Reverse DNS Lookup PTR
A reverse DNS lookup finds the hostname registered for an IP address by reading its PTR record. Enter an IPv4 or IPv6 address to see the hostname, and whether that hostname points back to the same address, which is the check mail servers apply to incoming connections.
How to read reverse DNS results
For each address the tool shows the PTR hostname and, underneath the address, the special name that was actually queried. For IPv4 the octets are reversed and .in-addr.arpa is appended, so 8.8.8.8 becomes 8.8.8.8.in-addr.arpa and 192.0.2.10 becomes 10.2.0.192.in-addr.arpa. IPv6 addresses are expanded to 32 hexadecimal digits, reversed digit by digit, and placed under ip6.arpa.
The forward-confirmed column answers a second question: does the hostname in the PTR record resolve back to the original address? If it does, the reverse and forward records agree (often written FCrDNS). If it does not, anyone could have put that hostname in the PTR record, so receivers do not trust it.
If you enter a hostname instead of an address, the tool first resolves it to its A and AAAA addresses and then reverses each one.
Why reverse DNS matters
- Email delivery. Gmail, Microsoft and most corporate mail systems reject or penalize mail from an address with no PTR record, or with one that is not forward-confirmed. The PTR hostname should also match the name the server announces in its SMTP greeting.
- Reading logs. Firewalls, web servers and traceroute use PTR records to show a name beside an address, which often reveals the network or provider involved.
- Investigating an address. The PTR name frequently encodes the provider, the region, or whether the address belongs to a residential or dynamic pool. Generic names of that kind are one reason a mail server may distrust a sender.
Common reverse DNS problems
- No PTR record. New cloud servers often have none, or a generic one. It is set in the hosting provider's control panel or by a request to the ISP, not in the domain's own DNS zone.
- PTR exists but is not forward-confirmed. The hostname in the PTR has no A or AAAA record, or that record points to a different address. Add or correct the forward record so both directions match.
- IPv6 forgotten. A server that sends mail over IPv6 needs a PTR for its IPv6 address as well. Mail that leaves over IPv6 with no reverse DNS is commonly rejected even when IPv4 is set up correctly.
- Several PTR records on one address. This is legal but unhelpful. Software usually reads only one, and which one is not predictable.
- Shared addresses. On shared hosting the PTR names the provider's server rather than your domain. That is expected, and it is one reason to send mail through a dedicated service.
If a sending address fails these checks, it is also worth running it through the blacklist check. The PTR record guide explains the record format and delegation of reverse zones.
Frequently asked questions
What is a reverse DNS lookup?
It is a DNS query that starts from an IP address and returns the hostname registered for it. The answer comes from a PTR record stored in a reverse zone under in-addr.arpa for IPv4 or ip6.arpa for IPv6.
Who controls the PTR record for an IP address?
The organization that the IP block is assigned to, which is normally the hosting provider, cloud platform or ISP. You set it through their control panel or support team. It cannot be created in your own domain's DNS zone unless the provider has delegated the reverse zone to you.
What is forward-confirmed reverse DNS (FCrDNS)?
It means the lookup works in both directions: the IP address has a PTR record naming a hostname, and that hostname has an A or AAAA record containing the same IP address. Mail servers use it as a basic sign that a sender is legitimately configured.
Why does my mail server need a PTR record?
Large mailbox providers require sending addresses to have valid reverse DNS and will reject or throttle mail without it. The PTR hostname should be forward-confirmed and should match the hostname your server uses in its SMTP greeting.
Why does an IP address have no reverse DNS?
Nobody has created a PTR record for it. That is common for unused addresses, newly provisioned cloud servers, and addresses that are not meant to host services. It does not mean the address is offline.
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