This post was most recently updated on December 11th, 2018
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.ServerAddress: 123.45.67.8>
To tell >nslookup to look up only MX records, [14] use the set command:
Beginning with V8.7 sendmail, you can also use the /mx
command in -bt
rule-testing mode 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 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: 123.45.67.8sendmail.org preference = 20, mail exchanger = mail1.reference.comsendmail.org preference = 30, mail exchanger = mail2.reference.comsendmail.org preference = 10, mail exchanger = mail.mailsite.rutgers.edumail1.reference.com inet address = 206.171.3.24mail2.reference.com inet address = 128.102.240.18mail.mailsite.rutgers.edu 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 mail.mailsite.rutgers.edu
. 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.
Now look up a real UUCP host, www . Enter its name as if it were a part of the yahoo.com domain:
>www.yahoo.com.
The output produced shows that www has an MX record:
www.yahoo.com preference = 5, mail exchanger = .yahoo.com.yahoo.com inet address = 128.32.201.15>
Mail sent to www.yahoo.com
is instead delivered to the machine named .yahoo.com
, which in turn forwards that mail over a dial-up line to the UUCP host www .
Machines that have MX records do not necessarily have A records. The host www is such a machine. You tell >nslookup (1) to look up an A record with the set command:
>set type=a
>www.yahoo.com.
*** No address information available for www.yahoo.com.
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.
Type | Description |
---|---|
a | IP address |
cname | Canonical name for an alias |
hinfo | Host CPU and operating system type |
mx | Mail exchanger records |
ns | Name server record |
any | Union of all records |
To exit >nslookup, just type exit.