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. )"
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.
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:
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.
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:
Protocols: HTTP (80)
From / Listener: hostname of your frontend server
To: External (Internet)