Chapter 16. Berkeley Internet Name Domain (BIND)
237
over others. The
MX
resource record with the lowest
preference value
is preferred over
E
E
the others, but you can set multiple email servers with the same value to distribute email traffic
between them.
The
email server name
may be a hostname or FQDN, as long as it points to the correct
E
E
system.
IN
MX
10
mail.domain.com.
IN
MX
20
mail2.domain.com.
In this example, the first
mail.domain.com
email server is preferred to the
mail2.domain.com
email server when receiving email destined for the
domain.com
domain.
  NS
  NameServer record, which announces the authoritative nameservers for a particular zone.
This is an example of an
NS
record:
IN
NS
nameserver name
E
I
The
nameserver name should be a FQDN.
E
E
Next, two nameservers are listed as authoritative for the domain. It is not important whether these
nameservers are slaves or if one is a master; they are both still considered authoritative.
IN
NS
dns1.domain.com.
IN
NS
dns2.domain.com.
  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 16.2.2.4 for more examples of
PTR
records in use.
  SOA
  Start Of Authority record, proclaims important authoritative information about a namespace
to the nameserver.
Located after the directives, an
SOA
resource record is the first resource record in a zone file.
The following example shows the basic structure of an
SOA
record:
@
IN
SOA
primary name server
hostmaster email
(
E
I
E
I
serial number
E
I
time to refresh
E
I
time to retry
E
I
time to expire
E
I
minimum TTL
)
E
I
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
E
E
person to contact about this namespace is substituted for the
hostmaster email .
E
E
The
serial number is incremented every time you change the zone file so that
named
will
E
E
know that it should reload this zone. The
time to refresh
tells any slave servers how
E
E
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serial number value is used by the slave to determine if it is using outdated zone data and
E
E
should refresh it.
The
time to retry
tells the slave nameserver the interval to wait before issuing another
E
E
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
E
E
requests concerning that namespace.
The
minimum TTL
requests that other nameservers cache the zone's information for at least
E
E
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
16 1 shows an amount of time in seconds and the equivalent time in another format.






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