MySQL service port are not meant to be accessible to the outside world as it would become a security concern to the administrator.
Although MySQL server (mysqld) by itself has a built-in mechanism to deny access from unauthorized ip-address, it still does not protect it from being overwhelmed by multiple malicious requests or buffer overflow attack directed to the server.
One of the solution is to limit the MySQL port access to trusted network using iptables
This assume your trusted network has the address within the range of 192.168.1.1-192.168.1.254
iptables -A INPUT -i eth0 -p tcp -s 192.168.1.0/24 --dport 3306 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 3306 -m state --state ESTABLISHED -j ACCEPT
MySQL server (mysqld) uses port 3306.
Note: Always assume the internet as the untrusted network!