Domain Authentication for IIS located in DMZ

I currently have a Windows Server 2003 doamin running Active
Directory(doamin.local) We setup and installed an IIS server in the DMZ for
customer to be able to access order status(using local ids and passwords) We
have now decided to setup a site for our Outside sales employees to be able
to access information while traveling. It would be nice to have them use
their already established domain username and passwords. Is the best
solution to join the IIS box to a seperate(dummy) domain such as domain.dmz
and then setup a one way trust between it and our current domain? What ports
do i need open in the firewall for windows authentication?