I solved my problem by using an OWIN middleware as a proxy, rather than using UrlRerwrite/ARR. This worked right away.

If someone comes up with a pure UrlRewrite/ARR solution, I am still interested. Here what works for me:

I found this ReverseProxyMiddleware for .Net Framework. Using this, my OwinStartup contains the following code:

    public override void Configuration(IAppBuilder app)
    {
        // this must come before the base implementation so proxy kicks in before Umbraco

        app.UseProxy(
            new List<ProxyRule> {

                // script proxy
                new ProxyRule {
                    Matcher = uri => 
                        uri.AbsoluteUri.Contains("localJs/proxiedScriptFile.js")
                    ,
                    Modifier = (req, user) => {
                        req.RequestUri = new Uri("https://thirdpartyserver.de/js/script.js");
                    },
                    RequiresAuthentication = false
                }
            },
            r =>
            {
            }
        );

        base.Configuration(app);

    }

Works like a charm. I post it here in case somebody else faces the same challenge.

Hi Mikael,

Thanks for the quick reply. How can I implement this in My Umbraco 8 application which is created using the .net framework? Is it possible to register this in Umbraco Composer?

Hi Diljjith,

no, this does not go into the Composer class.

You create a custom OWIN startup class which inherits from UmbracoDefaultOwinStartup. You then register your custom startup class in web.config. As far as I can remember, that's all which is needed.

My complete custom startup class looks like this:

using Microsoft.AspNetCore.Builder;
using Microsoft.Owin;
using Owin;
using ReverseProxyMiddleware;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Web;
using System.Web.Hosting;
using Umbraco.Core;
using Umbraco.Web;

[assembly: OwinStartup("DhyMikOwinStartup", typeof(DhyMikOwinStartup))]

namespace Website;

public class DhyMikOwinStartup : UmbracoDefaultOwinStartup
{
    public override void Configuration(IAppBuilder app)
    {
        // this must come before the base implementation so proxy kicks in before Umbraco

        app.UseProxy(
            new List<ProxyRule> {

                // script proxy
                new ProxyRule {
                    Matcher = uri => 
                        uri.AbsoluteUri.Contains("localJs/proxiedScriptFile.js")
                    ,
                    Modifier = (req, user) => {
                        req.RequestUri = new Uri("https://thirdpartyserver.de/js/script.js");
                    },
                    RequiresAuthentication = false
                }
            },
            r =>
            {
            }
        );

        base.Configuration(app);
    }
}

And this is what I have in web.config:

<configuration>
    ....
    <appSettings>
        ...
    <add key="owin:appStartup" value="DhyMikOwinStartup" />
        ....
    </appSettings>
</configuration>

Hope this clarifies it!

Kind regards, Mikael

Many thanks! This solved my problem.

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