Dynamic IP Detection

I have a m0n0Wall behind a DMZ zone from my cable Modem/Router wich
is NOT running on bridge mode, dynamic DNS on m0n0wall detects the
lan IP of the cable modem, but not the wan IP, does anybody
knows a work arround ?

Cable modem running on NAT mode NOT bridge. 
I know that if m0n0wall is not connected directly to the internet it 
need's to get the IP from an outside source, something like
http://checkip.dyndns.org, or zoneedit etc

Is m0n0wall doing that ? 

Thanks.


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