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Add a rule: source internet, destination the DMZ Server then
add a NAT rule: Virtual host your firewall IP real host the
DMZ Server.
By both set the service to all and you have a DMZ.
Make sure the DMZ is connected to a a network card only for
the DMZ en you don't give the DMZ access to your firewall,
why should you build a DMZ?

Grtz,

S.