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.

DNS Record Types

A full list of DNS Record Types may be obtained from IANA DNS Parameters.

RR	Value	RFC	Description
A	1	RFC 1035	IPv4 Address record. An IPv4 address for a host.
AAAA	28	RFC 3596	IPv6 Address record. An IPv6 address for a host. Current IETF recommendation for IPv6 forward-mapped zones.
A6	38	RFC 2874	Experimental. Forward mapping of IPv6 addresses. An IP address for a host within the zone.
AFSDB	18	RFC 1183	Location of AFS servers. Experimental - special apps only.
CNAME	5	RFC 1035	Canonical Name. An alias name for a host.
DNAME	39	RFC 2672	Experimental. Delegation of reverse addresses (primarily IPv6).
DNSKEY	48	RFC 4034	DNSSEC.bis. DNS public key RR.
DS	43	RFC 4034	DNSSEC.bis. Delegated Signer RR.
HINFO	13	RFC 1035	Host Information - optional text data about a host.
ISDN	20	RFC 1183	ISDN address. Experimental = special applications only.
KEY	25	RFC 2535	Public key associated with a DNS name.
LOC	29	RFC 1876	Stores GPS data. Experimental - widely used.
MX	15	RFC 1035	Mail Exchanger. A preference value and the host name for a mail server/exchanger that will service this zone. RFC 974 defines valid names.
NAPTR	35	RFC 3403	Naming Authority Pointer Record. Gross misnomer. General purpose definition of rule set to be used by applications e.g. VoIP
NS	2	RFC 1035	Name Server. Defines the authoritative name server(s) for the domain (defined by the SOA record) or the subdomain.
NSEC	47	RFC 4034	DNSSEC.bis. Next Secure record. Ssed to provide proof of non-existence of a name.
NXT	30		DNSSEC Next Domain record type. Obsolete use NSEC.
PTR	12	RFC 1035	IP address (IPv4 or IPv6) to host. Used in reverse maps.
RP	17	RFC 1183	Information about responsible person. Experimental - special apps only.
RRSIG	46	RFC 4034	DNSSEC.bis. Signed RRset.
RT	21	RFC 1183	Through-route binding. Experimental - special apps only.
SIG	24	RFC 2931//2535	DNSSEC. Obsolete use RRSIG. SIG(0) is used as a special meta RR in DDNS and zone transfer security.
SOA	6	RFC 1035	Start of Authority. Defines the zone name, an e-mail contact and various time and refresh values applicable to the zone.
SPF	99	RFC 4408	Sender Policy Framework (v1). Defines the servers which are authorized to send mail for a domain. Its primary function is to prevent identity theft by spammers.
SRV	33	RFC 2872	Defines services available in the zone, for example, ldap, http etc..
TXT	16	RFC 1035	Text information associated with a name. The SPF record should be defined using a TXT record and may (as of April 2006) be defined using an SPF RR. DKIM (RFC 4871 also makes use of the TXT RR for authenticaing email. How to define DKIM/ADSP RRs.
WKS	11	RFC 1035	Well Known Services. Deprecated in favour of SRV.
X25	19	RFC 1183	X.25 address. Experimental - special apps only.

Zone File Format

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.
The textual format has the following generic form:

name  ttl  class  type  type-specific-data

Where:

name	The name (or label) of the node in the zone file to which this record belongs. The name field may also take one of the following values: @ ; 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
ttl	32 bit value. The Time to Live in seconds (range is 1 to 2147483647) and indicates how long the RR may be cached. The value zero indicates the data should not be cached.
class	A 16 bit value which defines the protocol family or an instance of the protocol. The normal value is IN = Internet protocol (other values are HS and CH both historic MIT protocols).
types	The resource record type which determines the value(s) of the type-specific-data field. Type takes one of the values below.
type-specific-data	Data content of each record is defined by the type and class values.

The generic binary or wire-format is:

name  ttl class type rdlen rdata

DNS Zone File Directives

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.

Directive	Description
$INCLUDE	Includes the defined file in-line.
$ORIGIN	Defines the base name (aka label) to be used for 'unqualified' name substitution.
$TTL	Defines the default Resource Record TTL value, used if no TTL is defined in a resource record.