Reverse DNS Lookup
Resolve an IPv4 or IPv6 address to its PTR record, with the in-addr.arpa or ip6.arpa name shown so you can see exactly what was queried.
Result
About Reverse DNS Lookup
Reverse DNS maps an address back to a name using the special in-addr.arpa and ip6.arpa zones. It matters most for mail: virtually every large receiver checks that a sending IP has a PTR record, and that the name it returns resolves back to the same address.
How the query name is built
For IPv4 the octets are reversed and appended to in-addr.arpa, so 8.8.4.4 becomes 4.4.8.8.in-addr.arpa. For IPv6 every nibble is reversed and dot-separated under ip6.arpa - 32 labels for a single address. The tool shows the generated name so you can see what was actually asked.
Forward-confirmed reverse DNS
FCrDNS is the check that matters: the PTR gives a hostname, and that hostname must resolve forward to the original address. Mail servers, SSH audit tooling and abuse desks all use it. A PTR pointing at a name that does not resolve back is treated as no PTR at all - sometimes worse.
Common use cases
- Diagnosing mail rejected for "no reverse DNS" or generic-looking PTR names.
- Identifying the operator of an address seen in firewall or web logs.
- Verifying an ISP delegated your reverse zone correctly after an allocation.
Edge cases and gotchas
- Reverse DNS is controlled by the address owner, not the domain owner - you usually have to ask your ISP or cloud provider to set it.
- A generic PTR such as 203-0-113-10.dsl.example.net signals a residential connection and is commonly used to reject mail.
- IPv6 reverse zones are rarely populated; for mail servers, set them explicitly.