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 directly for 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" server the one you started out with to 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 respond it's not even running a name server it 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 directly server 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 up like 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.