Not too long ago in the Linux world, firewall rules were complex. iptables did its job very well, but managing rules was daunting for a newcomer. Debian 9 introduces some changes that make it pretty simple to add a firewall rule.
Usually firewall rules are taken care of automatically, when you install a program it takes care of opening up the required ports for itself. In some cases, software can conflict and that is what happened in my case. Virtualmin blocked the port for Zbbix (10050), since it overwrote all the firewall rules.
To add a new rule, we’ll use the newer
firewall-cmd command. To add a rule,
firewall-cmd --permanent --zone=public --add-port=10050/tcp replacing
10050 and tcp with your port the protocol to open. When done, reload the
firewall with service firewalld restart and your new rule should be applied.
That’s it! Really! If you Google how to add Firewall rules, you’ll see loads of examples for creating init scripts that add iptables rules when your machines starts, but firewall-cmd does all that heavy lifting now.