The zone transfer is the method a secondary DNS server uses to update its information from the primary DNS server. DNS servers within a domain are organized using a master-slave method where the slaves get updated DNS information from the master DNS. One should configure the master DNS server to allow zone transfers only from secondary (slave) DNS servers but this is often not implemented. By connecting to a specific DNS server and successfully issuing the ls –d domain-name > file-name you have initiated a zone transfer.