Install shorewall-init by entering the following commands in the terminal:
sudo apt update sudo apt install shorewall-init
Description:
Shorewall initialization
This package provides two related features: . a) It allows the firewall to be closed prior to bringing up network devices. This insures that unwanted connections are not allowed between the time that the network comes up and when the firewall is started. . b) It integrates with NetworkManager and distribution ifup/ifdown systems to allow for 'event-driven' startup and shutdown. . The two facilities can be enabled separately. . When Shorewall-init is first installed, it does nothing until you configure it.
Homepage: http://www.shorewall.net
Version: 5.1.12.2-1
Section: universe/net