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12.7 Troubleshooting nslookup Problems
The
last thing you want is to have problems with your troubleshooting
tool. Unfortunately, some types of failures render the
troubleshooting tool mostly useless. Other types of
nslookup failures are, at best, confusing
because they don't give you any direct information
to work with. Although there may be a few problems with
nslookup itself, most of the problems you
encounter will be with name server configuration and operation.
We'll cover a few odd problems here.
12.7.1 Looking Up the Right Data
This isn't really a problem, per se, but it can be
awfully confusing. If you use nslookup to look
up a type of data for a domain name and the domain name exists but no
data of the type you're looking for exists,
you'll get an error like this:
C:\> nslookup
Default Server: terminator.movie.edu
Address: 192.249.249.3
> movie.edu.
Name: movie.edu
>
Huh? It looks like we got an empty answer. In fact,
that's exactly what happened: there are no A records
for movie.edu, and the response from the name
server has no records in the Answer section of the message.
nslookup renders this empty response from the
name server as an empty response to us. It's not
very helpful or clear (previous versions of
nslookup printed a better response).So what types of records do exist? You can use
set type=any to find out:
> set type=any
> movie.edu.
Server: terminator.movie.edu
Address: 192.249.249.3
movie.edu nameserver = terminator.movie.edu
movie.edu nameserver = wormhole.movie.edu
movie.edu
primary name server = terminator.movie.edu
responsible mail addr = hostmaster.movie.edu
serial = 21
refresh = 900 (15 mins)
retry = 600 (10 mins)
expire = 86400 (1 day)
default TTL = 3600 (1 hour)
movie.edu MX preference = 10, mail exchanger = wormhole.movie.edu
terminator.movie.edu internet address = 192.249.249.3
wormhole.movie.edu internet address = 192.249.249.1
wormhole.movie.edu internet address = 192.253.253.1
wormhole.movie.edu internet address = 192.253.253.1
wormhole.movie.edu internet address = 192.249.249.1
Why are the IP addresses for terminator and
wormhole returned? If you receive the NS records
for movie.edu listing these two hosts as that
zone's name servers, chances are the next thing
you'll want are those hosts' IP
addresses. The name server anticipates that and sends along address
records in the Additional section. The same thing goes for the
movie.edu MX record pointing to
wormhole: if you get that record,
you'll want
wormhole's IP address next.
That explains why wormhole's IP
addresses show up twice, but this is arguably a bug in the Microsoft
DNS Server.
12.7.2 No PTR Data for Name Server's Address
Here's a cryptic message:
C:\> nslookup
*** Can't find server name for address 192.249.249.3: Non-existent domain
Default Server: UnKnown
Address: 192.249.249.3
>
The
"Non-existent
domain" message means that there's
no PTR record for 3.249.249.192.in-addr.arpa. In
other words, nslookup couldn't
find the name for 192.249.249.3, which is the first name server the
resolver is configured to query. The only reason
nslookup looks
up this address is
to print the "Default Server"
startup message. Obviously, this name server's data
is messed up, at least for the
249.249.192.in-addr.arpa zone, so
nslookup prints
"UnKnown."
12.7.3 Timeouts
What if
your
resolver is pointing to a name server that isn't
running or a host that can't be reached?
Here's what happens:
C:\> nslookup
DNS request timed out.
timeout was 2 seconds.
*** Can't find server name for address 192.249.249.4: Timed out
Default Server: UnKnown
Address: 192.249.249.4
>
The resolver is configured to use the name server 192.249.249.4 (and
only that name server). nslookup tries valiantly
to contact it but times out, prints
"UnKnown" for the default server,
and gives you a prompt. You can't really do anything
productive without changing servers at this pointafter all, no
server is running at that IP addressbut at least
you've got a prompt.Occasionally you'll see timeouts during the course
of an nslookup session. If you are looking up
some remote information, the name server could fail to respond
because it is still trying to look up the item and
nslookup gave up waiting. How can you tell the
difference between a name server that isn't running
and a name server that is running but didn't
respond? nslookup's responses
point out the difference. In this case, the response indicates no
name server process is running:
C:\> nslookup
Default Server: terminator.movie.edu
Address: 192.249.249.3
> movie.edu.
Server: terminator.movie.edu
Address: 192.249.249.3
*** terminator.movie.edu can't find movie.edu.: No response from server
The "No response from server"
message is quite misleading because nslookup
actually did get a response from the server. What actually happened
was this: nslookup sent a DNS query in a UDP
packet addressed to port 53 on terminator. Since
no name server was running on terminator, there
was no process listening on UDP port 53 and the TCP/IP software on
terminator responded with an ICMP destination
port unreachable message. nslookup received this
response and printed the misleading message shown previously.If a name server is simply not responding, you'll
see the following timeout message:
C:\> nslookup
Default Server: terminator.movie.edu
Address: 192.249.249.3
> movie.edu.
Server: terminator.movie.edu
Address: 192.249.249.3
DNS request timed out.
timeout was 2 seconds.
*** Request to terminator.movie.edu timed-out
12.7.4 Query Refused
You generally see a
"query refused" error message under
two conditions. The first is when you attempt a zone transfer and the
server refuses for security reasons (for example, based on the
settings in the Zone Transfers tab
of the zone properties window). This is what you'll
see:
C:\> nslookup
Default Server: terminator.movie.edu
Address: 192.249.249.3
> ls movie.edu This attempts a zone transfer
[terminator.movie.edu]
*** Can't list domain movie.edu.: Query refused
The DNS server refused to transfer the zone movie.edu. to your computer. If this
is incorrect, check the zone transfer security settings for movie.edu. on the DNS
server at IP address 192.249.249.3.
>
You might also see a "query
refused" error from a name server running a recent
version of BIND, which has the ability to restrict queries to
different zones based on the querier's source IP
address.
