Free reverse DNS lookup tool (also known as PTR record lookup or rDNS check). Given an IPv4 or IPv6 address, returns the PTR record hostname. Useful for verifying mail-server reverse DNS, identifying who owns an IP, and reverse-resolving log entries.
A forward lookup turns a name into an IP; reverse DNS does the opposite, turning an IP back into a hostname through a PTR record. The lookup works by querying a special zone: an IPv4 address is reversed and placed under in-addr.arpa, while IPv6 uses ip6.arpa. Enter any IPv4 or IPv6 address and this tool returns the PTR hostname, so you can put a name to an address from a log line, a firewall entry, or a connection you do not recognise.
Reverse DNS is most important for email. Receiving mail servers routinely check that a sending IP has a PTR record and that it matches the forward record, a test known as FCrDNS (forward-confirmed reverse DNS). An IP with no PTR, or one whose PTR does not line up with its HELO name, is far more likely to be greylisted or rejected as spam. Because PTR records are controlled by whoever owns the IP block, usually your hosting provider or ISP, you often have to request the change rather than set it yourself.
A reverse DNS lookup takes an IP address and returns the hostname stored in its PTR record. It is the opposite of a normal forward lookup, which turns a hostname into an IP.
A PTR (pointer) record maps an IP address back to a hostname. It lives in the reverse DNS zone (in-addr.arpa for IPv4 or ip6.arpa for IPv6) and is what a reverse lookup returns.
Many receiving mail servers reject or flag messages from IPs that have no PTR record or whose PTR does not match the forward hostname. Correct reverse DNS improves deliverability and is a common anti-spam check.
PTR records are controlled by the owner of the IP address block, which is usually your hosting provider or ISP. You typically request the reverse DNS entry from them rather than editing it in your own DNS zone.
