13.3. Zone Files
Zone files contain information about a namespace and are stored in the named working directory, /var/named/, by default. Each zone file is named according to the file option data in the zone statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone.
Each zone file may contain directives and resource records. Directives tell the nameserver to perform tasks or apply special settings to the zone. Resource records define the parameters of the zone and assign identities to individual hosts. Directives are optional, but resource records are required to provide name service to a zone.
All directives and resource records should go on their own individual lines.
Comments can be placed after semicolon characters (;) in zone files.
13.3.1. Zone File Directives
Directives begin with the dollar sign character ($) followed by the name of the directive. They usually appear at the top of the zone file.
The following are commonly used directives:
$INCLUDE — Configures named to include another zone file in this zone file at the place where the directive appears. This allows additional zone settings to be stored apart from the main zone file.
$ORIGIN — Appends the domain name to unqualified records, such as those with the hostname and nothing more.
For example, a zone file may contains the following line:
$ORIGIN example.com
Any names used in resource records that do not end in a trailing period (.) will have example.com appended to them.
Note The use of the $ORIGIN directive is unnecessary if the zone is specified in /etc/named.conf because the zone name is used as the value for the $ORIGIN directive by default.
$TTL — Sets the default Time to Live (TTL) value for the zone. This is the length of time, in seconds, a zone resource record is valid. Each resource record can contain its own TTL value, which overrides this directive.
Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to proliferate resource record changes.
13.3.2. Zone File Resource Records
The primary component of a zone file is its resource records.
There are many types of zone file resource records. The following are used most frequently:
A — Address record, which specifies an IP address to assign to a name, as in this example:
<host> IN A <IP-address>
If the <host> value is omitted, then an A record points to a default IP address for the top of the namespace. This system is the target for all non-FQDN requests.
Consider the following A record examples for the example.com zone file:
IN A 10.0.1.3 server1 IN A 10.0.1.5
Requests for example.com are pointed to 10.0.1.3, while requests for server1.example.com are pointed to 10.0.1.5.
CNAME — Canonical name record, maps one name to another. This type of record is also known as an alias record.
The next example tells named that any requests sent to the <alias-name> should point to the host, <real-name>. CNAME records are most commonly used to point to services that use a common naming scheme, such as www for Web servers.
<alias-name> IN CNAME <real-name>
In the following example, an A record binds a hostname to an IP address, while a CNAME record points the commonly used www hostname to it.
server1 IN A 10.0.1.5 www IN CNAME server1
MX — Mail eXchange record, which tells where mail sent to a particular namespace controlled by this zone should go.
IN MX <preference-value> <email-server-name>
In this example, the <preference-value> allows numerical ranking of the email servers for a namespace, giving preference to some email systems over others. The MX resource record with the lowest <preference-value> is preferred over the others. However, multiple email servers can possess the same value to distribute email traffic evenly among them.
The <email-server-name> may be a hostname or FQDN.
IN MX 10 mail.example.com. IN MX 20 mail2.example.com.In this example, the first mail.example.com email server is preferred to the mail2.example.com email server when receiving email destined for the example.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>
The <nameserver-name> should be an FQDN.
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.example.com. IN NS dns2.example.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 13.3.4 Reverse Name Resolution Zone Files for more examples of PTR records in use.
SOA — Start Of Authority resource 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 resource record:
@ IN SOA <primary-name-server> <hostmaster-email> ( <serial-number> <time-to-refresh> <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 hostname of the primary nameserver that is authoritative for this domain is the <primary-name-server> directive, and the email of the person to contact about this namespace is the <hostmaster-email> directive.
The <serial-number> directive is a numerical value incremented every time the zone file is altered to indicate it is time for named to reload the zone. The <time-to-refresh> directive is the numerical value slave servers use to determine how long to wait before asking the master nameserver if any changes have been made to the zone. The <serial-number> directive is a numerical value used by the slave servers to determine if it is using outdated zone data and should therefore refresh it.
The <time-to-retry> directive is a numerical value used by slave servers to determine the length of time to wait before issuing a refresh request in the event the master nameserver is not answering. If the master has not replied to a refresh request before the amount of time specified in the <time-to-expire> directive elapses, the slave servers stop responding as an authority for requests concerning that namespace.
The <minimum-TTL> directive is the quantity of time other nameservers cache the zone's information.
When configuring BIND, all times are specified in seconds. However, it is possible to use abbreviations when specifying units of time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). The table in Table 13-1 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 259200 3D 604800 1W 31536000 365D Table 13-1. Seconds compared to other time units
The following example illustrates the form an SOA resource record might take when it is populated with real values.
@ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; serial 21600 ; refresh after 6 hours 3600 ; retry after 1 hour 604800 ; expire after 1 week 86400 ) ; minimum TTL of 1 day
13.3.3. Example Zone File
Seen individually, directives and resource records can be difficult to grasp. However, when placed together in a single file, they become easier to understand.
The following example shows a very basic zone file.
$ORIGIN example.com
$TTL 86400
@ IN SOA dns1.example.com. hostmaster.example.com. (
2001062501 ; serial
21600 ; refresh after 6 hours
3600 ; retry after 1 hour
604800 ; expire after 1 week
86400 ) ; minimum TTL of 1 day
IN NS dns1.example.com.
IN NS dns2.example.com.
IN MX 10 mail.example.com.
IN MX 20 mail2.example.com.
IN A 10.0.1.5
server1 IN A 10.0.1.5
server2 IN A 10.0.1.7
dns1 IN A 10.0.1.2
dns2 IN A 10.0.1.3
ftp IN CNAME server1
mail IN CNAME server1
mail2 IN CNAME server2
www IN CNAME server2 |
In this example, standard directives and SOA values are used. The authoritative nameservers are set as dns1.example.com and dns2.example.com, which have A records that tie them to 10.0.1.2 and 10.0.1.3, respectively.
The email servers configured with the MX records point to server1 and server2 via CNAME records. Since the server1 and server2 names do not end in a trailing period (.), the $ORIGIN domain is placed after them, expanding them to server1.example.com and server2.example.com. Through the related A resource records, their IP addresses can be determined.
FTP and Web services, available at the standard ftp.example.com and www.example.com names, are pointed at the appropriate servers using CNAME records.
13.3.4. Reverse Name Resolution Zone Files
A reverse name resolution zone file is used to translate an IP address in a particular namespace into an FQDN. It looks very similar to a standard zone file, except that PTR resource records are used to link the IP addresses to a fully qualified domain name.
A PTR record looks similar to this:
<last-IP-digit> IN PTR <FQDN-of-system> |
The <last-IP-digit> is the last number in an IP address points to a particular system's FQDN.
In the follow example, IP addresses 10.0.1.20 through 10.0.1.25 are pointed to corresponding FQDNs.
$ORIGIN 1.0.10.in-addr.arpa
$TTL 86400
@ IN SOA dns1.example.com. hostmaster.example.com. (
2001062501 ; serial
21600 ; refresh after 6 hours
3600 ; retry after 1 hour
604800 ; expire after 1 week
86400 ) ; minimum TTL of 1 day
IN NS dns1.example.com.
IN NS dns2.example.com.
20 IN PTR alice.example.com.
21 IN PTR betty.example.com.
22 IN PTR charlie.example.com.
23 IN PTR doug.example.com.
24 IN PTR ernest.example.com.
25 IN PTR fanny.example.com. |
This zone file would be called into service with a zone statement in the named.conf file which looks similar to the following:
zone "1.0.10.in-addr.arpa" IN {
type master;
file "example.com.rr.zone";
allow-update { none; };
}; |
There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.