Reverse DNS (rDNS or RDNS) is a Domain Name Service (DNS) lookup of a domain name from an IP address. A regular DNS request would resolve an IP address given a domain name; hence the name “reverse.”
A special PTR-record type is used to store reverse DNS entries. The name of the PTR-record is the IP address with the segments reversed + “.in-addr.arpa”.
For example the reverse DNS entry for IP 1.2.3.4 would be stored as a PTR-record for “4.3.2.1.in-addr.arpa”.
Reverse DNS requests are often used for filtering spam. Spammers can easily set the sending email address using any domain name they want, including legitimate domain names like those of banks or trusted organizations.
Receiving email servers can validate incoming messages by checking the sending IP address with a Reverse DNS request. If the email is legitimate, the rDNS resolver should match the domain of the email address. The downside with this technique is that some legitimate mail servers don’t have the proper rDNS records setup on their end to respond properly because in many cases their ISP has to set these records up.
