Fixing Win32 Error 0x000006D4 (RPC_S_UNKNOWN_AUTHN_LEVEL)

The error code 0x000006D4 translates to RPC_S_UNKNOWN_AUTHN_LEVEL. This error message indicates that the RPC call failed because the client attempted to use an authentication level that the server does not recognize or support.

Understanding Authentication Levels in RPC:

• In RPC communication, authentication mechanisms are used to verify the identity of the client application or user trying to access the server.
• Beyond the choice of authentication service (e.g., NTLM, Kerberos), some services can offer different authentication levels that specify the extent of verification performed. For example, a higher level might involve encryption in addition to basic user verification.
• The RPC_S_UNKNOWN_AUTHN_LEVEL error occurs when the client specifies an authentication level within the chosen service that the server doesn’t support.

Reasons Behind RPC_S_UNKNOWN_AUTHN_LEVEL:

There are a couple of reasons why you might encounter this error:

• Misconfigured Client: The client application might be configured to use an authentication level that is not supported by the server for the chosen authentication service.
• Outdated Client Software (less common): In rare cases, if using outdated client software, it might be programmed to use an older authentication level no longer supported by the server.
• Server-Side Authentication Changes (less common): If the server administrator recently made changes to the supported authentication levels for a service on the server, the client might be unaware of the update and still attempt to use an unsupported level.

Resolving RPC_S_UNKNOWN_AUTHN_LEVEL:

1. Verify Client Authentication Settings:

   • Examine the client application’s configuration settings related to RPC authentication. Ensure it’s set to use an authentication level that is supported by the server for the chosen authentication service.

2. Consult Program Documentation:

   • Refer to the documentation for the program or service you’re trying to access remotely. It should specify the supported authentication levels for the chosen authentication service.

3. Update Client Software (if applicable):

   • Consider updating the client software to the latest version. This might include bug fixes or support for newer authentication level options compatible with the server.

4. Server Administrator Communication (if possible):

   • If possible, communicate with the server administrator to determine the supported authentication levels for the chosen service on the server. This can help you configure the client accordingly.

5. Software Vendor Support:

   • If the issue persists, consider contacting the software vendor’s support for the program or service. They might have insights into compatible authentication level settings or troubleshooting steps for their software.

Additional Tips:

• Some RPC development tools or libraries might offer functionalities to configure authentication settings for RPC calls, including the authentication level.
• Debuggers or logging mechanisms can help capture details about the authentication level being used in the client code and any error messages related to unsupported authentication levels.

By understanding the concept of authentication levels in RPC and the reasons behind unknown authentication level errors, you can identify the cause of the incompatibility. Verifying client authentication settings, consulting program documentation, updating client software (if applicable), communicating with the server administrator (if possible), and contacting software vendor support can help address this error.