Substring/RemoteForwarder Error on Cloud Service Classic

Question

Using Azure Cloud Service Classic with WorkerRoles and ran into this error. I'm not sure what exactly is going on.

Message="System.ArgumentOutOfRangeException: Length cannot be less than zero.
Parameter name: length

Server stack trace: 
   at System.String.Substring(Int32 startIndex, Int32 length)
   at Microsoft.WindowsAzure.Plugins.RemoteForwarder.RdpRemoteEndpointFinder.GetRemoteEndpoint(Socket clientSocket, Stream peekBuffer)

Exception rethrown at [0]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Microsoft.WindowsAzure.Plugins.RemoteForwarder.IRemoteEndpointFinder.GetRemoteEndpoint(Socket clientSocket, Stream peekBuffer)
   at Microsoft.WindowsAzure.Plugins.RemoteForwarder.RemoteForwardingService.ClientConnected(Object sender, ConnectionEventArgs e)
   at Microsoft.WindowsAzure.Plugins.RemoteForwarder.TcpSocketServer.OnClientConnected(ConnectionEventArgs e)"

Answer 1

The method System.String.Substring(Int32 startIndex, Int32 length) is invoked with a negative length parameter. This likely occurs because the code is trying to parse or manipulate data (like an RDP packet or stream) where the expected format is incorrect or the data is incomplete.

1. Involved Components:
   • Microsoft.WindowsAzure.Plugins.RemoteForwarder: A plugin that enables remote access or diagnostics in Azure Cloud Service roles.
   • GetRemoteEndpoint: This method is attempting to identify the endpoint details (IP, port, etc.) from the Socket or data buffer.
2. Possible Triggers:
   • Corrupted or malformed network data.
   • Unexpected changes in the RDP connection protocol or structure.
   • A bug or compatibility issue in the RemoteForwarder plugin.

---

Steps to Resolve

1. Update the Azure Cloud Services SDK

• Ensure you're using the latest version of the Azure SDK for Cloud Services, as older versions may contain bugs or compatibility issues.
  • Update the NuGet packages for Microsoft.WindowsAzure.Plugins.RemoteForwarder.

2. Verify Data Integrity

• Inspect the data being passed to the GetRemoteEndpoint method:
  • Ensure the socket data (peek buffer) contains valid and complete RDP connection details.
  • Check if the buffer handling logic can produce invalid lengths.
• Add logging to trace the startIndex and length values used in the Substring call.

3. Handle Edge Cases in the Plugin

• Add a custom try-catch block around the code where GetRemoteEndpoint is invoked to log more details.
  • If modifying the plugin is not feasible, implement network-level validation before passing the buffer to RemoteForwarder.

4. Test with Diagnostics Disabled

• Temporarily disable the Remote Desktop plugin in your service configuration:

  
       <Imports>
  
           <!-- Comment or remove the following line -->
  
           <!--<Import moduleName="RemoteAccess" />-->
  
           <!--<Import moduleName="RemoteForwarder" />-->
  
       </Imports>
  
  

  • If the error disappears, the issue is related to the RemoteForwarder plugin. Consider replacing it with a custom implementation or migrating to a newer service.

5. Consider Migrating to Azure App Services or Virtual Machines

• Azure Cloud Services Classic is legacy infrastructure and will eventually be deprecated. If feasible, consider migrating your solution to a modern Azure platform such as:
  • Azure App Services
  • Azure Virtual Machines
  • Azure Kubernetes Service (AKS)
• This approach avoids legacy plugin limitations and improves long-term maintainability.