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.
<p/>
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_in</b> and <b>mark_out</b> options. With the <b>set_mark_in</b> option, the
corresponding mark is applied to the inbound packets after decryption. Return traffic is
marked via <b>iptables -t mangle</b> rules in the PREROUTING chain.
<p/>
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>.
