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The protective techniques that firewalls employ sometimes create unexpected side effects for legitimate applications. One problem that can crop up involves Outlook Web Access (OWA). Normally OWA works perfectly. But sometimes, for no discernible reason, certain messages will not display.
When a user tries to access a message, Internet Explorer produces the nondescript "This page cannot be displayed" error, with no further information. The message may be accessible from inside the organization or over a VPN, though, which makes it all the more puzzling.
The answer lies in how some firewalls handle lengthy URLs. ("Lengthy" in this case means a thousand or more characters.) Even if a firewall is configured to allow access through port 80, it may still perform traffic analysis on HTTP requests passed through it.
Some firewalls have standing rules about how long a GET or POST request can be as a way of limiting possible buffer-overflow attacks through massively lengthy or malformed URLs. If these rules are too strict, they can cause OWA to fail at random. (Note that the length of the URL itself has nothing to do with the size of the page requested by the URL.)
There should be a way to edit your firewall's settings to allow longer URLs -- 2,000 characters at most should do it. If not, check with your firewall manufacturer for updated firmware that might solve the problem or add more functionality. Also, when you do this, make sure the Exchange system in question has been brought up to date with all available security patches.
Finally, some firewalls also object to the lengths of particular headers within a HTTP response. I am not aware of OWA experiencing problems specifically because of oversized headers in the responses it provides to clients, but it's something worth keeping in mind if the problem persists.
About the author: Serdar Yegulalp is editor of the Windows 2000 Power Users Newsletter and a regular contributor to SearchExchange.com.
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