Outlook Web Access provides Web browser access to e-mail, scheduling (including group scheduling), contacts, and collaborative information stored in Microsoft Exchange Storage System folders. Outlook Web Access is used by remote, home, and roving users.
When you publish Outlook Web Access servers through ISA Server, you are protecting the Outlook Web Access server from direct external access because the name and IP address of the Outlook Web Access server are not accessible to the user. The user accesses the ISA Server computer, which then forwards the request to the Outlook Web Access server according to the conditions of your mail server publishing rule.
Further, ISA Server enables you to easily configure forms-based authentication and to control e-mail attachment availability, to protect your corporate resources when accessed through Outlook Web Access.
The ISA Server Outlook Web Access publishing feature also enables you to publish Outlook Mobile Access and Exchange ActiveSync. Outlook Mobile Access provides users with access to Outlook from mobile devices. Using Exchange ActiveSync, you can synchronize with high levels of security, directly to your Exchange mailboxes from Microsoft Windows® Mobile™-based devices, such as Pocket PC, Pocket PC Phone Edition, and Smartphone.
Forms-Based Authentication
Forms-based authentication is a type of ASP.NET-based authentication in which an unauthenticated user is directed to an HTML form. After the user provides credentials, the system issues a cookie containing a ticket. On subsequent requests, the system first checks the cookie to see if the user was already authenticated, so that the user does not have to supply credentials again.
Most importantly, the credential information is not cached on the client computer. This is particularly important in a scenario where users are connecting to your Outlook Web Access server from public computers, where you would not want user credentials to be cached. Users are required to reauthenticate if they close the browser, log off from a session, or navigate to another website. Also, you can configure a maximum idle session time-out, so that if a user is idle for a prolonged period of time, reauthentication is required.
We recommend that when using forms-based authentication, you use HTTPS for all communications with the site to prevent hackers from stealing the user’s cookie. HTTPS is recommended in general for Outlook Web Access server publishing.
The procedure for configuring forms-based authentication is provided in Secure Outlook Web Access through the Listener in this document.
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ISA Server supports forms-based authentication for Exchange Server 2003, Exchange 2000 Server, and Exchange Server 5.5.
When you use ISA Server 2004 with Exchange Server 2003, you must choose to use forms-based authentication of only one of the products. If you use ISA Server forms-based authentication, you retain the ISA Server functionality to inspect response bodies, as well as request URLs, request headers, request bodies, and response headers. ISA Server forms-based authentication provides the additional benefits of authentication at the edge of the network and RADIUS-based authentication without domain membership. However, if you use ISA Server forms-based authentication, you cannot use the Exchange data compression feature.
If you use Exchange Server 2003 forms-based authentication, ISA Server inspects request URLs, request headers, request bodies, and response headers, but does not inspect response bodies. However, you retain the Exchange data compression feature.
When you use ISA Server 2004 with Exchange Server 2000 or Exchange Server 5.5, which do not provide forms-based authentication or data compression, we recommend that you use the ISA Server forms-based authentication feature.
Outlook Web Access includes optional functionality that allows a user to change the password. If a user changes the password during an Outlook Web Access session, the cookie provided after the user initially logged on will no longer be valid. When forms-based authentication is configured on ISA Server, the user who changes the password during an Outlook Web Access session will receive the logon page the next time a request is made.
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Controlling Attachment Availability
Because Outlook Web Access is often used from public computers, you may want to control the user’s ability to view and save attachments, so that private corporate information is not cached or saved to a public computer. ISA Server provides a mechanism for blocking e-mail attachments for users on public (shared) computers or users on private computers (or both). This prevents users from opening or saving attachments, although the attachments can be seen by the users. The procedure for blocking e-mail attachments is provided in Secure Outlook Web Access through the Listener in this document.
If you do not block attachments, note that some attachments, such as Windows Media® files and Excel spreadsheets, cannot be opened directly by a client connected remotely to an Outlook Web Access server. An attempt to open such a file will result in a failure of the application associated with the file. Those files must be saved locally and can then be opened. You can avoid this problem by configuring Exchange Server 2003 or Exchange 2000 Server to force users to save attachments. This feature is not available on Exchange Server 5.5. Configuring Exchange to force the saving of attachments is described in Require the Saving of Attachments in Exchange in this document.
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Exchange 2003 provides an attachment blocking feature, which blocks some type of files even if the feature is disabled.
When publishing Exchange Server 2007 with ISA Server 2004 with Service Pack 3 installed, note the following:
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When publishing using Exchange Server 2007, we recommend you configure attachment blocking on the Exchange server, instead of enabling attachment blocking in the ISA Server 2004 Web listener. For more information about configuring attachment blocking on Exchange Server 2007, see Exchange Server 2007 product Help.
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For authentication to succeed, the Exchange Client Access server must be configured for Basic authentication. For details about configuring the Exchange Client Access server, see Exchange Server 2007 product Help.
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If your ISA Server firewall policy already includes a Web publishing rule for Exchange Server, you cannot modify the existing rule to publish Exchange Server 2007. You must delete the existing rule and run the Web Publishing Wizard to create a new rule.