Wildcard DNS record

A wildcard DNS record is a record in a DNS zone that will match requests for non-existent domain names. A wildcard DNS record is specified by using a "*" as the leftmost label (part) of a domain name, e.g. *.example.com. The exact rules for when a wild card will match are specified in RFC 1034, but the rules are neither intuitive nor clearly specified. This has resulted in incompatible implementations and unexpected results when they are used.

Definitions of DNS wildcards

A wildcard DNS record in a zone file looks similar to this example:

This wildcard DNS record will cause DNS lookups on domain names ending in example.com that do not exist to have MX records synthesized for them. So, a lookup for the MX record for somerandomname.example.com would return an MX record pointing to host1.example.com.

Wildcards in the DNS are much more limited than other wildcard characters used in other computer systems. Wildcard DNS records have a single "*" (asterisk) as the leftmost DNS label, such as *.example.com. Asterisks at other places in the domain will not work as a wildcard, so neither *abc.example.com nor abc.*.example.com work as wildcard DNS records. Moreover, the wildcard is matched only when a domain does not exist, not just when there are no matching records of the type that has been queried for. Even the definition of "does not exist" as defined in the search algorithm of RFC 1034 section 4.3.2 can result in the wild card not matching cases that one might expect with other types of wildcards.

Domain Name System

The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates more readily memorized domain names to the numerical IP addresses needed for the purpose of locating and identifying computer services and devices with the underlying network protocols. By providing a worldwide, distributed directory service, the Domain Name System is an essential component of the functionality of the Internet.

The Domain Name System deligates the responsibility of assigning domain names and mapping those names to Internet resources by designating authoritative name servers for each domain. Network administrators may delegate authority over sub-domains of their allocated name space to other name servers. This mechanism provides distributed and fault tolerant service and was designed to avoid a single large central database.