Default Firewall in Fedora Server 26

[Stephen Gallagher]

One thing that I thought of while writing up my overseer email was this: We
decided a couple weeks ago that the plan for Fedora 26 was to deliver two
slightly different versions of Fedora Server: one with a GUI (Cockpit) and one
that only included the capability for another Cockpit instance to manage it.
This implies that we need two different default firewall configurations; one for
Server w/ GUI and one without.[1]

I see two ways that we could accomplish this:

 * Create two variants for use with /etc/os-release such as `VARIANT_ID=server`
and `VARIANT_ID=server-gui`. This would allow us to deliver a different default
systemd presets and firewall configuration to the system. If the Cockpit GUI was
installed later, the administrator would need to explicitly enable
cockpit.socket and grant firewall permission to allow it as a separate action.

 * Create a helper systemd service that would run as a dependency of the
cockpit.socket service and open the firewall while that service is running.
(This helper could be installed as part of the fedora-release-server package, so
it wouldn't take effect on systems that aren't Fedora Server).

I'm leaning towards the second one (though I'll probably have to come up with
new packaging guidelines for how it would work). My concern with this option is
that it *is* a deviation from how Fedora generally handles the installation of
services; normally installation and starting them are independent. That said, I
think an argument can be made that if someone is installing `cockpit-ws` on a
Fedora Server system, we can make an assumption that they intend to use it to
manage that system.

I don't really like the first option because I'm worried that it will set a
precedent that will lead to a combinatorial explosion of variants we need to
test. But it *does* give us the option to avoid auto-starting cockpit-ws if we
install it later.

Other ideas are of course most welcome.


[1] This is because Cockpit operates on a non-privileged port (9090) and
therefore if we leave this port available by default, there's an opportunity for
a user process to claim it.