[YARP] How to set up a reverse proxy for another full web server in a sub path?

[AbdallahR99]

I have a Qliksense server on some domain and I have its certificate (pfx) and I want to make a reverse proxy to it with ASP.NET + YARP
the issue is when I set the proxy prefix as /qlik for example
and go to localhost/qlik
it would gives me 404 because the qlik server would read /qlik as a route and will try to fine the page that called qlik
but when I remove the prefix of /qlik in the transform section
then go to localhost/qlik
the QlikSense server redirects me to another path called /hub
so it would be like localhost/qlik => localhost/hub
and this time my backend app itself is who would gives me 404 error because it will search of the /hub route which only exist on the QlikSense server and not my app

so I need a dynamic way to make all of the QlikSense paths be of sub path /qlik
like it should be when I go to localhost/qlik => localhost/qlik/hub and the qliksense server would read the path as /hub not /qlik/hub
or anyway to fix this

I heard about something in the transformation like:

{
            "ResponseHeader": "Location",
            "Set": "{Scheme}://{Host}/qlik{Headers.Location}",
            "When": "Response"
          }

but it gives me error, not sure if I'm using it correctly

[MihaZupan]

The backend server has to cooperate with a reverse proxy. For example, it should use relative paths in hrefs/redirects, or it should generate those paths by honoring headers like "X-Forwarded-Prefix".

If the backend service isn't cooperative / you can't control it, you then have to fix up any such responses.
The configuration file transforms don't currently support the sort of syntax you tried to use to generate dynamic header values.
You could achieve this sort of transformation from code though, e.g. something like

builder.Services.AddReverseProxy()
    .LoadFromConfig(builder.Configuration.GetSection("ReverseProxy"))
    .AddTransforms(context =>
    {
        if (context.Route.RouteId == "myQlikRoute")
        {
            context.AddResponseTransform(context =>
            {
                if (context.ProxyResponse.Headers.Location is { } location)
                {
                    context.HttpContext.Response.Headers[HeaderNames.Location] = "...";
                }

                return ValueTask.CompletedTask;
            });
        }
    });