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vb.net passive tcp port monitoring of another application

I have a vendor-provided VoIP contact center application. There is a softphone installed on the local PC which communicates with a CTI server application. The CTI server is accessed via a constant IP address and port. The port used on the local PC is dynamically assigned.

Using Wireshark, I have documented all of the XML command/responses that occur between the softphone and the CTI application. What I am trying to do is to reverse-engineer a passive API that sits on the local PC and monitors this softphone-CTI server traffic. I intend to use this information to record events and drive activities in our CRM application.

I've created a vb.net application that can monitor a port and capture the activity. The problem is that although I am communicating to the CTI application using a fixed server port, the local TCP port is dynamic, so I don't know which port to listen to. It is this local port that initiates the conversation.

Is there a way to passively monitor all active ports for traffic going TO a specific IP/port and then use that to determine which local port to monitor for outgoing/incoming traffic?

Thanks.

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andr_gin

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asrobins

ASKER

Building a simple proxy is a good solution. Unfortunately, aynchronous socket programming is not altogether that simple!

andr_gin

1.) You can check TcpClient.Available to see, if there is data available in the buffer to read:

Do
If ClientTcpClient.Available>0 Then
BytesRead = ClientStream.Read(Buffer,0,Buffersize)
ServerStream.Write(Buffer,0,BytesRead )
EndIf
If ServerTcpClient.Available>0 Then
BytesRead = ServerStream.Read(Buffer,0,Buffersize)
ClientStream.Write(Buffer,0,BytesRead )
EndIf
Loop

2.) Dont try to read and write the same TcpClient/Networkstream in two threads at the same time. This will cause an error.