Reverse DNS

In computer technology, reverse Domain Name Server (DNS) lookup or resolution is used to determine a domain name that is associated with an IP address using the Internet DNS lookup service. The most common of these is whois.net. This process is sometimes noted as rDNS. Computer networks commonly use domain names to determine an IP address in a process called forward DNS resolution; this process is completed every time an internet user enters a web address into their browser. Reverse DNS is the exact opposite.

Reverse DNS may be useful for a number of reasons from network troubleshooting to authenticating relationships between an owner of a domain name and the owner of the server given for an IP address. This validation process is often strong enough for whitelisting purposes as spammers’ zombie computers cannot pass this verification. However, reverse DNS is also useful in another anti-spam technique which checks domain names to see if they are from inexpensive internet services such as dial-up. The owners of these IP addresses often name them with generic rDNS names. As the vast majority of spam originates with these generically named IP addresses, many spam filters may refuse emails originating from such locations.