[pptp-server] NAT on firewall?

[Adam Haeder]

This question isn't about pptpd per se, but since it relates to the
network setup, I thought I would ask it.
I've currently got a Sparc machine running RedHat 6.0 with kernel 2.2.5
doing ip forwarding/masquerading for about 30 users. It also does some
ip forwarding from valid internet IPs through to non-routable
(192.168.x.x) IPs. My questions is this: in the current PoPToP HOWTO,
they talk about setting up a firewall and a pptpd server behind it on a
non-routable ip. The author then goes through setting up NAT using
natadm to be able to forward GRE info through the firewall to the
internal pptpd server, since it's the only ip redirector I know of that
can handle non-tcp or udp traffic. 
My question is this: can I use my existing masq box as this firewall?
I'm concerned about trying to use both ip forwarding/masq and nat on the
same box. Can it be done? I'm thinking the solution to my problem right
now is to setup a separate firewall system that does nat and then setup
an internal pptpd server. I was planning on keeping this solution down
to one server, but it looks like I might not be able to.
Any advice is appreciated!

--
Adam Haeder
Technical Coordinator, AIM Institute
adamh at omaha.org