17.2.2.2. Common Resource Records

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
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
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
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
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
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.
NS
The Nameserver record announces authoritative nameservers for a particular zone. It takes the following form: 
IN NS nameserver-name
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.
PTR
The Pointer record points to another part of the namespace. It takes the following form: 
last-IP-digit IN PTR FQDN-of-system
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 )
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.

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