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12.5 Common Tasks
You'll come to use nslookup for
little chores almost every day: for example, finding out the IP
address or MX records for a given domain name or querying a
particular name server for data. We'll cover these
common tasks before moving on to the more occasional stuff.
12.5.1 Looking Up Different Data Types
By default,
nslookup looks up the address for a name or the
name for an address. You can look up any data type by changing the
querytype, as we show in this example:
C:\> nslookup
Default Server: terminator.movie.edu
Address: 192.249.249.3
> misery Look up address.
Server: terminator.movie.edu
Address: 192.249.249.3
Name: misery.movie.edu
Address: 192.253.253.2
> 192.253.253.2 Look up name.
Server: terminator.movie.edu
Address: 192.249.249.3
Name: misery.movie.edu
Address: 192.253.253.2
> set type=mx Look up MX data.
> wormhole
Server: terminator.movie.edu
Address: 192.249.249.3
wormhole.movie.edu MX preference = 10, mail exchanger = wormhole.movie.edu
wormhole.movie.edu internet address = 192.249.249.1
wormhole.movie.edu internet address = 192.253.253.1
> set q=any Look up data of any type.
> diehard
Server: terminator.movie.edu
Address: 192.249.249.3
diehard.movie.edu internet address = 192.249.249.4
diehard.movie.edu MX preference = 10, mail exchanger = diehard.movie.edu
diehard.movie.edu internet address = 192.249.249.4
These are only a few of the valid DNS data types, of course. For a
more complete list, see Appendix A.
12.5.2 Authoritative Versus Nonauthoritative Answers
If you've
used nslookup before, you might have noticed
that it sometimes precedes its answers with the phrase
"Non-authoritative answer":
C:\>nslookup
Default Server: terminator.movie.edu
Address: 192.249.249.3
> slate.mines.colorado.edu.
Server: terminator.movie.edu
Address: 192.249.249.3
Non-authoritative answer:
Name: slate.mines.colorado.edu
Address: 138.67.1.38
This phrase indicates that the name server is not authoritative for
the data in the answer. (Recall that a name server is authoritative
for data when it's a primary or secondary for the
zone containing the data.) You'll see a
nonauthoritative response for one of two reasons. The first is that
the name server you queried didn't have the data you
were looking for and had to query a remote name server to get it. The
remote name server is authoritative for the data
(that's the reason it was queried!) and returns it
with the "authoritative answer" bit
set in the DNS message header. The Microsoft DNS Server you queried
puts this data in its cache and returns it to you marked
nonauthoritative. If you ask for the same data again, this time the
name server can answer from its cache and will mark the data
nonauthoritative: that's the second reason
you'll see a nonauthoritative answer.Authoritative answers are not announced by
nslookup: the absence of the nonauthoritative
message means the answer is authoritative.Notice that we ended the domain name with a trailing dot. The
response would have been the same had we left it off. Sometimes it is
critical that you use the trailing dot while debugging, but not
always. Rather than stopping to decide if this
name needs a trailing dot, we always add one if we know the name is
fully qualified (except, of course, for the example where we turn off
the search list).
12.5.3 Switching Servers
Sometimes
you want to query another name server directlyfor example, if
you think it is misbehaving. You can switch servers with
nslookup by using the
server or lserver commands.
The difference between server and
lserver is that lserver
queries your "local"
serverthe one you started out withto get the address of
the server you want to switch to; server uses
the default server instead of the local server. This difference is
important because the server that you just switched to may not be
responding, as we'll show in this example:
C:\> nslookup
Default Server: relay.hp.com
Address: 15.255.152.2
When we start up, our first server,
relay.hp.com, becomes our
lserver (this will matter later on in this
session):
> server galt.cs.purdue.edu.
Default Server: galt.cs.purdue.edu
Address: 128.10.2.39
> cs.purdue.edu.
Server: galt.cs.purdue.edu
Address: 128.10.2.39
*** galt.cs.purdue.edu can't find cs.purdue.edu: No response from server
At this point we try to switch back to our original name server. But
there is no name server running on galt to look
up relay's address:
> server relay.hp.com.
*** Can't find address for server relay.hp.com: No response from server
Instead of being stuck, though, we use the
lserver command to have our local server look up
relay's address:
> lserver relay.hp.com.
Default Server: relay.hp.com
Address: 15.255.152.2
>
Since the server on galt did not
respondit's not even running a name
serverit wasn't possible to look up the
address of relay to switch back to using
relay's name server.
Here's where lserver comes to
the rescue: the local name server, relay, was
still responding, so we used it. Instead of using
lserver, we could have recovered by using
relay's IP address
directlyserver 15.255.152.2.You can even change servers on a per-query basis. To specify that
you'd like nslookup to query a
particular server for information about a given domain name, you can
specify the server as the second argument on the line, after the
domain name to look uplike so:
C:\> nslookup
Default Server: relay.hp.com
Address: 15.255.152.2
> saturn.sun.com. ns.sun.com.
Server: ns.sun.com
Address: 192.9.9.3
Name: saturn.sun.com
Address: 192.9.25.2
And, of course, you can change servers from the command line. You can
specify the server to query as the argument after the domain name to
look up, like this:
C:\> nslookup -type=mx fisherking.movie.edu. terminator.movie.edu.
This instructs nslookup to query
terminator.movie.edu for MX records for
fisherking.movie.edu.To specify an alternate default server and enter interactive mode,
you can use a hyphen in place of the domain name to look up:
C:\> nslookup - terminator.movie.edu.
