Configure URL Rewrite using server variable to sup

Question

I was dealing with CORS issue a couple days ago where I need to support multiple origins. I did some research and found a couple posts pointing me to this great tool (URL Rewrite). I got the solution I wanted following Paco Zarate tip here: Access-control-allow-origin with multiple domains using URL Rewrite. I also found another post here: http://www.carlosag.net/articles/enable-cors-access-control-allow-origin.cshtml showing me how to achieve the same solution with a different method of configuration within URL Rewrite.

Paco Zarate solution works like a charm. Why am I still asking question? It's just for learning purposes. And also I think the second post configuration yields a more elegant list of origins/domains I want to white-listed. For ease of maintainability I can just go to the Rewrite Maps and see all of my AllowedOrigins.

When attempted the second post's solution, I got this message: ERR_CONNECTION_RESET under Console tab of the browser debugger tool. And the request header under Network tab says "Provisional headers are shown"

Many thanks in advance.

My config file:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
    <httpErrors errorMode="Detailed" />
    <urlCompression doStaticCompression="true" doDynamicCompression="true" />
    <httpProtocol>
        <customHeaders>
            <add name="Access-Control-Allow-Headers" value="Origin, Content-Type, Accept" />
            <add name="Access-Control-Request-Method" value="POST" />
            <add name="Access-Control-Allow-Credentials" value="true" />
            <add name="Access-Control-Allow-Origin" value="http://localhost:8080" />
        </customHeaders>
    </httpProtocol>
    <rewrite>
        <rules>
            <rule name="Capture Origin Header"> 
        <match url=".*" /> 
        <conditions>
                    <add input="{HTTP_ORIGIN}" pattern=".+" /> 
        </conditions> 
        <serverVariables>
                    <set name="CAPTURED_ORIGIN" value="{C:0}" /> 
        </serverVariables> 
        <action type="None" /> 
        </rule>
            <rule name="Preflight Options" enabled="false" stopProcessing="true">
                <match url=".*" />
                <conditions>
                    <add input="{REQUEST_METHOD}" pattern="^OPTIONS$" />
                </conditions>
                <action type="CustomResponse" statusCode="200" statusReason="Preflight" statusDescription="Preflight" />
            </rule>
        </rules>
        <rewriteMaps>
            <rewriteMap name="AllowedOrigins">
                <add key="http://somedomain:8080" value="http://localhost:8080" />
            </rewriteMap>
        </rewriteMaps>
         <outboundRules> 
            <rule name="Set-Access-Control-Allow-Origin for known origins"> 
                <match serverVariable="RESPONSE_Access-Control-Allow-Origin" pattern=".+" negate="true" /> 
                <conditions> 
                    <add input="{AllowedOrigins:{CAPTURED_ORIGIN}}" pattern=".+" /> 
                </conditions> 
                <action type="Rewrite" value="{C:0}" /> 
            </rule> 
        </outboundRules> 
    </rewrite>
    <tracing>
        <traceFailedRequests>
            <add path="*">
                <traceAreas>
                    <add provider="WWW Server" areas="Rewrite" verbosity="Verbose" />
                </traceAreas>
                <failureDefinitions timeTaken="00:00:00" statusCodes="500" verbosity="Error" />
            </add>
        </traceFailedRequests>
    </tracing>
</system.webServer>