What reverse DNS is
Ordinary DNS turns a name into an address. Reverse DNS goes the other way: it asks which hostname an IP address claims as its own.
The answer is stored in a PTR (pointer) record. To find it, the address is rewritten as a name under a special domain. For IPv4 the four numbers are reversed and .in-addr.arpa is added, so 192.0.2.25 becomes 25.2.0.192.in-addr.arpa. For IPv6 all 32 hexadecimal digits are written out, reversed, separated by dots and placed under .ip6.arpa. This tool builds that name for you, asks a public resolver for its PTR record over DNS-over-HTTPS, and shows exactly what came back.
Who controls a PTR record
Not the owner of the domain. Reverse zones are handed out together with the addresses themselves: from the regional registry to whoever holds the address block, which is usually an internet provider, a hosting company or a cloud platform. Only that holder can publish a PTR record.
This is why you cannot set reverse DNS in your own domain's DNS zone, even when the hostname you want is in your domain. It is also why a PTR record proves little by itself. The address holder can point it at any name at all, including a name they have nothing to do with. The proof comes from checking both directions.
Reading the result
| Result | What it means | What to do |
|---|---|---|
| Forward-confirmed | The address names a hostname, and that hostname resolves back to the same address | Nothing. This is what mail servers want to see |
| Does not match | The PTR record names a hostname that resolves to a different address, or to nothing | Fix the A or AAAA record of the hostname, or have the PTR record changed |
| No PTR record | The address holder has not published a hostname | Normal for many home and unused addresses. Request one if you send mail from it |
A name such as host-203-0-113-42.example.net is generated automatically by a provider for every customer address. It tells you which provider holds the address and little else.
Why mail servers need forward-confirmed reverse DNS
A receiving mail server sees only the IP address of whoever connects to it. It looks up the PTR record of that address, then resolves the hostname it got and checks that the original address is among the answers. This two-way check is called forward-confirmed reverse DNS, or FCrDNS. Spam sent from infected home computers almost never passes it, so it is a cheap and effective filter.
Many receivers refuse mail, or file it as spam, when the sending address has no PTR record or fails the forward check. Large mailbox providers, Gmail among them, list valid forward and reverse DNS in their requirements for senders. For the best result, the PTR hostname should also be the name your mail server announces in its SMTP greeting.
How to request a PTR record
- Find the address holder. A WHOIS lookup on the IP address shows which provider the block is registered to.
- Create the forward record first. Add an A record (AAAA for IPv6) for the hostname, such as
mail.example.com, pointing at the address. Many providers will not set a PTR record until this exists. - Ask the holder to set the PTR record. Cloud and hosting companies usually have a reverse DNS field in the control panel next to the address. Business connections with a static address need a support request. Home connections with a changing address generally cannot have a custom PTR record.
- Check both directions here. Resolvers cache the old answer until its TTL runs out, so allow some time before looking again.