DNS and BIND 4th Edition-DNS and BIND 4th Edition

I l@ve RuBoard

12.6 Less Common Tasks

Let's move on to some tricks you'll probably use less often but are still handy to have in your repertoire. Most of these will be helpful when you are trying to troubleshoot a DNS or BIND problem; they'll enable you to grub around in the messages the resolver sees and mimic a BIND name server querying another name server or transferring zone data.

12.6.1 Showing the Query and Response Messages

If you need to, you can direct nslookup to show you the queries it sends out and the responses it receives. Turning on debug shows the responses. Turning on d2 shows the queries as well. When you want to turn off debugging completely, you have to use set nodebug, since set nod2 turns off only level 2 debugging. After the following trace, we'll explain some parts of the output. If you want, pull out your copy of RFC 1035, turn to page 25, and read along with our explanation.

% nslookup 
Default Server:  terminator.movie.edu
Address:  0.0.0.0

> set debug 
> wormhole 
Server:  terminator.movie.edu
Address:  0.0.0.0

------------
Got answer:
    HEADER:
        opcode = QUERY, id = 6813, rcode = NOERROR
        header flags:  response, auth. answer, want recursion,
        recursion avail.  questions = 1,  answers = 2,
        authority records = 2,  additional = 3

    QUESTIONS:
        wormhole.movie.edu, type = A, class = IN
    ANSWERS:
    ->  wormhole.movie.edu
        internet address = 192.253.253.1
        ttl = 86400 (1D)
    ->  wormhole.movie.edu
        internet address = 192.249.249.1
        ttl = 86400 (1D)
    AUTHORITY RECORDS:
    ->  movie.edu
        nameserver = terminator.movie.edu
        ttl = 86400 (1D)
    ->  movie.edu
        nameserver = wormhole.movie.edu
        ttl = 86400 (1D)
    ADDITIONAL RECORDS:
    ->  terminator.movie.edu
        internet address = 192.249.249.3
        ttl = 86400 (1D)
    ->  wormhole.movie.edu
        internet address = 192.253.253.1
        ttl = 86400 (1D)
    ->  wormhole.movie.edu
        internet address = 192.249.249.1
        ttl = 86400 (1D)

------------
Name:    wormhole.movie.edu
Addresses:  192.253.253.1, 192.249.249.1

> set d2 
> wormhole 
Server:  terminator.movie.edu
Address:  0.0.0.0

 This time the query is also shown.

------------
SendRequest(  ), len 36
    HEADER:
        opcode = QUERY, id = 6814, rcode = NOERROR
        header flags:  query, want recursion
        questions = 1,  answers = 0,  authority records = 0,
      additional = 0

    QUESTIONS:
        wormhole.movie.edu, type = A, class = IN

------------
------------
Got answer (164 bytes):

 The answer is the same as above.

The lines between the dashes are the query and response messages. As promised, we'll go through the contents of the messages. DNS packets comprise five sections: Header, Question, Answer, Authority, and Additional.

Header section

The header section is present in every query and response message. The operation code nslookup reports is always QUERY. There are other opcodes for asynchronous notification of zone changes (NOTIFY) and for dynamic updates (UPDATE), but nslookup doesn't see those because it just sends regular queries and receives responses.

The ID in the header is used to associate a response with a query and detect duplicate queries or responses. You have to look in the header flags to see which messages are queries and which are responses. The string want recursion means that this is a recursive query. The flag is parroted in the response. The string auth. answer means that this response is authoritative. In other words, the response is from the name server's authoritative data, not from its cache. The response code, rcode, can be one of no error, server failure, name error (also known as nxdomain or nonexistent domain), not implemented, or refused. The server failure, name error, not implemented, and refused response codes cause the nslookup "Server failed," "Nonexistent domain," "Not implemented," and "Query refused" errors, respectively. The last four entries in the header section are counters—they indicate how many resource records there are in each of the next four sections.

Question section: There is always one question in a DNS message; it includes the domain name and the requested datatype and class. There is never more than one question in a DNS message—the capability of handling more than one would require a redesign of the message format. For one thing, the single authority bit would have to be changed because the answer section could contain a mix of authoritative and nonauthoritative answers. In the present design, setting the authoritative answer bit means that the name server is authoritative for the zone that contains the domain name in the question section.

Answer section: This section contains the resource records that answer the question. There can be more than one resource record in the response. For example, if the host is multihomed, there will be more than one address resource record.

