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230 Chapter 17. Berkeley Internet Name Domain (BIND)

IN NS dns1.domain.com.

IN NS dns2.domain.com.

Figure 17-14. Example NS records

• PTR — PoinTeR record, designed to point to another part of the namespace.

PTR records are primarily used for reverse name resolution, as they point IP addresses back to a

particular name. See Section 17.2.2.4 for more examples of PTR records in use.

• SOA — Start Of Authority record, proclaiming important authoritative information about the names-

pace to the nameserver.

Located after the directives, an SOA record is the ﬁrst resource record in a zone ﬁle.

@ IN SOA

primary-name-server hostmaster-email (

serial-number

time-to-refresh

time-to-retry

time-to-expire

minimum-TTL )

Figure 17-15. Sample SOA record conﬁguration

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serial-number value is used by the slave to determine if it is using outdated zone data and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 (in seconds).

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

17-1 shows an amount of time in seconds and the equivalent time in another format.

Seconds Other Time Units

60 1M

1800 30M

3600 1H

10800 3H

21600 6H

43200 12H

86400 1D

1 2 ... 225 226 227 228 229 230 231 232 233 234 235 ... 281 282

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