SharePoint XML Web Part and proxy setting

XML Web Part is quite useful when you want to collect data from other sites (e.g. RSS). Yesterday I wanted to display local weather information using XML Data Feed. I followed these steps:
  1. Registered to to download the SDK (to use weather images)
  2. Copied SDK images to TEMPLATE\Images\weather\
  3. Added XML Web Part to WSS site and set URL to*&prod=xoap
    &unit=e&par=null&key=(license key)
    [Update] has changed the URL to[zipcode]?cc=*&dayf=5
  4. Set up XSLT to display weather images
But I got errors on different WSS machines:
  1. "Cannot retrieve the URL specified in the XML Link property"
  2. "The web part has timed out"
The reason was my WSS sites were behind a proxy server. WSS XML Web Part need know proxy settings, otherwise it can not access the URL.

The first error happens when the WSS application pool does not use a domain account; the second error happens when a domain account is used.

After I added proxy settings in web.config, the problem was solved:

<!-- Proxy setting -->
<defaultproxy useDefaultCredentials="true">
<proxy usesystemdefault="false"