snat differant ip from dmz interface only ??

[simpletone@datapimp.org]

I would like to masq/snat a differant ip for all traffic coming into the
firewall from a dmz interface..

so, 3 nics.

eth0 EXTernal
eth1 LAN
eth2 DMZ


any traffic out EXT from LAN SNAT's to y.y.y.y

any traffic out EXT from DMZ SNAT's to x.x.x.x

currently i cant get this working, it SNAT's from the real external IP,
not the aliased ip (eth0:1)..

any help would be much appreciated.