Domain Name System (DNS) zone files

A DNS zone file is a text database file that describes a portion of the domain name system (DNS) called a DNS zone. A zone contains mappings between domain names and IP addresses and other resources, organized in form of resource records (RR).

There are different types of DNS Resource Records (RRs). The most common types of Resource Records (RRs) are listed below.

"A" (Address) type Resource Record

An "A" (Address) type Resource Record is an "IPv4 host record" and it is the most common type. An "A" type is simply a static mapping of a hostname to an IPv4 address.

"AAAA" (IPv6 Address) type Resource Record

An "AAAA" (IPv6 Address) type Resource Record is an "IPv6 host record", which is used to map a host name to an IPv6 address.

"MX" (Mail eXchanger) Resource Record

An "MX" (Mail eXchanger) Resource Record is specifically for mail servers. It's a special type of service-specifier record. It identifies a mail server for the domain.

"CNAME" (Canonical Name) Resource Record

A "CNAME" (Canonical Name) Resource Record is an alias record. By creating a CNAME record, we can make the same physical server respond to two different hostnames.

"PTR" (Pointer) type Resource Record

A "PTR" (Pointer) type Resource Record resolve IP addresses into hostnames.

"NS" (Name Server) record

"NS" (Name Server) records specify the authoritative DNS servers for a domain.

"SOA" (Start Of Authority) record

"SOA" (Start Of Authority) record is the first record in the zone file. An "SOA" record is only present in a zone file located on authoritative DNS servers. "SOA" (Start Of Authority) specifies

• The primary authoritative DNS server for the zone (domain).

• The e-mail address of the zone's (domain's) administrator.

• Timing related information to the secondary DNS servers like refresh or expire interval, serial number to indicate the version of the zone file etc.

A sample forward lookup zone directive and forward lookup zone file is copied below. You can see the different Resource Records (RRs) like SOA, NS, A, and CNAME in the zone file.

Sample forward lookup zone directive in /etc/named.conf file

zone "omnisecu.com" IN {
        type master;
        file "omnisecu.com.zone.db";
        allow-query { 192.168.1.0/24; };
};

Corresponding forward lookup zone file

$ORIGIN .
$TTL 86400      ; 1 day
omnisecu.com            IN SOA  RHEL04.omnisecu.com. admin.omnisecu.com. (
                                2009072845 ; serial
                                10800      ; refresh (3 hours)
                                900        ; retry (15 minutes)
                                604800     ; expire (1 week)
                                86400      ; minimum (1 day)
                                )
                        NS      RHEL04.omnisecu.com.
                        NS      RHEL05.omnisecu.com.
$ORIGIN omnisecu.com.
ftp                     CNAME   RHEL04.omnisecu.com
RHEL04                  A       192.168.1.104
RHEL05                  A       192.168.1.105
RHEL06                  A       192.168.1.106
www                     A       192.168.1.104

A sample reverse lookup zone directive in /etc/named.conf file and corresponding reverse lookup zone file is copied below.

Sample reverse lookup zone directive in /etc/named.conf file

zone "1.168.192.in-addr.arpa" IN {
type master;
file "1.168.192.in-addr.arpa";
allow-update { none; };
};

Corresponding reverse lookup zone file

$TTL    86400
@       IN SOA RHEL04.omnisecu.com. root.RHEL04.omnisecu.com.(
                                      2009072852 ; Serial
                                      28800      ; Refresh
                                      14400      ; Retry
                                      3600000    ; Expire
                                      86400 )    ; Minimum
        IN      NS      RHEL04.omnisecu.com.
104     IN      PTR     RHEL04.omnisecu.com.
105     IN      PTR     RHEL05.omnisecu.com.
106     IN      PTR     RHEL06.omnisecu.com.