Forward-confirmed reverse DNS
Forward-confirmed reverse DNS (FCrDNS) is a two-step check some receiving mail servers run on a connecting IP before accepting mail: look up the IP's PTR record to get a claimed hostname, then look up that hostname's own A or AAAA record and confirm it resolves back to the same IP. Checking only the PTR record has a gap, since whoever controls an IP block can publish any hostname there; confirming the forward record agrees closes that gap without needing a cryptographic signature, though it's still a hygiene signal rather than proof of identity. FCrDNS predates SPF, DKIM and DMARC, and survives because it's cheap and catches abuse infrastructure -- compromised devices, throwaway cloud instances -- that was never configured with matching forward and reverse records in the first place. AdminKit's reverse DNS and DNS lookup tools together let you run both halves of an FCrDNS check manually.
By AdminKit team, Netbase JSC · Updated