This tool is available at
The tool allows you to do Forward and Reverse DNS lookups. Given a host name, the tool finds its IP. Conversely, given an IP the tool finds the corresponding host.
Forward DNS lookup resolves a host name to an IP address (A record). The process of reverse resolving an IP address uses the pointer DNS record type (PTR record).
Thus, the tool does Forward-confirmed reverse DNS (FCrDNS) lookups. This is a networking parameter configuration where a given IP address has both forward (name-to-address) and reverse (address-to-name) Domain Name System (DNS) entries that match each other.
Unlike similar tools which do Forward/Reverse DNS lookups on a single host, our tool does lookups on multiple hosts, saving users time and effort.
To use the tool, enter one host name (or IP) per line, ending each line by pressing the Enter key.
Forward DNS lookups are faster than Reverse DNS lookups so for the latter you may want to do a few checks at once.
Depending on DNS server configurations, lookups with or without the www alias can produce dissimilar results. For instance yahoo.com with and without www returns different results.
Our tool can be used to identify Internet service providers (ISPs) who do not provide properly matching DNS and rDNS records. It can also be used to find shared hosting and, when misconfigured, forwarders information leaks.
FCrDNS verification can also be used for whitelisting purposes because spammers and phishers cannot usually by-pass this verification when they use zombie computers for email spoofing. That is, the reverse DNS might verify, but it will usually be part of another domain than the claimed domain name.