A connection between the subnets behind the gateways <b>moon</b> and <b>sun</b>
is set up using GRE interfaces.
<p/>
The gateways use <b>route-based forwarding</b> with <b>GRE tunnels</b>, with
firewall rules to allow traffic to pass. The IPsec traffic selector is limited
to the GRE protocol, specific routing is achieved with routes on the GRE
interfaces. The IKE daemon is configured to not install routes with
<em>charon.install_routes=0</em>, and static routes are installed for the
target subnets on the VTI interfaces.
<p/>
Client <b>alice</b> behind gateway <b>moon</b> pings client <b>bob</b> located
behind gateway <b>sun</b>.