Authority section: The authority section is where name server records are returned. When a response refers the querier to some other name servers, those name servers are listed here.

Additional section: The additional records section adds information that may complete the information included in other sections. For instance, if a name server is listed in the authority section, the name server's address may be included in the additional records section. After all, to contact the name server, you need to have its address.

For you sticklers for detail, there is a time when the number of questions in a DNS message isn't one: in an inverse query, when it's zero. In an inverse query, there is one answer in the query message, and the question section is empty. The name server fills in the question. But, as we said, inverse queries are almost nonexistent.

12.6.2 Querying Like a BIND Name Server

You can make nslookup send out the same query message a name server would. Name servers' query messages aren't that much different from resolvers' query messages in the first place. The primary difference in the query messages is that resolvers request recursive resolution and name servers seldom do. Requesting recursion is the default with nslookup, so you have to explicitly turn it off. The difference in operation between a resolver and a name server is that the resolver applies the search list, and the name server doesn't. By default, nslookup applies the search list, so that must be explicitly turned off as well. Of course, judicious use of the trailing dot will have the same effect.

In raw nslookup terms, this means that to query like a resolver, you use nslookup's default settings. To query like a name server, use set norecurse and set nosearch. On the command line, that's nslookup -norecurse -nosearch.

When a BIND name server receives a query, it looks for the answer in its authoritative data and in its cache. If it doesn't have the answer and it is authoritative for the zone, the name server responds that the name doesn't exist or that there are no records of the type sought. If the name server doesn't have the answer and it is not authoritative for the zone, it starts walking up the namespace looking for NS records. There are always NS records somewhere higher in the namespace. As a last resort, it uses the NS records for the root zone, the highest level.

If the name server has received a nonrecursive query, it responds to the querier by returning the NS records that it found. On the other hand, if the original query was a recursive query, the name server queries the remote name servers in the NS records that it found. When the name server receives a response from one of the remote name servers, it caches the response and, if necessary, repeats this process. The remote server's response either has the answer to the question or contains a list of name servers lower in the namespace and closer to the answer.

Let's assume for our example that we are trying to satisfy a recursive query and that we didn't find any NS records until we checked the gov zone. That is, in fact, the case when we ask the name server on relay.hp.com about www.whitehouse.gov—it doesn't find any NS records until the gov zone. From there we switch servers to a gov name server and ask the same question. It directs us to the whitehouse.gov servers. We then switch to a whitehouse.gov name server and ask the same question:

% nslookup 
Default Server:  relay.hp.com
Address:  15.255.152.2

> set norec             —Query like a name server: turn off recursion
> set nosearch          —Turn off the search list
> www.whitehouse.gov    —We don't need to dot-terminate since we've turned
                       —search off
Server:  relay.hp.com
Address:  15.255.152.2

Name:    www.whitehouse.gov
Served by:
- I.ROOT-SERVERS.NET
          192.36.148.17
          gov
- E.ROOT-SERVERS.NET
          192.203.230.10
          gov
- D.ROOT-SERVERS.NET
          128.8.10.90
          gov
- B.ROOT-SERVERS.NET
          128.9.0.107
          gov
- C.ROOT-SERVERS.NET
          192.33.4.12
          gov
- A.ROOT-SERVERS.NET
          198.41.0.4
          gov
- H.ROOT-SERVERS.NET
          128.63.2.53
          gov
- G.ROOT-SERVERS.NET
          192.112.36.4
          gov
- F.ROOT-SERVERS.NET
          192.5.5.241
          gov

Switch to a gov name server (you may have to turn recursion back on temporarily if your name server doesn't have the address of the gov name server already cached):

> server e.root-servers.net
Default Server:  e.root-servers.net
Address: 192.203.230.10

Ask the same question of the gov name server. It will refer us to name servers closer to our desired answer:

> www.whitehouse.gov.
Server:  e.root-servers.net
Address:  192.203.230.10

Name:    www.whitehouse.gov
Served by:
- DNSAUTH1.SYS.GTEI.NET

          whitehouse.gov
- DNSAUTH2.SYS.GTEI.NET

          whitehouse.gov
- DNSAUTH3.SYS.GTEI.NET

          whitehouse.gov

Switch to a whitehouse.gov name server—any of them will do:

> server dnsauth2.sys.gtei.net.
Default Server:  dnsauth2.sys.gtei.net
Address:  4.2.49.3

> www.whitehouse.gov.
Server:  sec1.dns.psi.net
Address:  38.8.92.2

Name:    www.whitehouse.gov
Addresses:  198.137.240.91, 198.137.240.92

Hopefully, this example gives you a feeling for how name servers look up domain names. If you need to refresh your understanding of what this looks like graphically, flip back to Figures Figure 2-12 and Figure 2-13.

