RBL and many DNS querys to my dns

Hi friends,

I've noticed that when I enable RBL in my qmail installation, the  
firewall show many connections to the default DNS server configured in  
my qmail server (about 600-800 connections permanently, with peaks of  
1000-2000).

When I receive spam attack, the DNS connections + smtp connections +  
http connections, kill my router.

Do you think that Should I configure djbdns as dns-cache?

Regards,
Spamis