Forward-Confirmed Reverse DNS (FCrDNS)

Verifies the round-trip: PTR ➡️ forward DNS ➡️ original IP. Most modern mail receivers require FCrDNS as part of identifying the sending host. A forward-confirmed PTR is one of the cheapest pieces of trust signal a mail server can present.

Applies to: IPv4 and IPv6 hosts.

How It Works

1. We do a PTR lookup on the IP.
2. For each PTR name returned, we do a forward A and AAAA lookup.
3. If any forward record matches the original IP, the check passes.

If no PTR exists, or no forward lookup matches, the check fails.

Fails On

• No PTR record published for the IP.
• The PTR target's A/AAAA records do not include the original IP.

Related Checks

• Reverse DNS (rDNS) - simpler version that just verifies any PTR exists.
• Generic PTR Pattern - flags PTRs that look like dynamic / consumer IP ranges.