Editing Domain Name System (section)

==Resource records==
{{anchor|resource records|resource record}}
The Domain Name System specifies a database of information elements for network resources. The types of information elements are categorized and organized with a [[list of DNS record types]], the resource records (RRs). Each record has a type (name and number), an expiration time ([[Time to live#DNS records|time to live]]), a class, and type-specific data. Resource records of the same type are described as a ''resource record set'' (RRset), having no special ordering. DNS resolvers return the entire set upon query, but servers may implement [[round-robin DNS|round-robin ordering]] to achieve [[load balancing (computing)|load balancing]]. In contrast, the [[Domain Name System Security Extensions]] (DNSSEC) work on the complete set of resource record in canonical order.

When sent over an [[Internet Protocol]] network, all records (answer, authority, and additional sections) use the common format specified in RFC 1035:{{Ref RFC|6895|rsection=3}}
{| class="wikitable" style="margin: 1em auto 1em auto"
|+ Resource record (RR) fields
|-
! Field !! Description !! Length ([[Octet (computing)|octets]])
|-
| NAME || Name of the node to which this record pertains || Variable
|-
| TYPE || Type of RR in numeric form (e.g., 15 for MX RRs) || 2
|-
| CLASS || Class code || 2
|-
| [[Time to live|TTL]] || Count of seconds that the RR stays valid (The maximum is 2<sup>31</sup>−1, which is about 68 years) || 4
|-
| RDLENGTH || Length of RDATA field (specified in octets) || 2
|-
| RDATA || Additional RR-specific data || Variable, as per RDLENGTH
|}

''NAME'' is the fully qualified domain name of the node in the tree.{{clarify|date=November 2017}} On the wire, the name may be shortened using label compression where ends of domain names mentioned earlier in the packet can be substituted for the end of the current domain name.

''TYPE'' is the record type. It indicates the format of the data and it gives a hint of its intended use. For example, the ''A'' record is used to translate from a domain name to an [[IPv4 address]], the ''NS'' record lists which name servers can answer lookups on a [[DNS zone]], and the ''MX'' record specifies the mail server used to handle mail for  a domain specified in an e-mail address.

''RDATA'' is data of type-specific relevance, such as the IP address for address records, or the priority and hostname for MX records. Well known record types may use label compression in the RDATA field, but "unknown" record types must not (RFC 3597).

The ''CLASS'' of a record is set to IN (for ''Internet'') for common DNS records involving Internet hostnames, servers, or IP addresses. In addition, the classes [[Chaosnet|Chaos]] (CH) and [[Hesiod (name service)|Hesiod]] (HS) exist.{{Ref RFC|6895|rp=11}} Each class is an independent name space with potentially different delegations of DNS zones.

In addition to resource records defined in a [[zone file]], the domain name system also defines several request types that are used only in communication with other DNS nodes (''on the wire''), such as when performing zone transfers (AXFR/IXFR) or for [[Extension Mechanisms for DNS|EDNS]] (OPT).

===Wildcard records===
The domain name system supports [[wildcard DNS record]]s which specify names that start with the ''asterisk label'', <code>*</code>, e.g., <code>*.example</code>.{{Ref RFC|1034}}{{Ref RFC|4592}} DNS records belonging to wildcard domain names specify rules for generating resource records within a single DNS zone by substituting whole labels with matching components of the query name, including any specified descendants. For example, in the following configuration, the DNS zone ''x.example'' specifies that all subdomains, including subdomains of subdomains, of ''x.example'' use the mail exchanger (MX) ''a.x.example''. The AAAA record for ''a.x.example'' is needed to specify the mail exchanger IP address. As this has the result of excluding this domain name and its subdomains from the wildcard matches, an additional MX record for the subdomain ''a.x.example'', as well as a wildcarded MX record for all of its subdomains, must also be defined in the DNS zone.

<syntaxhighlight lang="zone">
x.example.       MX   10 a.x.example.
*.x.example.     MX   10 a.x.example.
a.x.example.     MX   10 a.x.example.
*.a.x.example.   MX   10 a.x.example.
a.x.example.     AAAA 2001:db8::1
</syntaxhighlight>

The role of wildcard records was refined in {{IETF RFC|4592}}, because the original definition in {{IETF RFC|1034}} was incomplete and resulted in misinterpretations by implementers.<ref name="rfc4592" />