In computer networks, a reverse DNS lookup or reverse DNS resolution (rDNS) is the querying technique of the Domain Name System to determine the domain name associated with an IP address – the reverse of the usual "forward" DNS lookup of an IP address from a domain name.

Forward-confirmed reverse DNS (FCrDNS), also known as full-circle reverse DNS, double-reverse DNS, or iprev, is a networking parameter configuration in which a given IP address has both forward (name-to-address) and reverse (address-to-name)