Wildcard Record
Wildcard Records give you the ability to map all (or a section) of the records in your domain to one IP. All Wildcard records are created with A records. If you create a record that is part of your domain (and inclusive of the wildcard set) then only that record is directed to the other IP and everything else will go to the wildcard record.
A wildcard DNS record is specified by using a "*" as the left most label (part) of a domain name, e.g. *.example.com.
Several domain name registrars have, at various times, deployed wild cards for the top-level domains that they serve. It has also become common for ISPs to synthesize A records to redirect typos to their advertising sites.
Wildcard DNS records in a zone file look similar to this:
*.example.com. 3600 MX 10 host1.example.com.
This wild card 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 left most 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. More over, the wild card 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 you might expect with other types of wildcards.
The original definition of how a DNS wildcard behaves is specified in RFC 1034 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, RFC 4592, "The Role of Wildcards in the Domain Name System" was written to help clarify the rules.
To quote RFC 1912, "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 wild card MX for *.example.com, and an A record (but no MX record) for www.example.com, the correct response (as per RFC 1034) to an MX request for www.example.com is "no error, but no data"; the expected response is the MX record attached to *.example.com.