If you do
systemctl disable ufw and then reboot the system, what is the
ufw status verbose output after the reboot?
I'm guessing that it will be
If that's true, then the explanation is as follows:
ufw enable = enable firewall rules right now, and make a record that the desired state is to have the firewall enabled from now on. (If a firewall management daemon is needed, this will also start it automatically.)
ufw disable = disable firewall rules right now, and make a record that the desired state is to have the firewall disabled until commanded otherwise.
systemctl start ufw = restore the previously-recorded desired state of the firewall (if the desired state is to have the firewall disabled, there will be nothing to do). Also, start any firewall management daemon if needed, in such a way that the management process will be recognized by
systemd as a background process that does not belong to any user session. If you want to have the firewall enabled persistently, you'll want this to happen automatically at boot time, and also have
ufw enable set.
systemctl stop ufw = stop the firewall management daemon; may or may not also disable any existing firewall rules (use
iptables -vnL or
nft list tables to check the actual rules in effect within the kernel, independently from
systemctl enable ufw = at any subsequent reboots, run
systemctl start ufw automatically as part of the boot process.
systemctl disable ufw = at any subsequent reboots, don't do anything to restore the firewall state and don't automatically start the firewall management daemon: go with the kernel default of no firewall rules at all.
For example, if you need to set up new firewall rules remotely and must ensure you won't lock yourself out, you might set
systemctl disable ufw, set up an automatic reboot in 15 minutes or so, then add your rules and use
ufw enable. If you can still connect to the system, the rules are correct as far as the remote access is concerned; you can set
systemctl enable ufw (so the rules will be effective also after a reboot) and remove the timed reboot job.
But if you made a mistake that disables your access, just wait for the reboot job to trigger, and the system will come up with no firewall rules (because of
systemctl disable ufw) and you will be able to log back in to fix the rule configuration.