What is a PTR record?

The record that maps an IP address back to a name. It lives in the reverse DNS zone for the address range, which is controlled by whoever owns that range.

Why does my mail get rejected for reverse DNS?

Either the sending address has no PTR record, or the name it gives does not resolve back to that address. Both look like a server that is not what it claims, which is a common shape for a spam source. Ask your provider to set the PTR to a name you control, and make sure that name has an A record pointing back.

Can I set a PTR record myself?

Only if you control the address range, which for most people means no. The record you can set is the forward one: the A or AAAA record for the name, so that the two match once your provider sets the PTR.

Why do some addresses have several names?

Because a reverse zone may hold more than one PTR record for an address. It is unusual, and receivers generally treat the address as confirmed if any of the names forward confirms.

Reverse DNS Lookup

Looks up the PTR record for an address, then checks the answer the way a mail server does: by resolving the name it gave and seeing whether it comes back to the same address.

The address you type is sent to Cloudflare or Google's public resolver to read its PTR record. Nothing is stored here and nothing else leaves your browser.

How to use Reverse DNS Lookup

1Type an IPv4 or IPv6 address.
2Press look up.
3Read the name the address claims.
4Check whether that name resolves back, which is the part that matters.

Why the check runs both ways

A PTR record can say anything. The owner of an address can point it at any name at all, including one belonging to somebody else, so a PTR record on its own proves nothing.

What makes it mean something is forward confirmation: take the name the PTR gave, resolve it, and see whether one of its addresses is the one you started with. Only the owner of that name could have arranged that, so the pair together is evidence. This is what receiving mail servers check, and it is why the result here says confirmed or not rather than just printing the name.

Mail, and the address with no name

An address sending mail with no PTR record at all is treated with suspicion by most large receivers, and some reject it outright. Setting one is not something you can do in your own DNS: the reverse zone belongs to whoever owns the address range, which means your hosting provider or ISP.

For a server you rent, the provider's control panel usually has a field for it. For a home connection, the answer is usually that you cannot, which is one of the reasons sending mail from home rarely works well.

How an address becomes a name

The reverse lookup is an ordinary DNS query against a strange-looking name. 8.8.4.4 is asked about as 4.4.8.8.in-addr.arpa, with the octets reversed so that the hierarchy runs the same way as it does in a domain name.

IPv6 does the same thing one hex digit at a time, which is why the query name for a v6 address is 32 digits separated by dots followed by ip6.arpa. Both are shown here, since seeing the query name explains the result when it is empty.

Limits

Only the owner of an address range can publish a PTR record for it, which is usually your provider rather than you.
A missing PTR record is a normal answer for most addresses, and only a problem for one that sends mail.
Forward confirmation uses the A or AAAA records for the name, which may themselves be behind a proxy or load balancer.

Frequently asked questions

What is a PTR record?: The record that maps an IP address back to a name. It lives in the reverse DNS zone for the address range, which is controlled by whoever owns that range.
Why does my mail get rejected for reverse DNS?: Either the sending address has no PTR record, or the name it gives does not resolve back to that address. Both look like a server that is not what it claims, which is a common shape for a spam source. Ask your provider to set the PTR to a name you control, and make sure that name has an A record pointing back.
Can I set a PTR record myself?: Only if you control the address range, which for most people means no. The record you can set is the forward one: the A or AAAA record for the name, so that the two match once your provider sets the PTR.
Why do some addresses have several names?: Because a reverse zone may hold more than one PTR record for an address. It is unusual, and receivers generally treat the address as confirmed if any of the names forward confirms.