The roadwarrior <b>alice</b> sitting behind the NAT router <b>moon</b> sets up a
tunnel to the subnet hiding behind the NAT router <b>sun</b>. All IKE and ESP traffic
directed to the router <b>sun</b> is forwarded to the VPN gateway <b>bob</b>
using destination NAT. UDP encapsulation is used to traverse the NAT routers.
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Upon the successful establishment of the IPsec tunnel, the updown script automatically
inserts iptables-based firewall rules that let pass the tunneled traffic.
In order to test the double NAT-ed IPsec tunnel <b>alice</b> pings the inner IP address
of the router <b>sun</b>.