A DNS server on the Internet should normally only answer queries for the domains it is authoritative for. But unless you configure it otherwise bind will pass on requests for other domains up the hierarchy. This means it can potentially be used in a Denial of Service attack against other DNS servers. You can prevent this by restricting lookup via other DNS servers only to devices you trust. To do this add another line to the options section in the form...

options {

  ...

  allow-recursion { trusted.IP.subnet; };

It will still answer queries from anywhere for domains for which it is the authoritative server, but will now only do lookups via other DNS servers for requests from the trusted subnet.