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17.2.2. Editing Zone Files

As outlined in Section 17.1.1, “Nameserver Zones”, zone files contain information about a namespace. They are stored in the named working directory located in /var/named/ by default, and each zone file is named according to the file option 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.
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Table 17.5. The named service zone files
Path Description
/var/named/ The working directory for the named service. The nameserver is not allowed to write to this directory.
/var/named/slaves/ The directory for secondary zones. This directory is writable by the named service.
/var/named/dynamic/ The directory for other files, such as dynamic DNS (DDNS) zones or managed DNSSEC keys. This directory is writable by the named service.
/var/named/data/ The directory for various statistics and debugging files. This directory is writable by the named service.
A zone file consists of 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. While the directives are optional, the resource records are required in order to provide name service to a zone.
All directives and resource records should be entered on individual lines.

17.2.2.1. Common Directives
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Directives begin with the dollar sign character followed by the name of the directive, and usually appear at the top of the file. The following directives are commonly used in zone files:
$INCLUDE
The $INCLUDE directive allows you to include another file at the place where it appears, so that other zone settings can be stored in a separate zone file.

Example 17.7. Using the $INCLUDE directive

$INCLUDE /var/named/penguin.example.com
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$ORIGIN
The $ORIGIN directive allows you to append the domain name to unqualified records, such as those with the host name only. Note that the use of this directive is not necessary if the zone is specified in /etc/named.conf, since the zone name is used by default.
In Example 17.8, “Using the $ORIGIN directive”, any names used in resource records that do not end in a trailing period are appended with example.com.

Example 17.8. Using the $ORIGIN directive

$ORIGIN example.com.
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$TTL
The $TTL directive allows you to set the default Time to Live (TTL) value for the zone, that is, how long is a zone record 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 propagate resource record changes.

Example 17.9. Using the $TTL directive

$TTL 1D
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17.2.2.2. Common Resource Records
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The following resource records are commonly used in zone files:
A
The Address record specifies an IP address to be assigned to a name. It takes the following form: 
hostname IN A IP-address
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If the hostname value is omitted, the record will point to the last specified hostname.
In Example 17.10, “Using the A resource record”, the requests for server1.example.com are pointed to 10.0.1.3 or 10.0.1.5.

Example 17.10. Using the A resource record

server1  IN  A  10.0.1.3
         IN  A  10.0.1.5
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CNAME
The Canonical Name record maps one name to another. Because of this, this type of record is sometimes referred to as an alias record. It takes the following form: 
alias-name IN CNAME real-name
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CNAME records are most commonly used to point to services that use a common naming scheme, such as www for Web servers. However, there are multiple restrictions for their usage:
  • CNAME records should not point to other CNAME records. This is mainly to avoid possible infinite loops.
  • CNAME records should not contain other resource record types (such as A, NS, MX, etc.). The only exception are DNSSEC related records (that is, RRSIG, NSEC, etc.) when the zone is signed.
  • Other resource record that point to the fully qualified domain name (FQDN) of a host (that is, NS, MX, PTR) should not point to a CNAME record.
In Example 17.11, “Using the CNAME resource record”, the A record binds a host name to an IP address, while the CNAME record points the commonly used www host name to it.

Example 17.11. Using the CNAME resource record

server1  IN  A      10.0.1.5
www      IN  CNAME  server1
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MX
The Mail Exchange record specifies where the mail sent to a particular namespace controlled by this zone should go. It takes the following form: 
IN MX preference-value email-server-name
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The email-server-name is a fully qualified domain name (FQDN). 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.
In Example 17.12, “Using the MX resource record”, 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.

Example 17.12. Using the MX resource record

example.com.  IN  MX  10  mail.example.com.
              IN  MX  20  mail2.example.com.
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NS
The Nameserver record announces authoritative nameservers for a particular zone. It takes the following form: 
IN NS nameserver-name
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The nameserver-name should be a fully qualified domain name (FQDN). Note that when two nameservers are listed as authoritative for the domain, it is not important whether these nameservers are secondary nameservers, or if one of them is a primary server. They are both still considered authoritative.

