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Replacing the DMZ for Outlook Web Access to Exchange 2010 CAS

Posted on 2014-02-24
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Last Modified: 2014-02-26
Greetings,

For the last few years I've been running clients on Exchange 2003 servers under Windows 2003, and for iphone, OWA and remote Outlook user access I've been using dedicated low-level servers that run a front-end RPC/HTTP Exchange server.  The RPC/HTTP server was always in a DMZ, and the only port I allowed in to that DMZ was HTTPS.  Between the RPC/HTTP server in the DMZ and the Exchange server and domain controllers on the LAN, I had approx. 20 ports open for protocols such as LDAP, DCE Endpoint, RPC Netlogon access etc.  The goal of this setup was to minimize the threat of a hacker getting through to and controlling the OWA server, since even if this happened, they'd still be in the DMZ, and would not have full access to the LAN.  Now looking at Exchange 2010, I am seeing that Microsoft specifically recommends against putting the CAS server in a DMZ.  What is the best practice today for securing the CAS server with regards to OWA and Outlook Anywhere access via HTTPS?  I've seen some references to a reverse proxy server on the DMZ which passes HTTPS through to the CAS server.  

Appreciate your feedback and suggestions,

many thanks,

jkirman
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Question by:jkirman
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Simon Butler (Sembee) earned 250 total points
ID: 39882819
You realise that your DMZ did not do what you thought it did?
Having an Exchange server in a DMZ has always been an idea - if an attacker compromised your Exchange server he could walk straight in to your live network through one of those 20 or so ports you had open.

Here is my comprehensive reasons why using a DMZ for Exchange is a bad idea:
http://blog.sembee.co.uk/post/Why-you-shouldnt-put-Exchange-2003-in-a-DMZ.aspx

Anyway, on to your question.
You basically have two options.

1. Open the Exchange server straight to the internet. You only need two ports - 443 and 25. The combination of Exchange and IIS on a dedicated box has never been compromised.

2. A reverse proxy. That would be something like TMG, although Microsoft have discontinued that product, so you may have to find something else from a third party.
You can do a kind of reverse proxying with the latest versions of Windows Server.
http://blogs.technet.com/b/exchange/archive/2013/07/19/reverse-proxy-for-exchange-server-2013-using-iis-arr-part-1.aspx
However I believe it is only supported for use with Exchange 2013.

Although do read this article from the Exchange team as well, about life after TMG and reverse proxies.
http://blogs.technet.com/b/exchange/archive/2013/07/17/life-in-a-post-tmg-world-is-it-as-scary-as-you-think.aspx

Simon.
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by:hecgomrec
ID: 39886039
No. I don't agree.

Microsoft recommend to have an Edge Transport Server on the DMZ, not even a DC, to do all the receiving and sending and passing that info to the Hub Transport Server.

This scenario is not necessary but recommended as Exchange can use only the Hub Transport Server for internet facing service.
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by:Simon Butler (Sembee)
ID: 39886728
The Edge Server role only does SMTP traffic, it has nothing to do with the CAS role.
The question was about the CAS functionality, which cannot go in a DMZ when you are using an Exchange server.

Simon.
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by:hecgomrec
ID: 39888681
Simon... you are right!!!

My intention was to make him understand that the only server on DMZ should be the Edge Transport, everything else should be behind the firewall protection with the proper ports open.

For more on securing your server roles/apps  go: http://technet.microsoft.com/en-us/library/bb400932(v=exchg.141).aspx
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Author Closing Comment

by:jkirman
ID: 39889985
Simon, thanks for the articles.  The Exchange Team blog was enlightening, as it basically says a lot of the intended security actually just makes access more difficult vs. enabling the secure connections actually needed.  All too true.  ISA and TMG are options I'll take a look at.  Finally I'll be using an IDS on the firewall (I use Sonicwall appliances) which does provide an additional level of protection at a relatively low annual subscription cost.

Regards,

jkirman
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Author Comment

by:jkirman
ID: 39889989
hecgomrec, thanks for your input, I already had the article you recommended.
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