IIS Server Farm - URL Rewrite

I am trying to create IIS Server Farm, and having issues with the URL Rewrite to work correctly.

Windows 10 Enterprise IIS 10

Below is the relevant configurations.

<applicationPools>
    <add name="always-blue" autoStart="true">
        <processModel pingingEnabled="true" pingResponseTime="00:01:30" />
    </add>
    <add name="always-green" autoStart="true">
        <processModel pingingEnabled="true" pingResponseTime="00:01:30" />
    </add>
</applicationPools>
<sites>
    <site name="always-blue" id="2" serverAutoStart="true">
        <application path="/" applicationPool="always-blue">
            <virtualDirectory path="/" physicalPath="C:\xxx\always-up-blue" />
        </application>
        <bindings>
            <binding protocol="http" bindingInformation="*:80:alwaysup-blue" />
        </bindings>
    </site>
    <site name="always-green" id="3" serverAutoStart="true">
        <application path="/" applicationPool="always-green">
            <virtualDirectory path="/" physicalPath="C:\xxx\always-up-green" />
        </application>
        <bindings>
            <binding protocol="http" bindingInformation="*:80:alwaysup-green" />
        </bindings>
    </site>
</sites>

<rewrite>
    <globalRules>
        <rule name="alwaysup rewrite" stopProcessing="false">
            <match url=".*" />
            <conditions>
                <add input="{HTTP_HOST}" pattern="^alwaysup$" />
                <add input="{SERVER_PORT}" pattern="^80$" />
            </conditions>
            <action type="Rewrite" url="http://alwaysup/{R:0}" />
        </rule>
    </globalRules>
</rewrite>

Both http://alwaysup-blue and http://always-green websites are up and work normal. However the server farm doesn't work. http://alwaysup -> Returns 502. I checked the logs with FailTracedURLs, but couldn't make much sense of that. Here are the logs ..

    <webFarms>
        <webFarm name="alwaysup" enabled="true">
            <server address="alwaysup-blue" enabled="true">
                <applicationRequestRouting hostName="alwaysup-blue" httpPort="8081" />
            </server>
            <server address="alwaysup-green" enabled="true">
                <applicationRequestRouting hostName="alwaysup-green" httpPort="8082" />
            </server>
            <applicationRequestRouting>
                <healthCheck url="http://alwaysup/up.html" responseMatch="up" />
                <loadBalancing />
                <protocol preserveHostHeader="false">
                    <cache enabled="true" validationInterval="00:01:00" />
                </protocol>
            </applicationRequestRouting>
        </webFarm>

Had to add the preserveHostHeader to webfarm section of configuration.