Iptables

to save firewall rules use:

sudo dpkg-reconfigure iptables-persistent

or (stretch):

sudo dpkg-reconfigure iptables-persistent

allow ssh:

sudo iptables -A INPUT -p tcp --dport 22 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT

sudo iptables -A OUTPUT -p tcp --sport 22 -m conntrack --ctstate ESTABLISHED -j ACCEPT

references:

https://www.digitalocean.com/community/tutorials/iptables-essentials-common-firewall-rules-and-commands