Before we move on, notice that we asked each of the servers the very same question: "What's the address of www.whitehouse.gov ?" What do you think would happen if the gov name server had already cached www.whitehouse.gov 's address itself? The gov name server would have answered the question out of its cache instead of referring you to the whitehouse.gov name servers. Why is this significant? Suppose you messed up a particular host's address in your zone. Someone points it out to you, and you clean up the problem. Even though your name server now has the correct data, some remote sites find the old, messed-up data when they look up the domain name of the host. One of the name servers that serves a zone higher up in the namespace, such as a root name server, has cached the incorrect data; when it receives a query for that host's address, it returns the incorrect data instead of referring the querier to your name servers. What makes this problem hard to track down is that only one of the "higher up" name servers has cached the incorrect data, so only some of the remote lookups get the wrong answer—the ones that use this server. Fun, huh? Eventually, though, the "higher up" name server will time out the old record. If you're pressed for time, you can contact the administrators of the remote name server and ask them to restart named to flush the cache. Of course, if the remote name server is an important, much-used name server, they may tell you where to go with that suggestion.

12.6.3 Zone Transfers

nslookup can be used to transfer a whole zone using the ls command. This feature is useful for troubleshooting, for figuring out how to spell a remote host's domain name, or just for counting how many hosts are in some remote zone. Since the output can be substantial, nslookup allows you to redirect the output to a file. If you want to bail out in the middle of a transfer, you can interrupt it by typing your interrupt character.

Beware: some name servers won't let you pull a copy of their zones, either for security reasons or to limit the load placed on them. The Internet is a friendly place, but administrators must defend their turf.

Let's look at the movie.edu zone. As you can see in the following output, all the zone data is listed—the SOA record is listed twice, which is an artifact of how the data is exchanged during the zone transfer. Since some nslookup s only show you address and name server records by default, we specify the -d option to retrieve the whole zone:

% nslookup
Default Server:  terminator.movie.edu
Address:  0.0.0.0

> ls -d movie.edu.
[terminator.movie.edu]
$ORIGIN movie.edu.
@                       1D IN SOA       terminator al.robocop (
                                        2000091400      ; serial
                                        3H              ; refresh
                                        1H              ; retry
                                        4W2D            ; expiry
                                        1H )            ; minimum

                        1D IN NS        terminator
                        1D IN NS        wormhole
wormhole                1D IN A         192.249.249.1
                        1D IN A         192.253.253.1
wh249                   1D IN A         192.249.249.1
robocop                 1D IN A         192.249.249.2
bigt                    1D IN CNAME     terminator
cujo                    1D IN TXT       "Location:" "machine" "room" "dog" "house"
wh253                   1D IN A         192.253.253.1
wh                      1D IN CNAME     wormhole
shining                 1D IN A         192.253.253.3
terminator              1D IN A         192.249.249.3
localhost               1D IN A         127.0.0.1
fx                      1D IN NS        bladerunner.fx
bladerunner.fx          1D IN A         192.253.254.2
fx                      1D IN NS        outland.fx
outland.fx              1D IN A         192.253.254.3
fx                      1D IN NS        huskymo.boulder.acmebw.com.
                        1D IN NS        tornado.acmebw.com.
dh                      1D IN CNAME     diehard
carrie                  1D IN A         192.253.253.4
diehard                 1D IN A         192.249.249.4
misery                  1D IN A         192.253.253.2
@                       1D IN SOA       terminator al.robocop (
                                        2000091400      ; serial
                                        3H              ; refresh
                                        1H              ; retry
                                        4W2D            ; expiry
                                        1H )            ; minimum

Now let's say you missed a record in the beginning of the zone data, one that flew off the top of your screen. nslookup lets you save the listing of a zone to a file:

 > ls -d movie.edu  > /tmp/movie.edu    —List all data into /tmp/movie.edu
[terminator.movie.edu]
Received 25 answers (25 records).

Some versions of nslookup even support a built-in view command that sorts and displays the contents of a zone listing from interactive mode. In the latest BIND 8 releases, though, view is broken, and it isn't supported by BIND 9's nslookup as of 9.1.0.

I l@ve RuBoard