All DNS information is only stored within the DNS server. This information is maintained in the form of resource records (RRs). Resource records are coded in ASCII and contain the following details:
Owner of the information
RR type
Class (e.g. IN for internet)
Validity duration
Data
The format of a Resource Record is described in detail on "Format of a DNS message".
The most important RR types include:
SOA (Start of Authority)
The SOA RR specifies the start of the zone serviced by the DNS name server on which the SOA RR is stored. A system file may contain only one SOA RR per zone. The zone ends at the start of the next zone, i.e. when a new SOA Resource Record is specified.
NS (Name Server)
The NS RR specifies the name of the server responsible for a specific DNS domain.
A (Address)
The Address RR defines the IP address assigned to a DNS name. There should be a separate Address RR for each IP address of a computer.
AAAA (quad A)
The AAAA RR defines the IPv6 address which is assigned to a DNS name. There should be a separate AAAA RR for each IPv6 address of a computer.
PTR (Domain Name Pointer)
The PTR RR defines special names as pointers to other names in the domain.
PTR RRs are primarily used in in-addr.arpa data records to map addresses (the special names) to computer names. PTR names should be unique within a zone.
MX (Mail Exchanger)
The MX RR defines the IP address assigned to a DNS name.
CNAME (Canonical Name)
The CNAME RR can be used to assign an alternative name to a standard host name. The alternative name must not be specified in the name field of other Resource Records. Changes in the standard host name remain transparent to all application programs using the alternative name, i.e. the application programs need not be updated on changing the standard host name.
Information on further Resource Record types is available, for example, in RFC1035. Comprehensive information on the RFCs is available on the home page of the Internet Engineering Task Force (IETF): https://www.ietf.org/