RE: Front End/Back End communication



At Monday, May 15, 2006 10:29 AM, timpacalypse@xxxxxxxxx wrote:

I'm curious about how people are implementing FE/BE Exchange
communication. It absolutely kills me that all of this traffic is
being transported through all of these ports via clear text.

IPSec is the recommended way to secure it.

I thought about encrypting all of it using IPSEC but we are using NAT
between the DMZ and the Internal firewall. So all the traffic will
get dropped.

You can pass IPSec through NAT firewalls: NAT-T (NAT Traversal).

However, that design has got to breaking other functionality. Your FE
server (in your DMZ) has to be a member of the AD domain/forest, which
means it needs to be able to initiate connections into the protected
network. At the very least, you need to get these two subnets on a
routing relationship with each other (that is, have your firewall
perform NAT if it's coming from the protected network destined
externally, but not between the protected network and the DMZ). Your
firewall people (if they're not you) will probably stop listening right
about now, but you should remind them that NAT *is not* a security
measure and is intended to conserve publicly routable IP addresses.

Another
question is, even if you do use IPSEC do you still need to open the
individual ports? My understanding is that you don't but someone is
telling me that you do.

What do you mean by "open the individual ports"? You're not using IPSec
to tunnel traffic in this config, so you can just say "Encrypt all
communications to IP address M.N.O.P" -- but then the intervening
firewall will need to allow the proper protocols through (and there are
a LOT of them for an FE in the DMZ).

Better yet, get an ISA Server 2004 box and put *that* in your DMZ. Then
you can move the FE back to the protected network where it belongs, with
all of the advantages:

1) You can easily apply a single IPsec policy via Group Policies for
your Exchange servers. Any new Exchange servers you stand up in the
future will automatically use IPSec just by being a member of the
correct OU.
2) Your firewall configuration between the DMZ and protected network
stops resembling Swiss cheese, as you can close down the holes for
Kerberos, LDAP, GC lookups, SMB, DNS, SMTP, MAPI, and more.
3) You get an application-level proxy that filters incoming SMTP and
HTTP requests and drops malformed/corrupt ones on the floor before they
ever get to your Exchange server.

Microsoft publishes some good guidance on the various FE/BE options and
their security implications. The first guide you should read (if you
haven't already) is the _Exchange Server 2003 and Exchange 2000 Server
Front-End and Back-End Topology_ guide:

http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/febet
op.mspx

--
Devin L. Ganger Email: deving@xxxxxxxxxx
3Sharp LLC Phone: 425.882.1032 x 109
15311 NE 90th Street Cell: 425.239.2575
Redmond, WA 98052 Fax: 425.702.8455
(e)Mail Insecurity: http://blogs.3sharp.com/blog/deving/

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