Prevent? If you figure it out, you'll become a very rich person.

sorry for my bad english, I thought some tool to block, filter or similar
I heard about haproxy
Anybody with expirience?

CloudFlare is good option (and they have a free plan).

While there is no way to "prevent" a DDoS attack, there are ways to mitigate it. Throttling the bandwidth/access per IP per amount of time can help.

Take a look at:
http://serverfault.com/questions/211135/how-to-prevent-a-loic-ddos-attack
http://blog.bodhizazen.net/linux/prevent-dos-with-iptables/

thank you,
I found in this article http://www.thefanclub.co.za/how-to/how-secure-ubuntu-1204-lts-server-part-1-basics
interesting stuff