How to write code that calls a WCF service and falls back from Kerberos to NTLM if needed?

Question

I need to call a WCF service programmatically. The service may be hosted with either NTLM or Kerberos authentication and needs to work under either. That is, if connecting to the service via Kerberos fails, then it should fall back to NTLM.

Here's the code I'm using for Kerberos auth (if relevant, the service is hosted in SharePoint 2010 and is being called from a web part):

public static SiteMembershipSvc.SiteMembershipServiceClient InitialiseSiteMembershipService(string url)
{
    var binding = new BasicHttpBinding();
    binding.Security.Mode = BasicHttpSecurityMode.TransportCredentialOnly;
    binding.Security.Transport.ClientCredentialType = HttpClientCredentialType.Windows;
    url = url.EndsWith("/") ? url + SiteMembershipAddress : url + "/" + SiteMembershipAddress;
    var endpoint = new EndpointAddress(url);
    var proxy = new SiteMembershipSvc.SiteMembershipServiceClient(binding, endpoint);
    proxy.ClientCredentials.Windows.AllowedImpersonationLevel = System.Security.Principal.TokenImpersonationLevel.Impersonation;
    return proxy;
}

Calling a method on the proxy when run in an NTLM environment gives the error:

The HTTP request is unauthorized with client authentication scheme 'Negotiate'. The authentication header received from the server was 'NTLM'.

Note: The URL may be in another web application on another server. I can't check what authentication the web part's web app runs under and assume it is the same as where the WCF service is hosted.

How can I (automatically or manually) ensure authentication falls back from Kerberos back to NTLM on failure?

Update:

As mentioned, the authentication error occurs when a web method is called. However I don't want to wait that long as there are several web methods in the service called from several places. I'd like to test the authentication at the point where the proxy is configured (in the code snippet above).

I've tried using proxy.Open() but that doesn't seem to cause the failure.

Answer 1

I haven't been able to find a way to do this automatically. Instead I've added UI to the application where the type of authentication must be chosen.