DNS - connection limit (without any extra hardware)

[Luke <very.luke () gmail com>]

Hi,
as a comsequence of a virus diffused in my customer-base, I often receive
big bursts of traffic on my DNS servers.
Unluckly, a lot of clients start to bomb my DNSs at a certain hour, so I
have a distributed tentative of denial of service.
I can't blacklist them on my DNSs, because the infected clients are too
much.

For this reason, I would like that a DNS could response maximum to 10
queries per second given by every single Ip address.
Anybody knows a solution, just using iptables/netfilter/kernel tuning/BIND
tuning, without using any hardware traffic shaper?

Thanks
Best Regards

Luke