Red Hat LINUX 7.2 Reference Manual page 233

Section 14.2:BIND Configuration Files
• $TTL — Sets the default Time to Live (TTL) value for the zone. This is the number, in seconds,
given to nameservers that tells how long the zone's resource records should continue to be valid.
A resource record can contains its own TTL value, which would override this directive.
Increasing this value tells remote nameservers to cache this zone's information for a longer time.
This reduces the number of queries made concerning this zone, but it also lengthens the amount
of time required to proliferate resource record changes.
Zone File Resource Records
Zone file resource records contain columns of data, separated by whitespace, that define the record.
All zone file resource records are assigned a particular type, which designates the record's purpose.
The following types of resource records are the most commonly used:
• A — Address record, which specifies an IP address to assign to a name.
Figure 14–7 Sample A record configuration
<host>
If the <host> value is omitted, then an A record points to a default IP address for the top of the
namespace. This system will be the target of all non-FQDN requests.
Consider the following A record examples for the domain.com zone file:
Figure 14–8 Example A records
server1
Requests for domain.com are pointed to 10.0.1.3, while requests for server1.domain.com
are pointed to 10.0.1.5.
• CNAME — Canonical name record, which tells the nameserver that one name is also known as
another.
Figure 14–9 Sample CNAME record configuration
<alias-name>
In Figure 14–9, Sample CNAME record configuration, any requests sent to the <alias-name>
will point to the host named <real-name> . CNAME records are most commonly used to point
services that use a common naming scheme to the correct host.
Consider the example in Figure 14–10, Example CNAME record, where an A record sets a particular
hostname to an IP address and a CNAME record points the commonly used www hostname to it.
Figure 14–10 Example CNAME record
server1
<IP-address>
IN A
IN A
IN A
IN CNAME
IN A
10.0.1.3
10.0.1.5
<real-name>
10.0.1.5
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