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Unit 6: Domain Name System

NS records are the same but there’s no A records. And because we only have one system notes
handling all three Web, e-mail, and FTP server functions we only need one PTR record. A PTR
(Pointer) record is the opposite of an A record. It has the host part of the IP address and gives the
corresponding hostname. Typically you want a PTR record for every A record in the forward-
lookup file provided the server is in the domain. We don’t have PTR records for the name servers
above because they’re in a different domain (and thus in a different address space).

Why is only the host part of the IP address required in this file? Because the network portion
of the IP address is used when naming the reverse-lookup zone file, and it’s reversed. Because
192.168.1.x is a Class C network, the first three octets make up the network portion of the IP
address so it’s used in the zone file name. Only the last octet specifies the individual host so it’s
used to specify the host in PTR records. With the above example IP address, the zone file would
be named:

db.1.168.192
The reverse-lookup zone file is also situated in the /etc/bind directory. There’s another place this
naming convention is used. Take a look at the start of the SOA record. The domain is specified
as

1.168.192.in-addr.arpa
in-addr.arpa is the default domain for all reverse lookups.

6.1.5 Dns tools, testing, and troubleshooting

When you’re testing changes to your DNS records things may not act the way you expect them
to. What you need is some endurance. Most DNS servers cache lookups. If you make a change to
a zone record on EasyDNS or dyndns.org, or the IP address you pulled from your ISP changes
and ddclient sends the update, it’ll take the DNS servers at EasyDNS or dyndns.org up to 15
minutes to update. Then the DNS server that your desktop system is using to resolve names may
cache the old information for another 20 to 30 minutes.
If you’re using a Windows system to test DNS changes remember that it also has a DNS cache.
You can clear it manually in a DOS window with the command:

ipconfig /flushdns
As a result, if you create a change to your zone records give it at least 45 minutes before you try to
see if the changes had the desired effect. Web browsers also cache name-to-address information.
If you’re using a Web browser to test your changes, you may want to go and delete all the files in
the browser’s cache directory as well.
The above make playing around with dynamic DNS when using a modem sort of a pain. You
have to keep the connection up for for at least 45 minutes because if you disconnect, you’ll pull
a different IP address when you reconnect and your DNS records will have invalid IP addresses.
That’s why I showed you how to run the ping command in the background to keep the dial-up
connection alive.
A DNS problem will likely be in one of three places:
1. The DNS server addresses specified in the TCP/IP configuration on the PC you are using
to do the pinging are not correct.
2. The registrar’s domain record does not have the correct name server hostnames and/or
addresses.

3. The authoritative DNS servers for the domain do not have the domain’s zone records
configured properly.

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