bind9 reference

http://www.zytrax.com/books/dns/ch7/xfer.html#allow-notify

http://www.microhowto.info/howto/configure_bind_as_a_slave_dns_server.html

https://www.digitalocean.com/community/tutorials/how-to-setup-dns-slave-auto-configuration-using-virtualmin-webmin-on-ubuntu