The crashes are not caused by the server software, but by people who are attacking the server. I've setup several firewall rules which COULD help. I've blocked the IP addresses using software firewall (iptables). I've also contacted serverloft support and asked them to block the attacking IP addresses from their hardware firewall, but you all know how customer support is ("we are working on it", "tech support is working on it", "i don't understand", ...) so I don't really expect so much from them.