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AuthenticationManager.CustomTargetNameDictionary Property

Gets the dictionary that contains Service Principal Names (SPNs) that are used to identify hosts during Kerberos authentication for requests made using WebRequest and its derived classes.

Namespace:  System.Net
Assembly:  System (in System.dll)

public static StringDictionary CustomTargetNameDictionary { get; }

Property Value

Type: System.Collections.Specialized.StringDictionary
A writable StringDictionary that contains the SPN values for keys composed of host information.

An SPN is a name by which a client uniquely identifies an instance of a service or application on a server for purposes of mutual authentication. Mutual authentication is requested by default, and you can require it by setting WebRequest.AuthenticationLevel to MutualAuthRequired in your request.

When a WebRequest requires mutual authentication, the SPN for the destination must be supplied by the client. If you know the SPN, you can add it to the CustomTargetNameDictionary before sending the request. If you have not added SPN information to this dictionary, the AuthenticationManager uses the RequestUri method to compose the most likely SPN; however, this is a computed value and might be incorrect. If mutual authentication is attempted and fails, you can check the dictionary to determine the computed SPN. No SPN is entered into the dictionary if the authentication protocol does not support mutual authentication.

To add an SPN value to this dictionary, use the AbsoluteUri of the RequestUri as the key. Internally, the key is truncated to include the Scheme, Host, and the Port if it is not the default port.


Accessing the methods and properties of the CustomTargetNameDictionary requires unrestricted WebPermission.


When Kerberos authentication is performed through a proxy, both the proxy and the ultimate host name need to be resolved to an SPN. The proxy name resolution is protected by a timeout. Resolution of the ultimate host name to a SPN requires a DNS lookup, and there is no timeout associated directly with this operation. Therefore synchronous operations may take longer to timeout. To overcome this, add the ultimate host's URI prefix to the SPN cache prior to making requests to it.

Version 3.5 SP1 now defaults to specifying the host name used in the request URL in the SPN in the NTLM (NT LAN Manager) authentication exchange when the CustomTargetNameDictionary property is not set. The host name used in the request URL may be different from the Host header specified in the System.Net.HttpRequestHeader in the client request. The host name used in the request URL may be different from the actual host name of the server, the machine name of the server, the computer's IP address, or the loopback address. In these cases, Windows will fail the authentication request. To address the issue, you may need to notify Windows that the host name used in the request URL in the client request ("contoso", for example) is actually an alternate name for the local computer.

The following code example demonstrates displaying the contents of the CustomTargetNameDictionary.

       public static void RequestResource(Uri resource)
            // Set policy to send credentials when using HTTPS and basic authentication.

            // Create a new HttpWebRequest object for the specified resource.
            WebRequest request=(WebRequest) WebRequest.Create(resource);
            // Supply client credentials for basic authentication.
            request.UseDefaultCredentials = true;
            request.AuthenticationLevel = AuthenticationLevel.MutualAuthRequired;
            HttpWebResponse response = (HttpWebResponse) request.GetResponse();
            // Determine mutual authentication was used.
            Console.WriteLine("Is mutually authenticated? {0}", response.IsMutuallyAuthenticated);

             System.Collections.Specialized.StringDictionary spnDictionary = AuthenticationManager.CustomTargetNameDictionary;
            foreach (System.Collections.DictionaryEntry e in spnDictionary)
                Console.WriteLine("Key: {0}  - {1}", e.Key as string, e.Value as string);
            // Read and display the response.
            System.IO.Stream streamResponse = response.GetResponseStream();
            System.IO.StreamReader streamRead = new System.IO.StreamReader(streamResponse);
            string responseString = streamRead.ReadToEnd();
            // Close the stream objects.
            // Release the HttpWebResponse.


The output from this example will differ based on the requested resource
and whether mutual authentication was successful. For the purpose of illustration,
a sample of the output is shown here:

Is mutually authenticated? True
Key:  - HTTP/



.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.