DMZ design - Exchange, SQL, & DCOM

["Michael Borkin" <borkin () netquest com>]

I have been called upon to re-design an existing network to allow the hosting of a web and e-mail server.  It is a pure 
Microsoft network (95/98, NT, and W2K) that will incorporate a checkpoint FW-1 firewall (actually VPN-1) as part of the 
design.  My main questions at this point have to do with the DMZ, what belongs there, and how to connect it to the 
firewall and the internet.  The connection to the internet will come in over an SDSL router (brand unknown at this 
time), but from there I have gotten conflicting advice.  

Should all traffic be passed back to the firewall which will have 3-nic cards (1- Internet, 2- DMZ, 3- Internal 
network), or should the router itself have two ethernet ports (1- Firewall, 2- DMZ) and the firewall only have two nic 
cards (1- Internet, 2- Internal Network) as well?  The argument for the 3-card configuration is that logging is better 
that way.  Meanwhile, the 2+2 argument is to keep as little traffic from being able to flow into and through the 
firewall machine as possible for both overhead and security reasons.  I am leaning towards the 3-card configuration 
based on the fact that it is the recommendation from Checkpoint (or at least their vendors), but I would like to know 
if anyone has any opinions before I decide.  As for the machines in the DMZ, other than the web server itself (IIS 4.0) 
I am not sure which ones need to reside there and which need to be placed on the internal network for the best security 
configuration.  Below is described the main services that I am concerned with at the moment.

E-mail is currently handled by an Exchange Server, but is also used for services besides just internet e-mail such as 
public folders and internal company mail.  One person therefore recommended setting up an SMTP box in the DMZ and 
having it dedicated to relaying internet based e-mail from the outside back through the firewall (and vice-versa) to 
protect the other information on the Exchange server.  That sounded good to me, but later when I was discussing this 
with another person I got a totally different opinion.  He said it was a bad idea to let another box handle the e-mail 
and that to have the Exchange box on the internal network would cause me to have to punch huge holes in the firewall to 
let certain services through.  Therefore, the Exchange box needed to reside in the DMZ rather than behind it.  What he 
said really didn't make sense to me, because I would think that it would be having the Exchange server in the DMZ that 
would cause me to have to punch holes rather than the other way around.  But, just because I don't understand his 
reasoning doesn't mean he is incorrect especially since he knows a lot more about firewalling than I do, so I ask which 
is the better way to go?

Next, is that the web server uses dynamic html for much of the website content.  This leverages both a SQL server and 
DCOM programming built through Visual InterDev to deliver the content to the web server.  This is where it really goes 
over my head at the moment, if it was just SQL server then I know to place it on the inside and let the calls from the 
web server come back through the firewall.  However from what I have been told by a developer, DCOM uses dynamic port 
allocation when establishing a stateful connection (although from what I have read it uses udp, so I don't know why 
there should be a stateful connection).  I honestly don't understand enough to know where the DCOM part of the process 
sits (although I am guessing it is on the web rather than the database server), and whether this means that I have to 
open up a port range for DCOM to work properly or to move the SQL server out to the DMZ (neither of which sounds like a 
good idea to me).  Also, I am not sure about what ports or rules would need to be incorporated to get this to function 
as securely as possible if everything other than the web server resides behind the firewall.

If anyone could either point me towards reference material and/or give me advice about how the DMZ portion of the 
network should be setup based on the factors explained above it will be greatly appreciated.  If you need any further 
information before making a suggestion or recommendation, please feel free to contact me either on or off list and I 
will be more than glad to do what I can to fill in the gaps.  

Thanks,

Mike