Nslookup command overview

Nslookup.exe (abbreviation for name server lookup) is a command line utility used for testing and troubleshooting DNS servers. It is built into Unix (including Linux and variants) and Windows. The main purpose of the utility is to query DNS servers to find DNS details, MX records for a domain, NS servers of a domain

In Windows, Nslookup.exe is automatically installed along with the TCP/IP protocol installation which is done by default during a new Windows installation or setup. The actual executable lies in the system directory c:\windows\system32 directory by default in Windows XP and most other versions of Windows.

To get started with Nslookup.exe, the following prerequisites need to be present: 

The TCP/IP protocol must be installed on the computer that you want to execute the Nslookup command without which the command will not be available

When you run the ipconfig /all command, at least one DNS server should exist in the list of DNS servers

The Nslookup command always devolves (delegates from) the domain name from the current context (depending on the DNS Server settings listed). If you fail to use a fully qualified domain name, i.e. ending the domain name with a trailing dot (.), the first query will append the DNS settings to your query domain name. For example, if you have your DNS settings listed as xyz.com and you run a query for www.bing.com, the actual query will go out as www.bing.com.xyz.com because of you entering an unqualified query. However, if you were to query for www.bing.com. [with the trailing dot (.)], then the query would rightly go out to www.bing.com only (diagram below). This strange behavior is specific to the Microsoft version of the Nslookup command. I do not know of how Nslookup behaves while run with other vendors.

If the DNS search list is being used in the Domain suffix search order in TCP/IP advanced properties DNS tab (diagram below), devolution will not take place. The query will be appended to the domain suffixes specified in the list. To override the search list, always use the Fully Qualified Domain Name in your query.

The command can be used directly (non-interactive mode) or with subcommands (interactive mode). Using Nslookup.exe in the non-interactive mode is useful when we just need the output of a specific query and only a single value needs to be returned. But when we need to get the output for multiple queries and actions it is better to use the interactive mode

The syntax of Non Interactive Mode is as follows:

nslookup [-option] [hostname] [server]

 nslookup [-opt ...]                          # interactive mode using default server

nslookup [-opt ...] – server          # interactive mode using ‘server’

nslookup [-opt ...] host                 # just look up ‘host’ using default server

nslookup [-opt ...] host server    # just look up ‘host’ using ‘server’

Option refers to the various options that are available covered below

Hostname refers to the query that we need information on

Server refers to the DNS server to be used to search for the host

 Look at the following output from a unix and windows based host

Command line output (without subcommands)

UNIX

unix% nslookup example.com

Server:        192.168.1.1
Address:    192.168.1.1#53

Non-authoritative answer:

Name:    example.com
Address: 202.7.18.16

 

Windows

C:\>nslookup microsoft.com.

Server:  PQRTVXXXXD002DNS076
Address:  210.213.34.3

Non-authoritative answer:

Name:    microsoft.com
Addresses:  207.46.197.32, 207.46.232.182

Using subcommands (unix example)

nslookup

> server ns1.com

Default Server:  ns1.com
Address:  172.204.22.25

> set
> example.com

Server:  ns1.com
Address:  202.7.18.16

example.com   MX preference = 10, mail exchanger = mail.example.com
> exit

After entering the interactive mode, typing ? or help will reveal all the options that are available

To interrupt interactive commands, press CTRL+C. To exit interactive mode and return to the command prompt, type exit at the command prompt. A number of different options can be set in Nslookup.exe by running the set command at the command prompt. A complete listing of these options is obtained by typing set all.

Looking up different data types: type and querytype 

We will discuss the two options, type and querytype that belong to the set command. To look at different query type options within the domain name space, we use the set type or set querytype command at the command prompt. Both of them are exactly the same and are interchangeable. For example, to query for mail exchanger records (mail server details) of a particular domain like yahoo.com, we can type the following:

C:\>nslookup

> set q=mx
> yahoo.com.

Server:  google-public-dns-a.google.com
Address:  8.8.8.8

Non-authoritative answer:
yahoo.com       MX preference = 1, mail exchanger = a.mx.mail.yahoo.com
yahoo.com       MX preference = 1, mail exchanger = b.mx.mail.yahoo.com
yahoo.com       MX preference = 1, mail exchanger = c.mx.mail.yahoo.com
yahoo.com       MX preference = 1, mail exchanger = e.mx.mail.yahoo.com
yahoo.com       MX preference = 1, mail exchanger = f.mx.mail.yahoo.com
yahoo.com       MX preference = 1, mail exchanger = g.mx.mail.yahoo.com

The first time a remote host is queried, the local DNS server contacts the DNS server that is authoritative for that domain. The local DNS server will then cache that information, so that subsequent queries are answered nonauthoritatively out of the local server’s cache. The first time a query is made for a remote name, the answer is authoritative, but subsequent queries are nonauthoritative.

Querying another name server directly: server  and lserver

If we wanted to use another DNS server on the internet to send our queries, we can use the server or lserver commands. For example, using the DNS server 8.8.8.8 that is the Google Public DNS server

C:\>nslookup

Default Server:  nameserver1.example.com
Address:  10.1.2.3

> server 8.8.8.8
Default Server:  google-public-dns-a.google.com
Address:  8.8.8.8

The difference between the server and lserver commands is that the server command is uses the current default server to get the address of the server to switch to, whereas the lserver uses the local server to get the same address. For example, if you have a broadband connection and get a DHCP IP address from your ISP, you would also automatically get some DNS servers (assume dns1.isp1.com), the lserver command forces the usage of these local dns servers that were retrieved from your ISP. However, while using the Nslookup.exe command, if you earlier changed the DNS server that is used to query for other domains (assume you changed it to dns1.somefreedns.com), the server command would use the current default server (dns1.somefreedns.com) and not the local dns servers (dns1.isp1.com)

Zone Transfers : LS command

Nslookup.exe can be used to transfer an entire zone by using the ls command. The best use of this command is if you want to store a list of all host names within a particular remote domain into a local file (example below)

The syntax of this command is

ls [opt] DOMAIN [> FILE] – list addresses in DOMAIN (optional: output to FILE)
    -a          -  list canonical names and aliases
    -d          -  list all records
    -t TYPE     -  list records of the given type (e.g. A,CNAME,MX,NS,PTR etc.)

Using ls with no arguments will return a list of all address and name server data. The -a switch will return alias and canonical names, -d will return all data, and -t will filter by type.

>ls example.com

[nameserver1.example.com]

nameserver1.example.com.    NS     server = ns1.example.com
nameserver2.example.com     NS     server = ns2.example.com
nameserver1                            A      10.0.0.1
nameserver2                            A      10.0.0.2

While this is obviously a security issue, zone transfers can be blocked at the DNS Server level and this would be the case most of the time on the internet (unless someone foolish enough would like you to see his internal structure and naming conventions). This command is useful if we want to see the structure of hosts within a LAN environment. If zone transfers have been blocked, we would get output like the following

> ls google.com.
*** Can’t list domain example.com.: Query refused

When we want to send the output to a file (the file will be saved in the location that was mentioned in the command prompt before the nslookup command was started, we can do so as follows

> ls google.com. > lsoutput.txt
Received 0 records.
*** Can’t list domain google.com.: Query refused

In the following example below, we want to view all mail server records within a particular domain and store the output to a file

> ls –t MX google.com. > lsoutput.txt
Received 0 records.
*** Can’t list domain google.com.: Query refused

In this article the Nslookup command was covered. While this was just a brief overview, there are many other issues like troubleshooting and whether using Nslookup itself is recommend which will be covered in future articles