Книга: Linux Network Administrator Guide, Second Edition

Verifying the Name Server Setup

Verifying the Name Server Setup

nslookup is a great tool for checking the operation of your name server setup. It can be used both interactively with prompts and as a single command with immediate output. In the latter case, you simply invoke it as:

$ nslookup
hostname

nslookup queries the name server specified in resolv.conf for hostname. (If this file names more than one server, nslookup chooses one at random.)

The interactive mode, however, is much more exciting. Besides looking up individual hosts, you may query for any type of DNS record and transfer the entire zone information for a domain.

When invoked without an argument, nslookup displays the name server it uses and enters interactive mode. At the › prompt, you may type any domain name you want to query. By default, it asks for class A records, those containing the IP address relating to the domain name.

You can look for record types by issuing:

› set type=type

in which type is one of the resource record names described earlier, or ANY.

You might have the following nslookup session:

$ nslookup
Default Server: tao.linux.org.au
Address: 203.41.101.121
› metalab.unc.edu
Server: tao.linux.org.au
Address: 203.41.101.121
Name: metalab.unc.edu Address: 152.2.254.81
›

The output first displays the DNS server being queried, and then the result of the query.

If you try to query for a name that has no IP address associated with it, but other records were found in the DNS database, nslookup returns with an error message saying " No type A records found." However, you can make it query for records other than type A by issuing the set type command. To get the SOA record of unc.edu, you would issue:

› unc.edu
Server: tao.linux.org.au
Address: 203.41.101.121
*** No address (A) records available for unc.edu
› set type=SOA
› unc.edu
Server: tao.linux.org.au
Address: 203.41.101.121
unc.edu
 origin = ns.unc.edu
 mail addr = host-reg.ns.unc.edu
 serial = 1998111011
 refresh = 14400 (4H)
 retry = 3600 (1H)
 expire = 1209600 (2W)
 minimum ttl = 86400 (1D)
unc.edu name server = ns2.unc.edu
unc.edu name server = ncnoc.ncren.net
unc.edu name server = ns.unc.edu
ns2.unc.edu internet address = 152.2.253.100
ncnoc.ncren.net internet address = 192.101.21.1
ncnoc.ncren.net internet address = 128.109.193.1
ns.unc.edu internet address = 152.2.21.1

In a similar fashion, you can query for MX records:

› set type=MX
› unc.edu
Server: tao.linux.org.au
Address: 203.41.101.121
unc.edu preference = 0, mail exchanger = conga.oit.unc.edu
unc.edu preference = 10, mail exchanger = imsety.oit.unc.edu
unc.edu name server = ns.unc.edu
unc.edu name server = ns2.unc.edu
unc.edu name server = ncnoc.ncren.net
conga.oit.unc.edu internet address = 152.2.22.21
imsety.oit.unc.edu internet address = 152.2.21.99
ns.unc.edu internet address = 152.2.21.1
ns2.unc.edu internet address = 152.2.253.100
ncnoc.ncren.net internet address = 192.101.21.1
ncnoc.ncren.net internet address = 128.109.193.1

Using a type of ANY returns all resource records associated with a given name.

A practical application of nslookup, besides debugging, is to obtain the current list of root name servers. You can obtain this list by querying for all NS records associated with the root domain:

› set type=NS
› .
Server: tao.linux.org.au
Address: 203.41.101.121
Non-authoritative answer:
 (root) name server = A.ROOT-SERVERS.NET
 (root) name server = H.ROOT-SERVERS.NET
 (root) name server = B.ROOT-SERVERS.NET
 (root) name server = C.ROOT-SERVERS.NET
 (root) name server = D.ROOT-SERVERS.NET
 (root) name server = E.ROOT-SERVERS.NET
 (root) name server = I.ROOT-SERVERS.NET
 (root) name server = F.ROOT-SERVERS.NET
 (root) name server = G.ROOT-SERVERS.NET
 (root) name server = J.ROOT-SERVERS.NET
 (root) name server = K.ROOT-SERVERS.NET
 (root) name server = L.ROOT-SERVERS.NET
 (root) name server = M.ROOT-SERVERS.NET
Authoritative answers can be found from:
A.ROOT-SERVERS.NET internet address = 198.41.0.4
H.ROOT-SERVERS.NET internet address = 128.63.2.53
B.ROOT-SERVERS.NET internet address = 128.9.0.107
C.ROOT-SERVERS.NET internet address = 192.33.4.12
D.ROOT-SERVERS.NET internet address = 128.8.10.90
E.ROOT-SERVERS.NET internet address = 192.203.230.10
I.ROOT-SERVERS.NET internet address = 192.36.148.17
F.ROOT-SERVERS.NET internet address = 192.5.5.241
G.ROOT-SERVERS.NET internet address = 192.112.36.4
J.ROOT-SERVERS.NET internet address = 198.41.0.10
K.ROOT-SERVERS.NET internet address = 193.0.14.129
L.ROOT-SERVERS.NET internet address = 198.32.64.12
M.ROOT-SERVERS.NET internet address = 202.12.27.33

To see the complete set of available commands, use the help command in nslookup.

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