I recently had a client with an externally facing IIS server using the Default Website, and multiple Virtual Directories. They wanted everyone accessing one particular Virtual Directory to be redirected to a completely separate website. The ultimate goal in mind was to ensure that only internal users connecting to http://hostname/virtualdirectory will have access to the page.
After look into the best way to make this work, I found that utilising URL Rewrite on their IIS server would give the best solution.
After replicating the setup in a test lab, I managed to get the URL Rewrite working, to only redirect to an external website when someone accessed http://subdomain.domain.com/virtualdirectory
The following config was entered into the web.config file for the specific Virtual Directory.
<system.webServer>
<rewrite>
<rules>
<rule name="External Redirect" stopProcessing="true">
<match url="Test4" negate="true" />
<conditions>
<add input="{HTTP_HOST}" ignoreCase="true" negate="true" pattern="IIS"/>
</conditions>
<action type="Redirect" url="http://www.google.com.au" redirectType="Found" />
</rule>
</rules>
</rewrite>
</system.webServer>
The items in bold require changing to match your specific IIS setup. These are currently set to replicate my test lab environment.
When navigating to http://iis/test4, it loads the page. When navigating to http://subdomain.domain.com/test4, it redirects to Google. The setting is specific to the Virtual Directory, and other VDs are not affected by this.