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DNS Diagnostics Tool

Reverse DNS (rDNS) Lookup

Query authoritative PTR records and test Forward-Confirmed Reverse DNS (FCrDNS) status.

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Why Reverse DNS and FCrDNS Matter for Security and Mail Deliverability

Standard forward DNS resolves domain names to numerical IP addresses. Reverse DNS (rDNS) performs the inverse: resolving an IP address back to its canonical domain pointer (PTR record).

While forward DNS is managed in the domain registrar's zone file, reverse DNS is delegated hierarchically through the Regional Internet Registries (ARIN, RIPE, APNIC, etc.) down to the owner of the IP subnet or Autonomous System.

Forward-Confirmed Reverse DNS (FCrDNS)

Anyone can configure a fake forward record or point a domain name at any IP. However, because only the legitimate IP block holder controls the reverse DNS PTR zone, receiving servers use FCrDNS to establish cryptographic and administrative authenticity:

  1. The receiving mail server inspects the connecting client's IP (e.g. 198.51.100.10).
  2. It queries 10.100.51.198.in-addr.arpa for PTR → returns mail.example.com.
  3. It queries forward DNS for mail.example.com A record → verifies it matches 198.51.100.10.

If step 3 fails, the connection is classified as an unverified dynamic residential line or spoofed origin, and emails are dropped into the spam folder or rejected with an SMTP 550 error.

Frequently Asked Questions

What is Reverse DNS (rDNS)?▼

Reverse DNS resolves an IP address back to its associated human-readable hostname using a DNS PTR (pointer) record. Unlike standard forward DNS (which maps example.com to an IP), rDNS answers: 'What hostname is authorized for this IP?'

What is Forward-Confirmed Reverse DNS (FCrDNS)?▼

FCrDNS is a verification technique where an IP is resolved to a hostname via PTR, and that hostname is then resolved forward via an A or AAAA record. If the resulting IP matches the original IP, the record is forward-confirmed. Major mail providers (Google, Microsoft, Yahoo) require FCrDNS to accept inbound SMTP mail.

How does IPv4 reverse DNS work?▼

IPv4 reverse DNS reverses the 4 octets and appends the special domain '.in-addr.arpa'. For example, 8.8.8.8 becomes 8.8.8.8.in-addr.arpa, which points to dns.google.

How does IPv6 reverse DNS work?▼

IPv6 reverse DNS expands the 128-bit address into 32 hexadecimal nibbles separated by dots, reverses their order, and appends '.ip6.arpa'. For example, 2001:4860:4860::8888 expands into 32 dotted characters under ip6.arpa.

Why does my home broadband have a generic PTR record?▼

ISPs automatically generate generic dynamic PTR records (like c-73-12-34-56.hsd1.ca.comcast.net) for residential subscribers. Businesses with static IPs can request custom PTR records from their ISP or hosting provider.