12.4.3.3 Reverse DNS Lookups

Keywords: forwardchecknone, forwardchecktag, forwardcheckdelete

The forwardchecknone, forwardchecktag, and forwardcheckdelete channel keywords can modify the effects of doing reverse DNS lookups. These keywords can control whether the MTA does a forward lookup of an IP name found using a DNS reverse lookup, and if such forward lookups are requested, specify what the MTA does if the forward lookup of the IP name does not match the original IP number of the connection.

The forwardchecknone keyword is the default, and means that no forward lookup is done. The forwardchecktag keyword tells the MTA to do a forward lookup after each reverse lookup and to tag the IP name with an asterisk (*), if the number found using the forward lookup does not match that of the original connection. The forwardcheckdelete keyword tells the MTA to do a forward lookup after each reverse lookup and to ignore (delete) the reverse lookup returned name if the forward lookup of that name does not match the original connection IP address; in this case, the MTA uses the original IP address instead.

Having the forward lookup not match the original IP address is normal at many sites, where a more “generic” IP name is used for several different IP addresses.