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We use a dedicated, non-routable, IPv4 subnet for IPSec. Furthermore
the subnet is nullrouted in the absence of xfrm lookup (i.e., when there
is no matching IPSec Security Association) to avoid data leaks.
Each host is associated with an IP in that subnet (thus only reachble
within that subnet, either by the host itself or by its IPSec peers).
The peers authenticate each other using RSA public key authentication.
Kernel traps are used to ensure that connections are only established
when traffic is detected between the peers; after 30m of inactivity
(this value needs to be less than the rekeying period) the connection is
brought down and a kernel trap is installed.
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