Re: DMZ design - Exchange, SQL, & DCOM

[Bill Pennington <billp () rocketcash com>]

>

Just my .02....


> Michael Borkin wrote:
>
> 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.

3 nics is the way to go for the highest degree of security. Most SDSL routers I have seen are a little weak when it 
comes to filtering, and even then it is just port filtering which is good but not perfect as far as a security stradigy 
goes. I don't understand a lot of your comments about the 2+2 config. You want all traffic inbound to flow through your 
firewall. This is commonly refered to as a choke point. If you only have a single point of entry to your network it is 
easier to build up defenses.


> 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?

Get a linux box put sendmail on it ans place it in the DMZ. Allow anyone to establish an SMTP connection to it from the 
outside. Then put a hole from the DMZ to your internal exchange server for SMTP traffic only. I think the other person 
you where speaking with is confused about big holes in your firewall. If you are using SMTP to pass mail then you only 
need to open up port 25 SMTP. If you are trying to do the full blown Exchange site using the Exchange MTA, then yes it 
gets ugly. You don't need to do that though.

> 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.

The web server should be in the DMZ as should the SQL sever IMHO. The SQL server should NOT be accessable from the 
outside at all. It should only talk to the web server and internal clients. Then open a hole from the inside to the SQL 
server for the SQL server traffic (port escapes at the moment). Add of course open up HTTP and HTTPS from the inside to 
the DMZ.

A couple of issues you should keep in mind....

1. Every box should be behind a firewall period.

2. Do whatever port filtering you can at your router.

3. Find the Checkpoint FW-1 install checklist. (I think you might be able to get it at www.securityfocus.com but I am 
not sure)

4. Get an stand alone backup solution for servers in your DMZ.

5. Try to keep DMZ>Inside holes to a minimum.

6. Get "Building Internet Firewalls" by Chesowick (sp?) and Bellovan (sp?). It has lots of good theory and examples.

7. Watch your logs!



> 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

--

Bill Pennington
IT Manager
Rocketcash
billp () rocketcash com
http://www.rocketcash.com