How to Masq multiple ethernet interfaces?

[Joey Dalton]

Hello,

I need to have a total of four NICs on my firewall (Netfilter 1.2.2 on 
RedHat Linux 7.1.)  One is the external interface, and three internal for 
three physically separate subnets.

How can I Masq traffic going from those internal interfaces to the 
external, but not Masq going from one internal to another?  In other words, 
traffic going from eth1, eth2, and eth3 to eth0 should be Masq'd.  Traffic 
going between eth1, eth2, and eth3 should not.  How can I do that?  If 
somebody wouldn't mind, could you tell me the exact command to type 
in.  (I'm a beginner at netfilter, having been rescued from an Elron 
firewall on Windoze.)

Best Regards,
Joey