NAS to NAS backup over wireless with firewalling, VPN or not!?

[gtr33m]

Hi,

I'd like to set up a remote backup between 2 identical NAS boxes using a wireless link between two locations, call them A and B.

Network A needs to be able to freely communication with NAS A, but not only NAS A should be able to communicate with NAS B, and vice versa. Basically Network A must not be able to communicate with Network B.

The catch is, the NAS boxes have only a single network interface. They are linux based, and to a degree open, but I have been unable to install OpenVPN, IPSEC, or pptp on them. SSH and rsync are both up and running.

Ultra high security is not a requirement as the 2 locations have a degree of trust, but I need a solution that won't let someone access the opposite network by simply changing their IP address to the subnet on the other network. Clients on the different networks should not know the other exists.

Any suggestions would be greatly appreciated. I'd prefer a low cost option. This is an extra backup, a 'nice to have' but the budget will be minimal.

Thanks

Mark