SharePoint 2010 RSS feed and TMG: “Proxy Authentication required” error

Problem:

When you configure a webpart for RSS feeds in a SharePoint 2010 site, you receive the following error:

ProtocolError occured trying to complete the request. The server returned a status code of : ProxyAuthenticationRequired and the status description is : "Proxy Authentication Required ( Forefront TMG requires authorization to fulfill the request. Access to the Web Proxy filter is denied. )"

Solutions:

There are two things you can do, but maybe just one will work. In my case, solution 2 was in the end the only solution after testing and investigating half a day.

Solution 1

1. Navigate to the virtual directory of the SharePoint site, like: C:\inetpub\wwwroot\wss\VirtualDirectories\yoursitename

2. In the root you will find the file web.config. Edit this file and search for the line with <system.net>. Now copy and paste the following lines and make sure you change the value proxyaddress to the address of your own proxy server:

<system.net>

    <defaultProxy enabled="true" useDefaultCredentials="true"> 

        <proxy bypassonlocal="true" proxyaddress=http://10.1.2.3:80 /> 

    </defaultProxy> 

</system.net>

3. Save the file and restart IIS. To reset IIS, click Start > All Programs > Accessories > right-click Command Prompt, select Run as administrator, type in iisreset, and press ENTER.

Solution 2

Configure ISA/TMG to allow unauthenticated traffic to the Internet. Just add a rule which allows the SharePoint frontend server to navigate to the Internet (network External) over port 80.  Rule settings:

Action: Allow

Protocols: HTTP (80)

From / Listener: hostname of your frontend server

To: External (Internet)