Document firewall configuration with firewalld, nftables and list open ports by default

[bgilbert]

This is something that people will want to do.

We don't ship firewalld. We should document systemd units or config files that should be used to set up rules.

[bgilbert]

I assume we should emphasize nftables rather than legacy iptables?

[magnusviri]

I commented showing how I got nftables and iptables to work. I'm more than willing to fork this repository, update it, and do a pull request if what I posted is the correct way.

[travier]

@magnusviri Your Butane config looks like a good start. Would you like to make a PR for a doc page with that example? Thanks!

[travier]

Related to coreos/fedora-coreos-tracker#1747, we decided that we should document:

• how to setup firewalld either via layering or via a container
• how to setup static nftables/iptables rules
• what ports and services are running / listening on the network by default in Fedora CoreOS
• why the default container networking model using network namespaces reduces the need for a firewall on Fedora CoreOS
• what can be done with cloud firewalls on some platforms