On the DMZ we will have a Web Server that needs access back to the
Mainframe on the LAN, and a Mail server that need access to another
mail server on the LAN.
Bad idea. You don't want hosts in the DMZ to be able to establish
connections into the LAN. That would be breaking the concept of a DMZ
(allow connections from a network with higher security level to a
network with lower security level, but not vice versa).
There are several ways to deal with this problem, e.g. replicate the
information from the servers into the DMZ, use bastion hosts, or put
the servers from the LAN into a second DMZ.
Don't take general rules too far.
You don't want connections from the outside connecting directly to
systems on the inside at all.
Specific systems in the DMZ accessing specific systems/services on the
LAN is normal and acceptable. Trying too hard to stick to this
general rule usually results in worse systems (replication impacting
integrity, additional complexity impacting availability, etc).
These DMZ systems should be minimized and hardened so in effect they
are the bastion host.
In some environments you would want additional segmentation on the
LAN, but it's probably not realistic or a good idea to move your
mainframe into a DMZ.
PIXs do stateful inspection.
The web server needs to have access to a mainframe. How would you
increase security if not with a DMZ?
You can do this well with one PIX - If I were you and had 2 PIX's I'd
use the other one for redundancy.
Internet Router --> PIX <--> WAN/LAN router. <---> LAN <---MainFrame
|
DMZ
|
Web Server
You can put Inet -> DMZ and DMZ -> LAN ACL's on the PIX - Logically
doing what you were doing with the two PIXs
There could be some value in sticking with 2 firewalls if they were
different vendors/technologies, but again, you need to be careful here
too. If your firewall admins are good at PIX and you throw something
they don't know into the mix you could easily be worse off.
As far as the NAT goes - I think generally you NAT/PAT to the outside
from the inside or the DMZ and "no NAT" between the inside and DMZ/DMZ
and inside so that is just routing.
