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Sharepoint cannot authenticate to ISA Server

This gave and gives me a hard time! Imagine MOSS 2007 running in a corporate intranet. Your requirement is to connect to datasources on the internet. There are various scenarios, the most common are to

• Consume RSS Feeds using the RSSFeed webpart
• Consume a WebService or Server-Side Script using the SharePoint Designer DataView Webpart

When using the DataView WebPart, you'll get the following error message when trying to show the data from the internet source:

“The server returned a non-specific error when trying to get data from the data source. …”

When connecting using the RSSFeed Webpart, the error message will be even less saying...

OK, what can we do? Make sure that you've set the proxy in the web.config of your SharePoint Site Collection, as documented by Microsoft here: http://support.microsoft.com/kb/923173

However, this might still not do the trick! The DataView WebPart still shows the same error message. However, the RSSFeed Webpart is this time a little bit more specific:

“ProxyAuthenticationRequired (The ISA Server requires authorization to fulfill the request…)"

So wat is the problem? It seems (and some other bloggers mentioned this explicitely) that SharePoint doesn not authenticate itself towards the ISA Server! The only solution to this problem is to allow requests from all the IP addresses of all SharePoint WFE Servers to pass through ISA! (wheew, I hear your customers start to question you louder than normal - at least this is what happened here ;))

Well, this is what I know so far and granting all requests of my SharePoint WFE's to pass through works just perfectly. I'll try to investigate in this a bit more internally and make sure that MOSS server-side authentication is included in Office 14...

Hope this helps some of you to save a lot of valuable time...

Read the complete post at http://www.sharepointblogs.com/michael/archive/2007/09/06/sharepoint-cannot-authenticate-to-isa-server.aspx