Syn-flood attacks are common these days. Syn-flood attack means that the attackers open a new connection to the server, but do not state what they want. They just want to take up servers resources and increase server load.
This type of attack can be controlled by adding a rule in IPTable to reject syn-flood attacks. Login to your server as root via SSH, and run following commands:
iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP sudo /etc/init.d/iptables save sudo /etc/init.d/iptables restart
This method works on a CentOS / RedHat Linux server.