Upgrade BIND in order to security flaw

[Carlos Alberto Bernat Orozco]

Hi group

I know that what I'm going to ask is very basic but I want to be sure
before to do any upgrade and the anser to this question will correct
my concepts about DNS

I have installed BIND 9.3.0 on Mandriva. We are an ISP with 600 users
aprox. Our users are configurated with ou own DNS IP servers so it
resolves pages for them.

The question is:

is our servers caching resolvers? why the expression caching
resolvers? do I have to upgrade then?

Sorry if this is a dumb question but I prefer to have everything clear
before make any move.

Thanks in advanced