Chapter 8. DNS Resource Records

DNS records have a binary representation which is used internally in a DNS application e.g. BIND and when transferred between DNS servers. They also also have a text format which is used in a zone files.

Zone File Format
DNS Generic Record Formats
List of Record Types

A - IPv4 Address Record
A6 - IPv6 Address Record
CNAME - Host Alias Record
DNAME - Delegate Reverse Name Record
HINFO - System Information Record
KEY - DNSSEC Public Key Record
MX - Mail Exchanger Record
NS - Name Server Record
NXT - DNSSEC Content Record
PTR - Pointer Record
SIG - DNSSEC Signature Record
SOA - Start of Authority Record
SRV - Services Record
TXT - Text Record

Zone File Directives

$ORIGIN
$INCLUDE
$TTL

The SPF record is defined using a TXT record but is not (July 2004) an IETF RFC.

The DNS system defines a number of Resource Records (RRs). The text representation of these records are stored in zone files.

Zone file example

; zone file for mydomain.com
$TTL 12h    ; default TTL for zone
@         IN      SOA   ns1.mydomain.com. root.mydomain.com. (
                        2003080800 ; se = serial number
                        3h         ; ref = refresh
                        15m        ; ret = update retry
                        3w         ; ex = expiry
                        3h         ; min = minimum
                        )
              IN      NS      ns1.mydomain.com.
              IN      MX  10  mail.anotherdomain.com.
joe           IN      A       192.168.254.3
www           IN      CNAME   joe 

The above example shows a very simple but fairly normal zone file. The following notes apply to zone files:

1. Zone files consist of Commnents, Directives and Resource Records
2. Comments start with ';' (semicolon) and are assumed to continue to the end of the line. Comments can occupy a whole line or part of a line as shown in the above example.
3. Directives start with '$' and are standardized (defined in RFC 1035) - $TTL, $ORIGIN and $INCLUDE. BIND additionally provides the non-standard $GENERATE directive.
4. There are a number of Resource Record types defined in RFC 1035 and augmented by subsequent RFCs. Resouce Records have the generic format:

   name ttl  class   rr     parameter
   

   The value of 'parameter' is defined by the record and is described for each Resource Record type in the sections below.
5. The $TTL should be present and appear before the first Resource Record (BIND 9).
6. The first Resource Record must be the SOA record.

Resource records are always represented by textual records. The generic representation of each Resource Record is shown below:

Note: The record format shown below is as defined in the RFCs and is used internally or when transferring information across a network e.g. during a DNS XFER. Do not confuse this with the format you use to define an entry in a zone source file.

Where:

The current DNS RFCs define the following Resource Record Types:

Note: There are a number of other record types which were defined over the years and are no longer actively supported these include MD, MF, MG, MINFO, MR, NULL. A full list of DNS Record Types may be obtained from IANA DNS Parameters.

Directives start with '$' and are standardized (defined in RFC 1035) - $TTL, $ORIGIN and $INCLUDE. BIND additionally provides the non-standard $GENERATE directive.