11.5. Zone-Description Files

According to the /etc/named.conf configurations file, there should be three files in the /var/named directory. These are the following:

named.ca Links to the root servers are stored in this file. This file is downloaded from the intenic.net server; therefore, it should not be edited, and I will not be considering it.
sitename.zone This file is responsible for resolving the sitename.com name to its IP address.
10.12.190.in-addr.arpa.zone This file is responsible for resolving the 190.12.10.* network addresses to their corresponding host names .

The sitename.zone file contents may look like the following:

 @     IN     SOA    ns.sitename.com root.sitename.com (                     1 ; serial                     28800 ; refresh                     7200 ; retry                     604800 ; expire                     86400 ; ttl                     ) IN    NS     ns.sitename.com. IN    MX     10 mail.sitename.com. ns    A      190.12.10.1 mail  A      190.12.10.2

The functions of the main directives used to configure DNS records are the following:

Start of authority ( SOA ) Specifies the main information, including the administrator's email address and such information as the frequency, with which records are updated, the TTL of cached records, and so on.
Address ( A ) Indicates the computer name and IP address.
Canonical name ( CNAME ) Specifies a synonym for the real domain name in the type A record.
Pointer ( PTR ) Shows a domain name by its IP address.
Text ( TXT ) Denotes freestyle descriptive information.
Responsible person ( RP ) Specifies the email address of the person responsible for the operation of the service.
Host information ( HINFO ) Designates information about the computer, such as the operating system type and equipment installed.

For security reasons, the HINFO and TXT records are not used; therefore, they will not reveal any information to hackers. Hackers should not be given any information about the computer, however innocent it may seem, and information about the computer's operating system and equipment is far from innocent. The HINFO and TXT records are purely informational and do not contain any data affecting the server's operation.

Now, return to the sitename.zone file and consider its contents. In the first records (of the IN SOA type), the zone is described. First, the name of the DNS server (ns.sitename.com) and the person responsible for the record ( root@sitename.com ) are given. The parameters in the parentheses are each specified on a separate line for convenience. The first parameter is the serial number. Increment this parameter by 1 after each modification or replace it with the date the record was last modified. By this value, other servers will find out whether the record was modified.

The refresh parameter sets the frequency, with which other servers must update their information. In case of an error, the server has to try again after the period specified in the retry parameter.

The expire parameter specifies when cached-zone information will no longer be valid. The ttl parameter defines the entry's minimum TTL on caching servers.

These parameters inform the rest of the DNS how to refresh the information about the zone controlled by your DNS server.

The next record is of the NS type; there can be several such records. In this case, NS stands for name server. This record describes the DNS servers responsible for this zone. All other DNS participants will use these servers to resolve the sitename.com symbolic name to its IP address.

Next, mail exchange (MX) records can follow. DNS servers use these records to determine where to send mail that comes to the sitename.com domain. In this example, this is the mail.sitename.com server. The number in front of the server name specifies the MX entry's priority. If there are multiple MX records, they will be used in the order of their priorities.

Note	The NS and MX entries must terminate in a period.

The last records are used for the reverse lookup. They are of the following format:

 name  A      address

There are two such records in the example:

 ns    A      190.12.10.1 mail  A      190.12.10.2

This means that the ns. servername .com and mail.servername.com symbolic names resolve to the 190.12.10.1 and 190.12.10.2 IP addresses, respectively.