21.4 How to Use nslookup

If your site is connected to the Internet, you can use the nslookup(1) program to interactively find MX and other records. To run nslookup, just type its name:

% nslookup

Note that you may have to give the full pathname. Under SunOS, nslookup lives in the /usr/etc directory; under Ultrix, in /usr/ucb; and under HP-UX, in /usr/bin.

Once nslookup is running, it prints the name of your default name server and the IP address for that machine, then a > character as a prompt and awaits input:

Server:  Your.Main.Server
Address:  123.45.67.8

>

To tell nslookup to look up only MX records, [14] use the set command:

[14] Beginning with V8.7 sendmail, you can also use the /mx command in -bt rule-testing mode (see Section 38.5.2, "Look Up MX Records with /mx") to look up MX records.

> set type=mx
>

Now look up some real hosts and domains. First look up the domain sendmail.org by entering its name at the prompt:

> sendmail.org.

Note the trailing dot that tells nslookup(1) that the local, default domain should not be appended prior to the lookup. The output produced by the above lookup looks like this:

> sendmail.org.
Server:  
Address:  123.45.67.8

sendmail.org    preference = 20, mail exchanger = mail1.reference.com
sendmail.org    preference = 30, mail exchanger = mail2.reference.com
sendmail.org    preference = 10, mail exchanger = knecht.oxford.reference.com
mail1.reference.com     inet address = 206.171.3.24
mail2.reference.com     inet address = 128.102.240.18
knecht.oxford.reference.com     inet address = 204.247.98.2
>

The first two lines again show the name and IP address of the local DNS server. The next three lines show that the domain sendmail.org has three MX records. Mail addressed to that domain is sent to the machine with the lowest preference (cost), which happens to be knecht.oxford.reference.com. [15] If that machine is down (or not accepting mail), the message is sent to the machine with the next higher cost, mail1.reference.com. The last three lines show the IP addresses (A records) for those machines.

[15] Note that case is not significant in domain names; all of the following are the same: org, OrG, ORG.

Now look up a real UUCP host, lady. Enter its name as if it were a part of the icsi.berkeley.edu domain:

> lady.icsi.berkeley.edu.

The output produced shows that lady has an MX record:

lady.icsi.berkeley.edu   preference = 5, mail exchanger = icsib.ICSI.Berkeley.EDU
icsib.ICSI.Berkeley.EDU inet address = 128.32.201.15
>

Mail sent to lady.icsi.berkeley.edu is instead delivered to the machine named icsib.ICSI.Berkeley.EDU, which in turn forwards that mail over a dial-up line to the UUCP host lady.

Machines that have MX records do not necessarily have A records. The host lady is such a machine. You tell nslookup(1) to look up an A record with the set command:

> set type=a
> lady.berkeley.edu.

*** No address information available for lady.berkeley.edu.

The nslookup(1) program is a useful tool for performing all the same lookups that are done by sendmail. Each type of lookup corresponds to a set type. The list of some available nslookup(1) types is shown in Table 21.1.

To exit nslookup(1), just type exit (or Control-D if that fails).