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8/3/2019 Use Nslookup
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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 setcommand:
[14] 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 domainsendmail.orgby 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 = mail.mailsite.rutgers.edu
mail1.reference.com inet address = 206.171.3.24
mail2.reference.com inet address = 128.102.240.18
mail.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
http://www.zoneedit.com/doc/nslo
5/18/2009
8/3/2019 Use Nslookup
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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 . [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, 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 forwardsthat 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 setcommand:
>
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 .
Table: Some nslookup Types
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 (1), just type exit(or Control-D if that fails).
http://www.zoneedit.com/doc/nslo
5/18/2009