With external recursion enabled, a DNS server can be exploited to attack other networks. You can disable recursion in BIND but making a change to the config file.

Using your favorite text editor, open the file /etc/named.conf

nano /etc/named.conf

Look for a line that says:

recursion yes;

Change that line to no:

recursion no;

This will disable recursive lookups and protect your server from being used to attack other servers.