The roadwarriors <b>alice</b> and <b>venus</b> sitting behind the NAT router <b>moon</b> set up
tunnels to gateway <b>sun</b>. UDP encapsulation is used to traverse the NAT router.
Since both roadwarriors possess the same 10.1.0.0/25 subnet, gateway <b>sun</b> uses Source NAT
after ESP decryption to map these subnets to 10.3.0.10 and 10.3.0.20, respectively.
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In order to differentiate between the tunnels to <b>alice</b> and <b>venus</b>, respectively,
<b>XFRM marks</b> are defined for both the inbound and outbound IPsec SAs and policies using
the <b>mark</b> parameter in ipsec.conf.
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<b>iptables -t mangle</b> rules are then used in the PREROUTING chain to mark the traffic to
and from <b>alice</b> and <b>venus</b>, respectively.
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The script designated by <b>leftupdown=/etc/mark_updown</b> automatically inserts 
iptables mangle rules that mark the inbound ESP_IN_UDP packets as well as iptables IPsec-policy rules 
that let pass the tunneled traffic. In order to test the tunnel, the NAT-ed hosts <b>alice</b> 
and <b>venus</b> ping the client <b>bob</b> behind the gateway <b>sun</b>.