Chapter 8. DNS Resource Records

DNS resource records (RRs) describe the characteristics of a zone (or domain) and have a binary or wire-format, which is used in queries and responses, and a text format used in zone files and which is described in this chapter.

Contents

Zone File Format
DNS Generic Record Formats

Zone File Directives

$ORIGIN
$INCLUDE
$TTL
$GENERATE (non-standard BIND only)

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 example.com
$TTL 2d    ; 172800 secs default TTL for zone
$ORIGIN example.com.
@             IN      SOA   ns1.example.com. hostmaster.example.com. (
                        2003080800 ; se = serial number
                        12h        ; ref = refresh
                        15m        ; ret = update retry
                        3w         ; ex = expiry
                        3h         ; min = minimum
                        )
              IN      NS      ns1.example.com.
              IN      MX  10  mail.example.net.
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 Comments, 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 - $ORIGIN and $INCLUDE (defined in RFC 1035) and $TTL (defined in RFC 2308). BIND additionally provides the non-standard $GENERATE directive.
4. There are a number of Resource Record (RR) types defined in RFC 1035 and augmented by subsequent RFCs.
5. The $TTL directive should be present and appear before the first RR (RFC 2308 implemented in BIND 9).
6. The first Resource Record must be the SOA (Start of Authority) record. The generic format is described below.

Resource Records have two representations. A textual format described in this chapter and a binary or wire-format described in Chapter 15.

The textual format has the following generic form:

name  ttl  class  type  type-specific-data

Where:

@
; replace with the current value of $ORIGIN
 
; blank/space or tab in which case the last name used or the value of $ORIGIN (or its default value) is substituted

The generic binary or wire-format is:

name  ttl class type rdlen rdata

The binary format is described in chapter 15 RR format

Directives start with '$' and are standardized - $ORIGIN and $INCLUDE (defined in RFC 1305) and $TTL (defined in RFC 2308). BIND additionally provides the non-standard $GENERATE directive.