Example 17.13. Using the NS resource record

IN  NS  dns1.example.com.
IN  NS  dns2.example.com.
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PTR
The Pointer record points to another part of the namespace. It takes the following form: 
last-IP-digit IN PTR FQDN-of-system
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The last-IP-digit directive is the last number in an IP address, and the FQDN-of-system is a fully qualified domain name (FQDN).
PTR records are primarily used for reverse name resolution, as they point IP addresses back to a particular name. See Section 17.2.2.4.2, “A Reverse Name Resolution Zone File” for more examples of PTR records in use.
SOA
The Start of Authority record announces important authoritative information about a namespace to the nameserver. Located after the directives, it is the first resource record in a zone file. It takes the following form: 
@  IN  SOA  primary-name-server hostmaster-email (
       serial-number
       time-to-refresh
       time-to-retry
       time-to-expire
       minimum-TTL )
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The directives are as follows:
  • 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-name-server directive is the host name of the primary nameserver that is authoritative for this domain.
  • The hostmaster-email directive is the email of the person to contact about the namespace.
  • The serial-number directive is a numerical value incremented every time the zone file is altered to indicate it is time for the named service to reload the zone.
  • The time-to-refresh directive is the numerical value secondary nameservers use to determine how long to wait before asking the primary nameserver if any changes have been made to the zone.
  • The time-to-retry directive is a numerical value used by secondary nameservers to determine the length of time to wait before issuing a refresh request in the event that the primary nameserver is not answering. If the primary server has not replied to a refresh request before the amount of time specified in the time-to-expire directive elapses, the secondary servers stop responding as an authority for requests concerning that namespace.
  • In BIND 4 and 8, the minimum-TTL directive is the amount of time other nameservers cache the zone's information. In BIND 9, it defines how long negative answers are cached for. Caching of negative answers can be set to a maximum of 3 hours (that is, 3H).
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). Table 17.6, “Seconds compared to other time units” shows an amount of time in seconds and the equivalent time in another format.
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Table 17.6. Seconds compared to other time units
Seconds Other Time Units
60 1M
1800 30M
3600 1H
10800 3H
21600 6H
43200 12H
86400 1D
259200 3D
604800 1W
31536000 365D

Example 17.14. Using the SOA resource record

@  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
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17.2.2.3. Comment Tags
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Additionally to resource records and directives, a zone file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to the user. Any text after the semicolon character to the end of the line is considered a comment. For example: 
   604800  ; expire after 1 week
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17.2.2.4. Example Usage
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The following examples show the basic usage of zone files.
17.2.2.4.1. A Simple Zone File
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Example 17.15, “A simple zone file” demonstrates the use of standard directives and SOA values.

Example 17.15. A simple 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.
dns1      IN  A      10.0.1.1
          IN  AAAA   aaaa:bbbb::1
dns2      IN  A      10.0.1.2
          IN  AAAA   aaaa:bbbb::2
;
;
@         IN  MX     10  mail.example.com.
          IN  MX     20  mail2.example.com.
mail      IN  A      10.0.1.5
          IN  AAAA   aaaa:bbbb::5
mail2     IN  A      10.0.1.6
          IN  AAAA   aaaa:bbbb::6
;
;
; This sample zone file illustrates sharing the same IP addresses
; for multiple services:
;
services  IN  A      10.0.1.10
          IN  AAAA   aaaa:bbbb::10
          IN  A      10.0.1.11
          IN  AAAA   aaaa:bbbb::11

ftp       IN  CNAME  services.example.com.
www       IN  CNAME  services.example.com.
;
;
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In this example, the authoritative nameservers are set as dns1.example.com and dns2.example.com, and are tied to the 10.0.1.1 and 10.0.1.2 IP addresses respectively using the A record.
The email servers configured with the MX records point to mail and mail2 via A records. Since these names do not end in a trailing period, the $ORIGIN domain is placed after them, expanding them to mail.example.com and mail2.example.com.
Services available at the standard names, such as www.example.com (WWW), are pointed at the appropriate servers using the CNAME record.
This zone file would be called into service with a zone statement in the /etc/named.conf similar to the following: 
zone "example.com" IN {
  type master;
  file "example.com.zone";
  allow-update { none; };
};
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17.2.2.4.2. A Reverse Name Resolution Zone File
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A reverse name resolution zone file is used to translate an IP address in a particular namespace into an fully qualified domain name (FQDN). It looks very similar to a standard zone file, except that the PTR resource records are used to link the IP addresses to a fully qualified domain name as shown in Example 17.16, “A reverse name resolution zone file”.

Example 17.16. A reverse name resolution zone file

$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.
;
1  IN  PTR  dns1.example.com.
2  IN  PTR  dns2.example.com.
;
5  IN  PTR  server1.example.com.
6  IN  PTR  server2.example.com.
;
3  IN  PTR  ftp.example.com.
4  IN  PTR  ftp.example.com.
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In this example, IP addresses 10.0.1.1 through 10.0.1.6 are pointed to the corresponding fully qualified domain name.
This zone file would be called into service with a zone statement in the /etc/named.conf file similar to the following: 
zone "1.0.10.in-addr.arpa" IN {
  type master;
  file "example.com.rr.zone";
  allow-update { none; };
};
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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.
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