About the Reverse DNS
This reverse DNS tool takes an IP address and looks up its PTR record — the hostname the address claims to be. It then performs a forward-confirmed reverse DNS (FCrDNS) check: it resolves that hostname forward again and confirms it points back to the original IP, the exact validation that receiving mail servers run on a sending IP.
The lookups run server-side against public resolvers. Use it to check whether a mail server's IP has correct, matching reverse DNS before sending email, to debug deliverability issues, or to verify that an IP's PTR and forward records are consistent. Mismatched or missing FCrDNS is a common reason mail gets flagged or rejected.
How to use it
- Enter the IPv4 or IPv6 address you want to check.
- Submit to fetch the PTR record (the reverse hostname).
- Review the FCrDNS result confirming the hostname resolves back to the IP.
- Note any mismatch or missing PTR that could affect mail delivery.
- Fix the PTR with your hosting or ISP if forward and reverse do not align.
Examples
- Check a mail server IP 203.0.113.25 and see PTR mail.example.com, which resolves forward back to 203.0.113.25 — FCrDNS passes.
- An IP returns PTR host.example.net but that name resolves to a different IP — FCrDNS fails and mail may be penalized.
- An IP with no PTR record at all returns no reverse hostname, a frequent deliverability problem for self-hosted mail.
Frequently asked questions
What is reverse DNS?
Reverse DNS maps an IP address back to a hostname using a PTR record, the opposite of a normal forward lookup. It answers 'what name does this IP claim?'.
What is FCrDNS (forward-confirmed reverse DNS)?
It is a two-step check: look up the PTR hostname for an IP, then resolve that hostname forward and confirm it returns the same IP. Matching both directions is what mail servers expect.
Why does reverse DNS matter for email?
Receiving mail servers check FCrDNS on the sending IP as an anti-spam signal. A missing or mismatched PTR makes legitimate mail more likely to be flagged or rejected.
How do I set or fix a PTR record?
PTR records are controlled by whoever owns the IP block, usually your hosting provider or ISP, not your domain's DNS. Request the correct PTR through them.
What is the difference between a PTR check and full FCrDNS?
A plain PTR lookup only reads the reverse hostname. FCrDNS additionally re-resolves that hostname forward to confirm it points back to the same IP, which a bare PTR does not guarantee.
Can I check IPv6 reverse DNS too?
Yes, the tool resolves PTR and runs FCrDNS for both IPv4 and IPv6 addresses, using public resolvers on the server.