asp.net - Forms authentication: disable redirect to the login page

Question

I have an application that uses ASP.NET Forms Authentication. For the most part, it's working great, but I'm trying to add support for a simple API via an .ashx file. I want the ashx file to have optional authentication (i.e. if you don't supply an Authentication header, then it just works anonymously). But, depending on what you do, I want to require authentication under certain conditions.

I thought it would be a simple matter of responding with status code 401 if the required authentication was not supplied, but it seems like the Forms Authentcation module is intercepting that and responding with a redirect to the login page instead. What I mean is, if my ProcessRequest method looks like this:

public void ProcessRequest(HttpContext context)
{
    Response.StatusCode = 401;
    Response.StatusDescription = "Authentication required";
}

Then instead of getting a 401 error code on the client, like I expect, I'm actually getting a 302 redirect to the login page.

For nornal HTTP traffic, I can see how that would be useful, but for my API page, I want the 401 to go through unmodified so that the client-side caller can respond to it programmatically instead.

Is there any way to do that?

Answer 1

ASP.NET 4.5 added the Boolean HttpResponse.SuppressFormsAuthenticationRedirect property.

public void ProcessRequest(HttpContext context)
{
    Response.StatusCode = 401;
    Response.StatusDescription = "Authentication required";
    Response.SuppressFormsAuthenticationRedirect = true;
}