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Exchange 2007 with ISA 2006

Today I have be working on publishing Microsoft Exchange Outlook WebAccess and Active Sync to the Internet. We had some discussions with some Microsoft Consultants about a secure way to publish Outlook Web Access to the Internet, especially the authentication part of such a solution.

Some people are talking about publishing OWA directly to the Internet. In my opinion, this results in a major security thread, because you directly publish a TCP/80 and TCP/443 connection from the Exchange server to the Internet. An vulnerability or exploit in these services could end up in an hacker who takes over the Exchange server.

A second solution is placing a front-end server in a DMZ segment, but making the server a domain member for authentication. In my opinion still a security leak, because somebody who hacks the DMZ server has maybe the ability to hack or corrupt the Active Directory.

The third solution, and the solution we advise, is using a Microsoft ISA 2006 server as a front-end server in the DMZ. We configure a RADIUS or LDAPS (if you would like the option to change the password) connection to a RADIUS server or a domain member on the internal LAN segment. This ensures a secure way of authenticating users and even if somebody hacks the ISA server, he still hasn’t hacked a domain member server or a vulnerability in TCP/80 or TCP/443 of the Exchange server.

I have had a lot of help of an article on isaserver.org from Thomas Shinder while configuring the solution. I had some problems with publishing Active Sync. Ended up with enabling Basic Authentication on the Active Sync virtual directory (Microsoft-Server-ActiveSync).

LDAP and eSafe Gateway

eSafe Gateway can be used for scanning incoming and outgoing SMTP connections for virusses and SPAM. Normally eSafe Gateway doesn’t check incoming mail addresses against a directory like Active Directory or Novell Directory Services.

This means that all mail addresses for a trusted domain are forwarded to the internal mail server. In the most ideal situation unknown mail addresses should be blocked at the eSafe Gateway. This feature will take away load from the internal mail server, because this mail server doesn’t have to generate NDR (Non-Delivery Reports) messages. Beside that, the eSafe Gateway also doesn’t have to process the NDR’s. LDAP (Lightweight Directory Access Protocol) provides this functionality.

With LDAP configured, the eSafe Gateway will synchronize all known mail objects from the directory services with the eSafe Gateway. By this, the eSafe Gateway knows all valid mail objects and can block invalid mail objects. There are some issues when configuring a LDAP query with Active Directory. By default Active Directory only allows 1000 objects in one query. Some customers have more mail object, so this settings needs to be added. Inside Active Directory, you should edit the LDAP Policy setting MaxPageSize. Look here for more information about editing the MaxPageSize variable.

Some organizations use PublicFolders in conjunction with Microsoft. These PublicFolders can be mail-enabled and should be added in the LDAP filter configuration inside eSafe Gateway. This is done by changing the default filter

(&(|(objectClass=person)(objectClass=contact)(objectClass=organizationalPerson))(!(objectClass=computer)))

in

(&(|(objectClass=person)(objectClass=contact)(objectClass=organizationalPerson)(objectClass=publicFolder))(!(objectClass=computer)))

This results in adding the mail object PublicFolder to the LDAP query.