For security reason, you should block all incoming null packets requests to your server. All TCP requests with flag NONE to your server should be blocked. These are generally bad bots looking for security holes on your server to exploit. This can be easily done by adding a rule in IPTable / firewall.
Login to your server as root via SSH and run following command:
iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP sudo /etc/init.d/iptables save sudo /etc/init.d/iptables restart
This works on a CentOS / RedHat Linux server.