Difference between "sudo systemctl disable openvpn@client.service" and "sudo systemctl disable openvpn"

Question

I have question about openVpn service. I want to disable the openVpn service, but I wonder what is the difference between "sudo systemctl disable openvpn@client.service" and "sudo systemctl disable openvpn"?

systemctl list-units | grep openvpn

and I get

openvpn.service                 loaded active exited    OpenVPN service        
openvpn@client.service          loaded active running   OpenVPN connection to client                                                 
system-openvpn.slice            loaded active active    system-openvpn.slice   

Answer 1

openvpn.service is mainly a historical leftover. Basically, it imitates /etc/init.d/openvpn for people who were used to older Debian versions having a single service that controls multiple VPN configurations in one shot, whereas the systemd approach is to have a separate "openvpn@foo.sevice" instance for each individual configuration.

In other words, the service doesn't start anything its own – its only purpose is to aggregate all openvpn@... instances. If you run systemctl cat openvpn@.service you'll see that it references openvpn.service, so systemctl reload openvpn would propagate the reload to all openvpn@... instances that you have running.

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However, note that Debian's custom openvpn@.service is actually being replaced by the newer openvpn-client@.service and openvpn-server@.service units which come directly from OpenVPN itself. Those two service units are also multiple-instance, but they don't reference the "central" openvpn.service at all.