Reverse DNS (PTR) Lookup
See an IP's hostname (its PTR record), then check that the name points back to the same IP and doesn't look generic, the two things receiving mail servers judge. Works for a list of IPs or a range up to a /24 too.
match: the PTR name points back to its IP (forward-confirmed). generic: the name is built from the IP or named after a customer pool.
Fetched over DNS-over-HTTPS from Cloudflare (fallback: Google). Answers are kept for up to an hour, so a PTR that just changed may not show yet.
About PTR records and reverse DNS.
What is a PTR record (reverse DNS)?
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Regular DNS turns names into IPs. Reverse DNS goes the other way, IP to name, through PTR records in the special in-addr.arpa zone for IPv4 or ip6.arpa for IPv6. For example, 8.8.8.8 has the PTR dns.google.
Why does a mail server need a PTR?
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Receiving mail servers check the sending IP's PTR. With no PTR, or one that doesn't point back to the same IP, mail is often rejected or lands in spam; Gmail requires both from every sender. Ideally the PTR is a name like mail.example.com whose A record points at the same IP, and matches the server's HELO name.
How do I change a PTR record?
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PTRs aren't set in your domain's DNS but by whoever holds the IP block: the ISP, VPS host, or cloud provider. VPS hosts usually have a "Reverse DNS" option in their panel; if not, open a ticket with the IP and the name you want. Make sure the name's A record already points at the IP. The single-IP check shows the reverse zone and its nameservers, so you can see who runs it.
What is forward-confirmed reverse DNS (FCrDNS)?
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The IP points to a name through its PTR, and that name points back to the same IP through an A or AAAA record. Whoever holds an IP can put any name in its PTR, so checking both directions proves they also control the name.
Can I check a whole range at once?
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Yes. Enter a CIDR range like 203.0.113.0/24 (up to a /24 for IPv4 and a /120 for IPv6), or several IPs separated by commas or spaces, up to 256 addresses.
Can I use it from a terminal?
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Yes. curl 'ipku.in/en/tools/reverse-dns-lookup?q=8.8.8.8' returns the check as plain text, and /api/ptr/8.8.8.8 or /api/ptr/1.1.1.0/24 returns JSON.