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 wildcard will match are specified in , 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: *.example.com. 3600 IN MX 10 host1.example.com. 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 section 4.3.3 can result in the wildcard not matching cases that one might expect with other types of wildcards. The original definition of how a DNS wildcard behaves is specified in sections 4.3.2 and 4.3.3, but only indirectly by certain steps in a search algorithm and as a result, the rules are neither intuitive nor clearly specified. As a result, 20 years later, , "The Role of Wildcards in the Domain Name System" was written to help clarify the rules. To quote , "A common mistake is thinking that a wildcard MX for a zone will apply to all hosts in the zone. A wildcard MX will apply only to names in the zone which aren't listed in the DNS at all." That is, if there is a wildcard MX for *.example.com, and an A record (but no MX record) for www.example.com, the correct response (as per ) to an MX request for www.example.com is "no error, but no data"; this is