Hypertext Transfer Protocol (HTTP) uses a series of commands, called "verbs," to describe what action

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is taking place between the client and server. Two of the most common HTTP verbs are GET and POST. The former is used for retrieving a Web document and the latter is used for sending form information to a server.

One of the newer HTTP verb sets is implemented in a standard called Distributed Authoring Version, or HTTP-DAV, which has been specified in RFC 2518.

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

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