Restrict query on my DNS

[Michel Dubois]

Hello,

I want to restrict the usage of my DNS servers and allow only access 
from my subnets.
In my configuration file I've created the following ACL :

acl "closed_dns" {
       xxx.xxx.xxx.0/24;
       yyy.yyy.yyy.0/24;
};

And in the option section :

allow-query { closed_dns; };

I've checked and from external subnet I'm not able to query my server 
(using nslookup) ... BUT i've discovered after several hours that some 
of domains are no longer responding. I've removed the ACL and it was 
working normally as before.
Could you please help me on this issue ?
Best regards,

Michel