Im running Nagios 3 and also OpenVPN
I want to test periodically that the connection is still available. Is nagios the way to do this. I see there is a check_vpn plugin but the documentation is brief.
Are other plugins better suited to the task?
I am a big nagios fan and we use it to monitor approx. 200 devices with around 800 services. The easiest thing to do is to ping an IP address at the other end of the tunnel. If you want to go beyond that, you have a range of options, depending on what kind of machine is available and which machines are running the OpenVPN clients.
In most cases the snmp plugin should provide a lot of freedom. You would probably want to "snmpwalk" the target (i.e. the two OpenVPN clients) to see what information they offer for the tunnel, and then from there write specific commands to just look at the relevant OIDs.
Obviously this requires an SNMP client on the target machines. If they are Windows boxes, I can tell you that versions prior to Windows XP or Windows Server 2003 have a limited range of OIDs they offer, the same is also true for the linux snmp clients prior to kernel version 2.6.26 (e.g. debian etch runs on 2.6.18 and offers quite limited data).