Windows 2003 SMTP Relay in a DMZ

Posted on 2004-10-07
Last Modified: 2008-01-09
How do I configure the networking for a SMTP Relay in a DMZ?  My DMZ sits behind a Cisco Pix 515.  As I see it I have 2 options.

1.  Either having 2 NIC, one to the DMZ and on the LAN?
2.  or set up routing rules and allow incoming and outgoing SMTP traffic on the PIX from the internet to the SMTP DMZ server and from the SMTP DMZ server to the LAN (Exchange Server).

Setting up the SMTP service (IIS, etc) should not be an issue but I'm unsure on the network setup.  I did try to setup the first senario but as soon as I set up the second nic (eather to the DMS or the Lan) I lose connectivity.

Question by:chilemoore
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LVL 71

Expert Comment

by:Chris Dent
ID: 12250444

The second option is the more secure of the two, it stops any direct connections to your internal LAN from anywhere but the DMZ.

Careful with the SMTP Server that comes with IIS though, I'm pretty sure it will bounce E-mail over 2Mb.

On the Exchange server you would set it to Forward all outbound mail to the SMTP Relay Host on the DMZ.
LVL 15

Expert Comment

ID: 12252349
>>My DMZ sits behind a Cisco Pix 515

DMZs usually do not sit behind firewalls, they are DeMilitarized.. could you explain your setup a little more?

Author Comment

ID: 12252450
Have my DMZ branching off my PIX.  Currently the only thing in the DMZ is my VPN that connects to the LAN via the Private Port, DMZ on the Public.  I do have some restrictions so it's not opened to the Internet.  Plan to put my SMTP relay in the DMZ also.  Any suggestions?  

The plan is to do option 2 above, but wondered if option 1 was duable since my VPN is set up that way.  Plus don't know if I can remotely administer the SMTP relay when it's in the DMZ due to some routing issues on the Pix 515.  That would be me VPNin (Remember VPN server is in the DMZ) and then trying to connect to the SMTP.  Not a big deal, more concerned on getting the SMTP Relay working correctly.

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Author Comment

ID: 12252458
Also thought it was strange that in the 2 nic senario i lost all connectivity when I had the 2 set up (routing?)
LVL 71

Assisted Solution

by:Chris Dent
Chris Dent earned 60 total points
ID: 12252492

The main problem with Option 1 is that it completely bypasses the Firewall, the idea of putting servers in a DMZ (aside from that you can make it as secure as possible) is that, if compromised, it still can't harm your internal network.

By giving a public machine an interface on both networks you give an attacker (if successful) *full* access to the internal network.

Author Comment

ID: 12252524
even though I only alow SMTP form the firwall to the SMTP Relay in the DMZ?  Well not realy Demileterized..
LVL 15

Expert Comment

ID: 12252595
What Chris-Dent is saying, is that if all you're gonna allow into the MailServer is SMTP, and you would physically connect it to your internal LAN, then it is now part of your internal network and not part of the LAN.  you might as well put in behind the firewall and use port forwarding.  If it's in your DMZ, by definition you want all communications with your LAN to go through te firewalls ACL
LVL 15

Accepted Solution

adamdrayer earned 65 total points
ID: 12252607
>>then it is now part of your internal network and not part of the LAN
should have been
then it is now part of your internal network and not part of the DMZ

Author Comment

ID: 12912496
I finally got it going, contacted Cisco and they gave me a hand.

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