Monitoring multiple sites all with the same IP-Range?

Hi,
I have to monitor multiple machines in multiple sites that are all in the same ip-range 192.168.0.0/24 from a central linux nagios server. Each of the sites ca e.g. open a reverse ssh tunnel to my nagios server so that I can reach the remote-site via an individual tcp-port on my nagios server,

I am looking for a clever and simple idea to avoid NATTing and VPNs. I have tried socks5 and ssh tunnels which works perfectly for tcp-checks but with udp and icmp i had problems because tsocks socks proxy could not tunnel icmp and udp. I also want to avoid additional hardware or nagios site-servers at the locations.

Does someone have a good idea how I could send arbitrary nagios-checks/ip-packets to the sites although they have the same ip target adresses ?

Thanks

Have you tried NRPE?