What is a Reverse DNS Lookup?

In order to find the Internet domain name associated with a particular internet protocol (IP) address, one may use a procedure called a reverse DNS lookup. This procedure utilizes a system called a domain name system (DNS) which links numeric computer addresses to domain names such as example.com. Further searches can reveal the name of the person or entity who registered a particular domain name, as well as contact information in some cases.

A person may use a reverse DNS lookup when he or she knows the numeric IP address of a website and wants to know the domain name associated with it. In such a case, one uses an Internet tool or website to type an IP address which is a series of numbers separated by periods, for example 123.456.123.456. As long as website designers have enabled the right pointer records within the programming for the domain name at the server level, a request sent regarding a particular IP address will return the correct information. This is the essence of a reverse DNS lookup, also called a reverse DNS resolution (rDNS).

A reverse DNS lookup is also used to help deter email spam. Email users are often inundated with unwanted emails called spam. Some Internet email servers use a technique called forward confirmed reverse DNS lookup to help weed out unsolicited messages from spammers who might broadcast an unwanted email to millions of people at once. This forward confirmed reverse DNS lookup means that a request is sent regarding a particular IP address, but it is taken a step further in that the result is then verified to make sure that the relationship is valid.

Every computer that connects to the Internet does so via an Internet protocol address. With the right hardware and software, this unique address can be traced to a particular computer anywhere on the planet. Numeric domain names can be difficult to remember, however, so letters and numbers are used to route a domain name to its numeric IP address. The series of alphanumeric domain names is called the domain name system, or DNS.

A separate database, called WHOIS, maintains contact information for the technical and administrative contacts for domain names registered within the DNS. In addition, WHOIS provides contact information for owners of blocks of IP addresses. A search similar to a reverse DNS lookup will reveal the person or entity that owns the domain name or IP addresses.