What is a reverse DNS lookup?
Normal DNS turns a name into an address (example.com → 93.184.215.14). Reverse DNS goes the other way: it asks which hostname belongs to an IP address. The answer is stored as a PTR record under a special domain: in-addr.arpa for IPv4 (with the octets reversed, so 192.0.2.10 becomes 10.2.0.192.in-addr.arpa) and ip6.arpa for IPv6, written nibble by nibble.
Why reverse DNS matters
- Email delivery. Most receiving mail servers reject or spam-folder messages from IPs without a valid PTR record, or whose PTR does not resolve back to the same IP.
- Logs and troubleshooting. Tools like
tracerouteand web server logs show hostnames, which often reveal the ISP, city or router role. - Security. Analysts use rDNS to quickly classify traffic from cloud providers, crawlers or residential ISPs.
What is forward-confirmed reverse DNS?
Anyone who controls an IP block can set its PTR to any name, even mail.google.com. FCrDNS closes that gap by resolving the PTR hostname forward again and checking that it points back to the original IP. We run this check automatically for every hostname found.
How to set or change a PTR record
PTR records are controlled by whoever owns the IP block, not by your domain's DNS host. For a home connection that is your ISP; for a server it is your hosting or cloud provider, which usually offers a "reverse DNS" setting in its control panel. Read the full reverse DNS guide.
Frequently asked questions
Why does my IP have no reverse DNS?
Many ISPs and cloud providers simply do not publish PTR records for every address. That is normal unless you run a mail server.
Can an IP have more than one PTR record?
Technically yes, but it is discouraged; mail servers expect exactly one hostname.