Microsoft Exchange Server uses HTTP-DAV to send and receive requests called Distributed Authoring and Versioning Searching and Locating (DASL), which help run Outlook Web Access (OWA). Internet Explorer versions 5.0 and up use HTTP-DAV, but other browsers -- such as earlier versions of Netscape -- do not.
Proxy servers or firewalls that closely screen HTTP traffic may unintentionally block legitimate OWA traffic if they don't allow the use of HTTP-DAV verbs. Under these conditions, the infamous "Loading …" screen will appear indefinitely when users try to access OWA.
NOTE: This issue only occurs with browsers that explicitly support HTTP-DAV; browsers that don't support HTTP-DAV will revert to a lower level of functionality.
The two most basic ways to work around this problem:
- Update the firewall or proxy server to support HTTP-DAV verbs. This isn't always
possible or practical, but it is the best first approach, since it may solve a number of other
problems as well.
- Set up Secure Sockets Layer (SSL) for OWA. The firewall or proxy server will not be able to screen encrypted traffic, so it will pass through as-is. This is useful if you can't upgrade the firewall or proxy server; it also adds another level of security to OWA.
ABOUT THE AUTHOR:
Serdar Yegulalp writes about personal computing and IT for a variety of publications, including (among others) Windows Magazine, InformationWeek and the TechTarget family of sites.
This was first published in September 2006