Ufw

ufw is the Uncomplicated Firewall that comes with Ubuntu. It allows a quick and easy firewall to be setup.

Status:

ufw status

Enable/Disable

ufw enable
ufw disable

Adding Rules based on application:

ufw allow openvpn
ufw allow openssh

Adding Rules based on port:

ufw allow 1000/udp

Allowing based on port and interface:

ufw allow in on tap0 to any port 5353 proto udp

IPv6 support can be enabled (not on by default) in /etc/default/ufw:

IPV6=yes