A tunnel that will connect the subnets behind the gateways moon
and sun, respectively, is preconfigured by installing a %trap eroute
on gateway moon by means of the setting auto=route in ipsec.conf.
A subsequent ping issued by client alice behind gateway moon to
bob located behind gateway sun triggers the %trap eroute and
leads to the automatic establishment of the subnet-to-subnet tunnel.
leftfirewall=yes automatically inserts iptables-based firewall rules
that let pass the tunneled traffic.