Olaf Kirch - Linux Network Administrator Guide, Second Edition

;I.ROOT-SERVERS.NET. 3600000 A 192.36.148.17

;. 3600000 NS J.ROOT-SERVERS.NET.

;J.ROOT-SERVERS.NET. 3600000 A 198.41.0.10

;. 3600000 NS K.ROOT-SERVERS.NET.

;K.ROOT-SERVERS.NET. 3600000 A 193.0.14.129

;. 3600000 NS L.ROOT-SERVERS.NET.

;L.ROOT-SERVERS.NET. 3600000 A 198.32.64.12

;. 3600000 NS M.ROOT-SERVERS.NET.

;M.ROOT-SERVERS.NET. 3600000 A 202.12.27.33

;

Example 6.11: The named.hosts File

;

; /var/named/named.hosts Local hosts at the brewery

; Origin is vbrew.com

;

@ IN SOA vlager.vbrew.com. janet.vbrew.com. (

2000012601; serial

86400; refresh: once per day

3600; retry: one hour

3600000; expire: 42 days

604800; minimum: 1 week

)

IN NS vlager.vbrew.com.

;

; local mail is distributed on vlager

IN MX 10 vlager

;

; loopback address

localhost. IN A 127.0.0.1

;

; Virtual Brewery Ethernet

vlager IN A 172.16.1.1

vlager-if1 IN CNAME vlager

; vlager is also news server

news IN CNAME vlager

vstout IN A 172.16.1.2

vale IN A 172.16.1.3

;

; Virtual Winery Ethernet

vlager-if2 IN A 172.16.2.1

vbardolino IN A 172.16.2.2

vchianti IN A 172.16.2.3

vbeaujolais IN A 172.16.2.4

;

; Virtual Spirits (subsidiary) Ethernet

vbourbon IN A 172.16.3.1

vbourbon-if1 IN CNAME vbourbon

Example 6.12: The named.local File

;

; /var/named/named.local Reverse mapping of 127.0.0

; Origin is 0.0.127.in-addr.arpa.

;

@ IN SOA vlager.vbrew.com. joe.vbrew.com. (

1; serial

360000; refresh: 100 hrs

3600; retry: one hour

3600000; expire: 42 days

360000; minimum: 100 hrs

)

IN NS vlager.vbrew.com.

1 IN PTR localhost.

Example 6.13: The named.rev File

;

; /var/named/named.rev Reverse mapping of our IP addresses

; Origin is 16.172.in-addr.arpa.

;

@ IN SOA vlager.vbrew.com. joe.vbrew.com. (

16; serial

86400; refresh: once per day

3600; retry: one hour

3600000; expire: 42 days

604800; minimum: 1 week

)

IN NS vlager.vbrew.com.

; brewery

1.1 IN PTR vlager.vbrew.com.

2.1 IN PTR vstout.vbrew.com.

3.1 IN PTR vale.vbrew.com.

; winery

1.2 IN PTR vlager-if2.vbrew.com.

2.2 IN PTR vbardolino.vbrew.com.

3.2 IN PTR vchianti.vbrew.com.

4.2 IN PTR vbeaujolais.vbrew.com.

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: