Reverse DNS lookup

In computer networking, reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name that is associated with a given IP address using the Domain Name System (DNS) of the Internet.

Computer networks use the Domain Name System to determine the IP address associated with a domain name. This process is also known as forward DNS resolution. Reverse DNS lookup is the inverse process, the resolution of an IP address to its designated domain name.

The reverse DNS database of the Internet is rooted in the Address and Routing Parameter Area (arpa) top-level domain of the Internet. IPv4 uses the in-addr.arpa domain and the ip6.arpa domain (previously ip6.int) is delegated for IPv6. The process of reverse resolving an IP address uses the pointer DNS record type (PTR record).

Informational RFC 1912 (Section 2.1) specifies that "Every Internet-reachable host should have a name" and that such names match with a reverse pointer record, but it is not a requirement of standards governing operation of the DNS itself.

Reverse domain name notation

Reverse domain name notation (or reverse-DNS) is a naming convention for the components, packages, and types used by a programming language, system or framework. A characteristic of reverse-DNS strings is that they are based on registered domain names, and are only reversed for sorting purposes. For example, if a company making a product called "MyProduct" has the registered domain name "example.com", they could use the reverse-DNS-ish string "com.example.MyProduct" to describe it. Reverse-DNS names are a simple way of reducing name-space collisions, since any domain name is registered by only one party at a time.

History

Reverse-DNS first became widely used with the Java platform, and has since been used for other systems, for example, ActionScript 3 packages and Android applications.

Examples

Examples of systems that use Reverse-DNS are Sun Microsystems' Java platform and Apple's Uniform Type Identifier or UTI. The Android operating system also makes use of the notation for classifying applications, as the Dalvik virtual machine made use of Java.