Surprising X-Forwarded-Prefix transform behavior

[brnls]

I have an asp.net core backend server with YARP in front. I am forwarding requests and stripping the path base when it forwards on. I am able to get it working but was confused on one bit.

IncomingHost:5000/backend1 -> DestinationHost:5001
IncomingHost:5000/backend2 -> DestinationHost:5002

new RouteConfig
{
    ClusterId = GetCluster(proxy.Path),
    RouteId = $"{proxy.Path}_route",
    Match = new RouteMatch
    {
        Path = $"/{proxy.Path}/{{**catchall}}",
        Hosts = new []
        {
            host
        }
    },
    Transforms = new List<Dictionary<string, string>> { }
}
.WithTransformPathRemovePrefix($"/{proxy.Path}")
.WithTransformRequestHeader("X-Forwarded-Prefix", $"/{proxy.Path}")
.WithTransformXForwarded()  // Why this is required?

Why is the WithTransformXForwarded required? If I don't provide it, the header is not sent. I understand I would use WithTransformXForwarded() if I wanted to set the header from the PathBase, but since I am setting it manually it seems unrelated and surprising that it is required.

Is this correct behavior? I may be misunderstanding correct usage of this API.

[benjaminpetit]

That's correct, if WithTransformXForwarded isn't called, all the X-Forwarded-* headers are stripped if present, and not added.

[brnls]

Okay, thanks for the reply.