you just have to set up a different network on the third adapter. For example:
eth0 is Internet eth1 192.168.0.0/255.255.255.0 (maybe yor LAN) eth2 192.168.1.0/255.255.255.0 (DMZ)
Then connect your FTP-Server with eth2(cross-connect or hub/switch) an create some filter-rules. In your case allow FTP to DMZ (or just to the server-IP) for the clients you want.
you just have to set up a different network on the third adapter. For example:
eth0 is Internet eth1 192.168.0.0/255.255.255.0 (maybe yor LAN) eth2 192.168.1.0/255.255.255.0 (DMZ)
Then connect your FTP-Server with eth2(cross-connect or hub/switch) an create some filter-rules. In your case allow FTP to DMZ (or just to the server-IP) for the clients you want.
First, you should understand what a DMZ zone is, the purpose of a DMZ zone is to allow the external world to acces that DMZ network/server, for this, a good rule is to have separate network segments fro your nwteork and for the DMZ network AND phisically separate the 2 of them.
Of course, you may want to allow the outside world to talk to the DMZ server for certain services as HTTP, FTP and so on.
That way, you will need to set some rules to allow the external traffic to enter your DMZ server. And that should be a DNAT rule.
