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reverse DNS(Reverse Domain Name System) Name resolution that is the opposite of the standard forward DNS query. Most of the time, the DNS is queried with a domain name to return the host's IP address. With reverse DNS, also called "inverse DNS" and "reverse mapping," the DNS system is sent an IP address, and the domain name is returned.
Reverse DNS is used to log incoming traffic by domain name for statistical purposes. It is also used to prevent spam by determining if the email message is coming from the domain name indicated in the message header. Reverse DNS is an option and not mandatory in a DNS server. See DNS records, DNS and SPF.