Networking / Beginners

Using the nslookup Command

The nslookup command is a powerful tool for diagnosing DNS problems. You know you're experiencing a DNS problem when you can access a resource by specifying its IP address but not its DNS name. For example, if you can get to www.ebay.com by typing 66.135.192.87 in your browser's address bar but not by typing www.ebay.com, you have a DNS problem.

Looking up an IP address

The simplest use of nslookup is to look up the IP address for a given DNS name. For example, how did I know that 66.135.192.87 was the IP address for ebay.com? I used nslookup to find out:

C:\>nslookup ebay.com
Server: ns1.orng.twtelecom.net
Address: 168.215.210.50
Non-authoritative answer:
Name: ebay.com
Address: 66.135.192.87
C:\>

As you can see, just type nslookup followed by the DNS name you want to look up. Nslookup issues a DNS query to find out. This DNS query was sent to the server named ns1.orng.twtelecom.net at 168.215.210.50. It then displayed the IP address that's associated with ebay.com: namely, 66.135.192.87.

Tip: In some cases, you may find that using an nslookup command gives you the wrong IP address for a host name. To know that for sure, of course, you have to know with certainty what the host IP address should be. For example, if you know that your server is 203.172.182.10 but Nslookup returns a completely different IP address for your server when you query the server's host name, something is probably wrong with one of the DNS records.

Using nslookup subcommands

If you use nslookup without any arguments, the nslookup command enters a subcommand mode. It displays a prompt character (>) to let you know that you're in nslookup subcommand mode rather than at a normal Windows command prompt. In subcommand mode, you can enter various subcommands to set options or to perform queries. You can type a question mark (?) to get a list of these commands. Table-2 lists the subcommands you'll use most.

The Most Commonly Used nslookup Subcommands

Subcommand	What It Does
name	Queries the current name server for the specified name.
server name	Sets the current name server to the server you specify.
root	Sets the root server as the current server.
set type=x	Specifies the type of records to be displayed, such as A, CNAME, MX, NS, PTR, or SOA. Specify ANY to display all records.
set debug	Turns on Debug mode, which displays detailed information about each query.
set nodebug	Turns off Debug mode.
set recurse	Enables recursive searches.
set norecurse	Disables recursive searches.
exit	Exits the nslookup program and returns you to a command prompt.

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Using the nslookup Command

Looking up an IP address

Using nslookup subcommands

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