"Running a public name server on the Internet today can be challenging, when it's exposed to a neverending flood of attacks against it. The situation is worse if you're an ISP that provides recursive lookups for customers. There are many ways to fight high traffic levels, such as adding more servers or buying specialized hardware solutions or firewalls that help to protect against abusive traffic. Another solution, if you're running your name server on a Linux system is PHREL, a Per Host RatE Limiter.
"The problem comes when customer hosts are compromised and used to send spam, which in turn sends thousands of DNS requests per minute to your name server as they seek their recipients' mail server addresses..."
