In my post, I've provided a batch script which configures blackhole routes for all private networks.
The effect is that if traffic is routed via a VPN tunnel, and the tunnel goes down, this traffic is discarded instead of being sent out via the default route. This will prevent data leaking out the WAN interface.
Second effect: there is no session built involving this traffic and the WAN interface, so that if the tunnel comes up again, it will immediately continue to transport this traffic through the tunnel.
Also consider enabling dynamic routing protocols on these tunnels, instead of the static routes - reliability of detecting tunnel being down is higher with dynamic protocols. Not rare a case when VPN tunnel is seen "up" but no traffic can actually pass inside it, dynamic routing protocol will detect this easily, static routes not so.
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