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.