Guide
Reverse DNS and forward-confirmed lookups for IP addresses
Ordinary DNS maps names to addresses. Reverse DNS goes the other way, and it is used in logs, in network diagnostics such as traceroute and, most importantly, by mail servers deciding whether to trust a sender. The page performs the lookup for an IPv4 or IPv6 address and then checks that the name it finds points back to the same address.
How the query is formed
A PTR query is an ordinary DNS query for a special name. For IPv4, reverse the four numbers and add in-addr.arpa: 192.0.2.10 becomes 10.2.0.192.in-addr.arpa. For IPv6, write the address in full, reverse its 32 hexadecimal digits, separate them by dots and add ip6.arpa. The reversal exists because DNS delegates from the right, so the part of the address that identifies the network has to come last. The page shows the query name it used, which is useful when you want to repeat the lookup with another tool.
Who controls the answer
The reverse zone belongs to whoever holds the address block: an internet provider for a home line, a cloud provider for a virtual machine, a company for its own range. Your domain registrar and your own DNS host cannot publish a PTR for an address you rent. You ask the provider, often through a control panel, to set it. If the provider has delegated a zone to you, you edit it yourself. The Reverse DNS Zone Generator helps you write such a zone.
Forward confirmation
Anyone who controls a reverse zone can put any name in it, including mail.bank.example. The PTR record alone therefore proves nothing. A name is trustworthy only if looking it up in forward DNS gives the original address back, which is forward-confirmed reverse DNS (FCrDNS). The page does this for every name it finds and marks each with a tick when the forward lookup returns the same address. A tick says the owner of the address block and the owner of the name agree, which is what mail receivers and some security tools require.
Why mail depends on it
Receiving servers treat a sending address with no PTR, a generic one, or one that does not confirm as suspicious, and many reject such mail. A mail server should have a PTR whose name matches the name it announces in its greeting and resolves back to its address. Providers that sell dedicated mail servers always let you set the PTR for that reason. A name that fails confirmation shows a mismatch in the page so you know whether to fix the PTR or the forward record.
A worked example
Press Sample to look up 8.8.8.8. The query name is 8.8.8.8.in-addr.arpa (the four numbers are identical, so reversing changes nothing), the host name is dns.google, and the forward lookup column shows a tick with the text “Yes: the name resolves back to this address (forward-confirmed)”. For 192.0.2.10 the page would build the query name 10.2.0.192.in-addr.arpa. A home connection typically shows a generated name or none.
Limits
Only PTR records published in public DNS are visible. The confirmation is done with the same resolver, so a mismatch may reflect caching right after a change. A confirmed name tells you who set up the address, not whether the traffic from it is wanted. Private addresses have no public reverse records; classify addresses first with the Private IP Checker.
Related tools
- Reverse DNS Zone Generator writes reverse zones and PTR records.
- Private IP Checker tells whether an address can have public records.
- MX Lookup shows the mail servers whose PTR records matter.
- A Record Lookup is the forward lookup used for confirmation.
Checked against the current version of the Reverse DNS Lookup on .