gistfile1.txt

# Run as root or use sudo
$ sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
$ sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT
$ sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT
$ sudo iptables -A INPUT -p tcp --dport 21 -j ACCEPT
$ sudo iptables -A INPUT -p tcp --dport 443 -j ACCEPT
$ sudo iptables -A INPUT -p tcp --dport 3306 -j ACCEPT
$ sudo iptables -A INPUT -j DROP
$ sudo iptables -I INPUT 1 -i lo -j ACCEPT

# Save iptables config
$ sudo iptables-save | sudo tee /etc/iptables/rules.v4
$ sudo service iptables-persistent restart

# Install and use so firewalls are saved through restarts
$ sudo apt-get install -y iptables-persistent
$ sudo service iptables-persistent start

// Source: https://serversforhackers.com/editions/2014/06/17/more-security/
// Source 2: http://fideloper.com/ubuntu-12-04-lemp-nginx-setup