You should be able to pull the temperature from the system by running the command:
sysctl -a | grep tempe
I've used the extend to graph OpenVPN users from Linux before and I guessing that can probably be used to graph the temperature of the CPUs from FreeBSD.
None of those are exposed over SNMP that I'm aware of. Of them all, the only one that might possibly be there is temperature, but that may depend on hardware and the modules loaded.
We include a standard MIB, plus some additional options for pf stats. For determining whether a connection is down, there isn't anything in SNMP that checks gateway status. You'd probably just use the interface counters and alarm when it has less throughput than what's normal for that day and time of day.
thanks for the reply. Looks like my used check_mk does not find those interfaces. I will take a closer look how I can tweak check_mk to find those MIB entries.