Editing Domain Name System (section)

===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" />