Configure web.config to redirect non-www requests to www for multi-site Drupal 7 Install
Some subtle differences between this and the rule above, but this officially works for my multi-site Drupal install on Azure IIS.

&lt;rule name="WWW Rewrite" enabled="true"&gt;
          &lt;match url="(.*)" /&gt;
            &lt;conditions&gt;
                &lt;add input="{HTTP_HOST}" negate="true" pattern="^www\.(.*)$" /&gt;
            &lt;/conditions&gt;
            &lt;action type="Redirect" url="http://www.{HTTP_HOST}/{R:0}" appendQueryString="true" redirectType="Permanent" /&gt;
&lt;/rule&gt;