UFW(uncomplicated firewall) is the simple easy firewall to manage and protect your server. Few command snippets to manage it.

• View configured rules in ufw: It’ll list all the allowed/rejected rules for ufw.
  • sudo ufw status
  • It might show Status: inactive. You need to enable ufw to view rules.
• View firewall added rules, while firewall is in in-active state or is not-enabled.
  • ufw show added
• Add a rule to firewall: It will allow the connection to machine given on specific port(here 7865).
  • sudo ufw allow 7865/tcp
  • sudo ufw allow 22/tcp
• Enable Ufw: By default ufw will be started on your machine, but ufw itself will not be in enabled state.
  • To enable it: sudo ufw enable

• Reject all connections to machine: It will implicitly reject all the connection, that are not allowed by the firewall.
  • sudo ufw default reject incoming
• To allow all outgoing connections.
  • sudo ufw default allow outgoing

References

• https://www.digitalocean.com/community/tutorials/how-to-setup-a-firewall-with-ufw-on-an-ubuntu-and-debian-cloud-server
• https://askubuntu.com/questions/30781/see-configured-rules-even-when-inactive