very clumsy way the query has to be entered, this is partly because IP addresses have the most sigificant part first whereas DNS addresses have the most significant part last. There are plenty of pieces of software that do reverse resolution without this clumsy interface.

nslookup Errors and Solutions

*** <server> can't find <hostname>: Non-existent domain

The host you queried can not be resolved using the current DNS server. If the current DNS server is not the source of authority for the domain in question, you should locate the SOA DNS server and try querying that host directly for information on the hostname.

*** No <record type> information is available for <hostname>

No such record type appears in the DNS system for the hostname you are querying. Again, try the SOA DNS server is you have doubts.

*** Request to <server> timed-out

The server which was specified with the last 'server' command is not accepting DNS queries. Either it is not up and running on the network, or it is not accepting packets on the DNS channel.

Web based nslookup Tools

http://www.kloth.net/services/nslookup.php

http://www.zoneedit.com/lookup.html

http://swhois.net/

http://centralops.net/co/

http://network-tools.com/nslook/