ufw

sudo ufw status numbered verbose - check ufw status
sudo ufw default deny incoming - deny incoming connections
sudo ufw default allow outgoing - enable outgoing connections
sudo ufw allow ssh - allow certian connection (you can also specify a port number instead of protocol!)
sudo ufw deny ssh - deny certian connection
sudo ufw allow/deny proto tcp from any to any port 80,443 - deny / allow all 80 and 443 connections
sudo ufw allow from 192.168.1.103 to any port 22- allow incomming ssh connections with specified ip.
sudo ufw allow from 192.168.1.1/24 to any port 22 - same thing, but now with specified subnet
sudo ufw delete <rule-id> - delete rule from ufw, get id with status command