Red Hat LINUX 7.2 Reference Manual page 235

Section 14.2:BIND Configuration Files
• SOA — Start Of Authority record, proclaiming important authoritative information about the
namespace to the nameserver.
Located after the directives, an SOA record is the first resource record in a zone file.
Figure 14–15 Sample SOA record configuration
@ IN
<hostmaster-email> (
<time-to-retry>
<time-to-expire>
<minimum-TTL> )
The @ symbol places the $ORIGIN directive (or the zone's name, if the $ORIGIN directive is not
set) as the namespace being defined by this SOA resource record. The primary nameserver that is
authoritative for this domain is used for the <primary-name-server> , and the email of the
person to contact about this namespace is substituted for the <hostmaster-email> .
The <serial-number> is incremented every time you change the zone file so that named will
know that it should reload this zone. The <time-to-refresh> tells any slave servers how
long to wait before asking the master nameserver if any changes have been made to the zone. The
<serial-number> value is used by the slave to determine if it is using outdated zone data and
should refresh it.
The <time-to-retry> tells the slave nameserver the interval to wait before issuing another
refresh request, if the master nameserver is not answering. If the master has not replied to a refresh
request before the <time-to-expire> elapses, the slave stops responding as an authority for
requests concerning that namespace.
The <minimum-TTL> requests that other nameservers cache the zone's information for at least
this amount of time.
With BIND, all times refer to seconds. However, you can also use abbreviations for other units
of time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). The table in
Table 14–1, Seconds compared to other time units shows an amount of time in seconds and the
equivalent time in another format.
Table 14–1 Seconds compared to other time units
Seconds
60
1800
<primary-name-server>
SOA
<serial-number>
<time-to-refresh>
Other Time Units
1M
30M
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