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Type of domain name 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 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.

To quote from , many DNS implementations diverge, in different ways, from the original definition of wildcards. Some of the variations include:

With djbdns, in addition to checking for wildcards at the current level, the server checks for wildcards in all enclosing superdomains, all of the way up to the root. In the examples listed above, the query for telnet. tcp. host1. example for an MX record would match a wildcard despite the domain tcp. host1. example existing. Microsoft's DNS server (if configured to do so) and MaraDNS (by default) have wildcards also match all requests for empty resource record sets; i. e., domain names for which there are no records of the desired type. In the examples listed above, the query for sub.*. example for an MX record would match *. example, despite sub.*. example explicitly existing with only a TXT Record.

Wildcard domains are widely used by blogging websites that allow users to create sub domains upon demand; e. g., sites such as WordPress or Blogspot. Another popular use is by Free Dynamic DNS websites that allow users to create a DNS name that changes to match their host IP as the IP address is changed periodically by their ISP's DHCP server.

New gTLDs are prohibited from publishing wildcards (or using equivalent name server mechanisms) by specification 6 of the ICANN New gTLD Base Registry agreement. However, ICANN's Name Collision Occurrence Management Framework (PDF), explicitly requires new gTLDs to publish (for at least 90 days) special MX, SRV, TXT, and 127.0.53.53 A record wildcards that warn of potential name collisions due to use of relative domain names with domain search paths.

Several domain name registrars have, at various times, deployed wildcard records for the top level domains to provide a platform for advertising, most notably VeriSign for com and net with its (now removed) Site Finder system. The museum TLD also had a wildcard record which has now been removed. as of 2018, top level domains using a wildcard A record (other than 127.0.53.53) are fm, la, ph, pw, vg and ws. The internationalized TLDs .中国 ( or for "China") and .გე ( for the Georgian letters for the Georgian country code "GE") also have wildcard A records. The *.中国 wildcard resolves to ibaidu. com (flagged by Chrome as unsafe), and the *.გე wildcard resolves to a website of the ge TLD. It has also become common for ISPs to synthesize address records for typos, for the same person, a practice called "catchall" typosquatting, but these aren't true wildcards, but rather modified caching name servers.

The Internet Software Consortium produced a version of the BIND DNS software that can be configured to filter out wildcard DNS records from specific domains. Various developers have produced software patches for BIND and for djbdns. Other DNS server programs have followed suit, providing the ability to ignore wildcard DNS records as configured.