Connecting IPSec from Behind a gateway

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I am trying to connect 2 FreeBSD 5.4 boxes with an IPSec tunnel using
racoon.   The problem is the second box is behind a nat'd gateway. 
The gateway router is a commercial box with "IPSEC Passthrough"
enabled.

What do I need to do to get this to work?
Which IP (global or private) should I use when configure the connection?

TIA,
Tomoki Taniguchi