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8.4. Using nslookup

nslookup is a debugging tool provided as part of the BIND software package. It allows anyone to query a name server directly and retrieve any of the information known to the DNS system. It is helpful for determining if the server is running correctly and is properly configured, or for querying for information provided by remote servers.

The nslookup program is used to resolve queries either interactively or directly from the command line. Here is a command-line example of using nslookup to query for the IP address of a host:

% nslookup crab.wrotethebook.com 
Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
Name:    crab.wrotethebook.com
Address:  172.16.12.1

Here, a user asks nslookup to provide the address of crab.wrotethebook.com. nslookup displays the name and address of the server used to resolve the query, and then it displays the answer to the query. This is useful, but nslookup is more often used interactively.

The real power of nslookup is seen in interactive mode. To enter interactive mode, type nslookup on the command line without any arguments. Terminate an interactive session by typing Ctrl-D (^D) or entering the exit command at the nslookup prompt. As an interactive session, the previous query shown is:

% nslookup 
Default Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
> crab.wrotethebook.com 
Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
Name:    crab.wrotethebook.com 
Address:  172.16.12.1 
 > ^D

By default, nslookup queries for A records, but you can use the set type command to change the query to another resource record type or to the special query type ANY. ANY is used to retrieve all available resource records for the specified host.[96]

[96]"All available" records can vary based on the server answering the question. A server that is authoritative for the zone that contains the host's records responds with all records. A nonauthoritative server that has cached information about the host provides all of the records it has cached, which might not be every record the host owns.

The following example checks MX records for crab and rodent. Note that once the query type is set to MX, it stays MX. It doesn't revert to the default A-type query. Another set type command is required to reset the query type.

% nslookup 
Default Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
> set type=MX 
> crab.wrotethebook.com 
Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
crab.wrotethebook.com    preference = 5, mail exchanger = crab.wrotethebook.com 
crab.wrotethebook.com    inet address = 172.16.12.1 
 
> rodent.wrotethebook.com 
Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
rodent.wrotethebook.com    preference = 5, mail exchanger = rodent.wrotethebook.com 
rodent.wrotethebook.com    inet address = 172.16.12.2
> exit

You can use the server command to control the server used to resolve queries. This is particularly useful for going directly to an authoritative server to check some information. The following example does just that. In fact, this example contains several interesting commands:

  • First we set type=NS and get the NS records for the zoo.edu domain.

  • From the information returned by this query, we select a server and use the server command to direct nslookup to use that server.

  • Next, using the set domain command, we set the default domain to zoo.edu. nslookup uses this default domain name to expand the hostnames in its queries in the same way that the resolver uses the default domain name defined in resolv.conf.

  • We reset the query type to ANY. If the query type is not reset, nslookup still queries for NS records.

  • Finally, we query for information about the host tiger.zoo.edu. Because the default domain is set to zoo.edu, we simply enter tiger at the prompt.

Here's the example:

% nslookup 
Default Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
> set type=NS 
> zoo.edu 
Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
Non-authoritative answer: 
zoo.edu nameserver = NOC.ZOO.EDU 
zoo.edu nameserver = NI.ZOO.EDU 
zoo.edu nameserver = NAMESERVER.AGENCY.GOV 
Authoritative answers can be found from: 
NOC.ZOO.EDU     inet address = 172.28.2.200 
NI.ZOO.EDU      inet address = 172.28.2.240 
NAMESERVER.AGENCY.GOV inet address = 172.21.18.31 
> server NOC.ZOO.EDU 
Default Server:  NOC.ZOO.EDU 
Address:  172.28.2.200 
 
> set domain=zoo.edu 
> set type=any 
> tiger 
Server:  NOC.ZOO.EDU 
Address:  172.28.2.200 
 
tiger.zoo.edu   inet address = 172.28.172.8 
tiger.zoo.edu   preference = 10, mail exchanger = tiger.ZOO.EDU 
tiger.zoo.edu   CPU=ALPHA OS=Unix 
tiger.zoo.edu   inet address = 172.28.172.8, protocol = 6 
         7 21 23 25 79 
tiger.ZOO.EDU   inet address = 172.28.172.8
> exit

The final example shows how to download an entire domain from an authoritative server and examine it on your local system. The ls command requests a zone transfer and displays the contents of the zone it receives.[97] If the zone file is more than a few lines long, redirect the output to a file and use the view command to examine the contents of the file. (view sorts a file and displays it using the Unix more command.) The combination of ls and view is helpful when tracking down a remote hostname. In this example, the ls command retrieves the big.com zone and stores the information in temp.file. Then view is used to examine temp.file.

[97]For security reasons, many name servers do not respond to the ls command. See the allow-transferoption in Appendix C, "A named Reference" for information on how to limit access to zone transfers.

rodent% nslookup 
Default Server:  rodent.wrotethebook.com 
Address:  172.16.12.2 
 
> server minerals.big.com 
Default Server:  minerals.big.com 
Address:  192.168.20.1 
 
> ls big.com > temp.file 
[minerals.big.com] 
######## 
Received 406 records. 
> view temp.file 
 acmite                         192.168.20.28 
 adamite                        192.168.20.29 
 adelite                        192.168.20.11 
 agate                          192.168.20.30 
 alabaster                      192.168.20.31 
 albite                         192.168.20.32 
 allanite                       192.168.20.20 
 altaite                        192.168.20.33 
 alum                           192.168.20.35 
 aluminum                       192.168.20.8 
 amaranth                       192.168.20.85 
 amethyst                       192.168.20.36 
 andorite                       192.168.20.37 
 apatite                        192.168.20.38 
 beryl                          192.168.20.23 
--More--q
> exit

These examples show that nslookup allows you to:

  • Query for any specific type of standard resource record

  • Directly query the authoritative servers for a domain

  • Get the entire contents of a domain into a file so you can view it

Use nslookup's help command to see its other features. Turn on debugging (with set debug) and examine the additional information this provides. As you play with this tool, you'll find many helpful features.



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