Mapping Port to PID for Transient Windows TCP Connections

Question

I am trying to reverse engineer a third-party TCP client / server Windows XP, SP 3 app for which I have no source available. My main line of attack is to use WireShark to capture TCP traffic.

When I issue a certain GUI command on the client side, the client creates a TCP connection to the server, sends some data, and tears down the connection. The server port is 1234, and the client port is assigned by the OS and therefore varies.

WireShark is showing that the message corresponding to the GUI command I issued gets sent twice. The two messages bear a different source port, but they have the same destination port (1234, as mentioned previosuly).

The client side actually consists of several processes, and I would like to determine which processes are sending these messages. These processes are long-lived, so their PIDs are stable and known. However, the TCP connections involved are transient, lasting only a few milliseconds or so. Though I've captured the client-side port numbers in WireShark and though I know all of the PIDs involved, the fact the connections are transient makes it difficult to determine which PID opened the port. (If the connections were long-lived, I could use netstat to map port numbers to PIDs.) Does anybody have any suggestions on how I can determine which processes are creating these transient connections?

Answer 1

I ended up creating a batch file that runs netstat in a tight loop and appends its output to a text file. I ran this batch file while running the system, and by combing through all of the netstat dumps, I was able to find a dump that contained the PIDs associated with the ports.

Answer 2

I can think of two things:

1. Try sysinternals' tcpview program. It gives a detailed listing of all tcp connections opened by all the processes in the system. If a process creates connections, you will be able to see them flash (both connect and disconnect are flashed) in tcpview and you will know which processes to start looking into.

2. Try running the binary under a debugger. Windbg supports multi-process debugging (so does visual studio I think). You may have only export symbols to work with but that should still work for calls made to system dlls. Try breaking on any suspected windows APIs you know will be called by the process to create the connections. MSDN should have the relevant dlls for most system APIs documented.

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