Using Nslookup Utility
An Effective DOS-Based Program

Let us now understand how to use the Nslookup utility for troubleshooting some common problems. The Nslookup's non-interactive mode of operation is the quickest way to figure out if the DNS name server is responding. The test can be performed by typing:

Nslookup 127.0.0.1 202.54.15.30

Here, 127.0.0.1 is the special Loopback IP address used to verify the responsiveness of the DNS name server. 202.54.15.30 is the DNS name server's IP address. If the DNS name server is working fine, you'll see the following:

Server: ns.dnserver.org
Address: 202.54.15.30
Name: local host
Address: 127.0.0.1

All the queries from the Nslookup utility are directed to the default DNS name server. With the interactive mode, you can also test other DNS name servers on your network. For instance, if you want to test the DNS name server with an IP address 202.54.15.31, while your default DNS name server is at 202.54.15.30, type:

Server 202.54.15.31

This will display:

Default Server: ns.otherdnserver.org
Address: 202.54.15.31

While using the Nslookup utility, you can also see what information is being sent to and received from the DNS name server. For this, the Nslookup utility has to be configured to use the debugging mode in its interactive mode. Type the Set subcommand with the keyword Debug. For more exhaustive information, use the keyword D2. The debugging mode lists every step involved in the execution of a command.

The Set subcommand with the keyword Nodebug lets you exit the debugging mode. To exit from the exhaustive debugging mode, use the keyword Nod2. This however reverts you to the standard debugging mode. The keyword Nodebug helps you exit the debugging mode completely.

Some common error messages you may come across while using the Nslookup utility are:

Timed Out: This error occurs when the DNS name server fails to answer a request following a certain time period and a certain number of retries. You can change the values of both, the timeout and retry using the Set command with the keywords timeout = value and retry = value, to get the required information.

No Response From Server: This message is displayed when the DNS name server is not operating on the server.

No Records: This message is shown if the DNS name server cannot find resource records for the type of the query entered. This occurs despite the valid host name being present in the query. Use the Set command with the keyword querytype to change the query type.

Non-existent Domain: This message appears if the host name or the DNS domain name is non-existent.

Connection Refused/Network Is Unreachable: This message is displayed if the utility fails to establish a connection with the DNS name server.

Server Failure: This message is displayed when the server encounters a bug in its database and subsequently fails to respond with a valid answer.

Refused: This message is displayed if the DNS name server does not process any request from you.

Format Error: This message appears if the DNS name server finds a glitch in the format of the request packet. This is however indicative of a problem in NSlookup and not the DNS server.

The Nslookup utility is definitely of great help compared to the DNS Manager or the DNS Console. The interactive mode's subcommands allow you to troubleshoot DNS problems more extensively.