What is a Reverse DNS Lookup?
A reverse DNS lookup turns an IP address back into a hostname. Where a normal A or
AAAA lookup asks "what IP serves example.com?", a reverse lookup asks "what name claims this
IP?" — the answer comes from a PTR record in the .in-addr.arpa (IPv4) or .ip6.arpa
(IPv6) DNS zone. PTR records are crucial for email deliverability (most mail servers reject SMTP connections from IPs
without a PTR), traceroute readability, server logs, and basic abuse-tracking.
What is FCrDNS?
Forward-Confirmed Reverse DNS (FCrDNS) is the verified version: take the PTR result and do a forward
A / AAAA lookup on it. If at least one forward result matches the original IP, the record is
"confirmed". Major email providers (Gmail, Outlook, Yahoo) require FCrDNS to accept inbound mail because a PTR alone is
owner-controllable and trivially spoofable — only FCrDNS proves the forward DNS owner agrees with the IP owner.
How this tool works
- IPv4 — the IP is reversed octet-by-octet and suffixed with
.in-addr.arpa. So1.2.3.4becomes4.3.2.1.in-addr.arpa. - IPv6 — the address is expanded to its full 32-nibble form, the nibbles are reversed, dot-separated, and suffixed with
.ip6.arpa. - The PTR query is sent to Cloudflare DNS-over-HTTPS — no records are proxied through our servers.
- When FCrDNS verification is enabled, each PTR hostname triggers a forward lookup; the result is
✓ confirmedonly if at least one returned IP equals the original. - Bulk mode caps concurrency at 8 parallel queries to be polite to the free resolver.
Tips
- Use the sample buttons to test against well-behaved infrastructure (Cloudflare, Google, Quad9) whose PTRs always FCrDNS-pass.
- IPs from residential ISPs often have generic
cust-xx-xx-xx-xx.example.netPTRs that fail FCrDNS because the home user doesn't control forward DNS — that's normal, not a bug. - Reverse DNS for IPv6 ranges can take a few hundred ms longer than IPv4 because the queried name is much deeper.