What is Wildcard DNS?

A wildcard DNS record is a resource record in a DNS zone that matches all query names where the corresponding owner name does not exist in the zone. It is written as an asterisk followed by a dot, then the domain name, for example *.example.com. The wildcard matches any name at its own level in the hierarchy, but it does not match the parent domain itself or names that already have an explicit record.

When a DNS resolver queries a name like random.example.com and no A, AAAA, or other record exists for that exact name, the authoritative server checks for a wildcard record at the *.example.com level. If found, it returns the record defined for the wildcard (often an A record pointing to a web server or an MX record for catch-all email). The wildcard expansion does not apply to the root of the zone (example.com) or to names that are already defined, nor does it match deeper levels if the intermediate name exists. For example, *.example.com will match foo.example.com but not bar.foo.example.com unless there is no foo.example.com record.

Wildcard DNS is commonly used for hosting providers that assign dynamic subdomains to users, or for redirecting all undefined subdomains to a single website. It is defined in RFC 1034 section 4.3.3 and further clarified in RFC 4592. The wildcard does not apply to all record types uniformly; it synthesizes a response only for the query type that the wildcard record itself defines. Operators must be careful not to create conflicting records that break expected behavior, such as blanking a zone with an unintended wildcard MX record in mail environments.