Forward-confirmed reverse DNS
Forward-confirmed reverse DNS (FcrDNS) or, more precisely, forward-confirmed reverse DNS lookup refers to two consecutive DNS queries in which the name associated with an IP address is determined first (so-called reverse DNS lookup ), and then the associated IP address.
If the result of the second query leads back to the original IP address, this is considered a forward confirmed .
meaning
A successful FcrDNS check is an indication of a connection between the owner of a domain and the owner of the CIDR address block from which the IP address originates.
It is also an indication that both owners have at least a basic understanding of the relevant RFCs and are able to configure their servers correctly.
application
A negative result could indicate poorly maintained servers or a zombie PC . A mail server, for example, can use this feature to ward off spam .