What is Reverse DNS (rDNS)?

Reverse DNS (rDNS) maps an IP address to a hostname. It is the opposite of forward DNS (which maps hostname to IP).

Forward: mail.tidalmail.com → 192.0.2.1

Reverse: 192.0.2.1 → mail.tidalmail.com

How it works: PTR records in special reverse DNS zones (in-addr.arpa for IPv4) store the mapping.

Email importance:

Receivers check if sending IP has valid reverse DNS

Missing or mismatched rDNS is a spam signal

Legitimate mail servers always have proper rDNS

Reverse DNS proves the IP owner has configured it properly. It is a basic trust indicator.

Verifying a ship's registration by looking up its hull number rather than its name.