allow-transfer defines a match list e.g. IP address(es) that are allowed to transfer (copy) the zone information from the server (master or slave for the zone). The default behaviour is to allow zone transfers to any host. While on its face this may seem an excessively friendly default, DNS data is essentially public (that's why its there) and the bad guys can get all of it anyway. However if the thought of anyone being able to transfer your precious zone file is repugnant, or (and this is far more significant) you are concerned about possible DoS attack initiated by XFER requests, then use the following policy.