Microsoft KB Archive/177074

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BUG: Starting and Stopping a Winsock Server Using SO_REUSEADDR

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Q177074

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The information in this article applies to:


 * Microsoft Windows NT Server version 4.0
 * Microsoft Windows NT Workstation version 4.0
 * Microsoft Windows 95

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SYMPTOMS
To close a Winsock server and rebind a new server to its old port you need to employ setsockopt on the server socket with SO_REUSEADDR. However, this becomes problematic if the server spawned incoming clients into child processes for handling. If any child processes are left open with their socket connection intact, any future client connection requests to any new server will be forcefully refused (WSAECONNREFUSED).

CAUSE
When the server calls closesocket, the Windows NT TCP/IP address object connect handler address is set to NULL, which causes TCP/IP to send a TCP/IP Reset (RST) when future clients attempt to connect to the server's port number.

When a new server is created and reopens the same port, this causes the Windows NT TCP/IP driver to create a duplicate address object with a valid connect handler address. However, connection requests are validated against the old TCP/IP address object, which still has a NULL connect handler.

The end result is that all future client connection requests are forcefully refused (WSAECONNREFUSED) as long as child processes from the original server maintain their socket connections through the original NT TCP/IP address object. Once they all close, the original object is destroyed and normal operation resumes.

STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

Steps to Reproduce Behavior

 * 1) Create a Winsock server application.
 * 2) Upon accepting a client connection, set its handle inheritable, spawn a child process. Actual use of the socket is irrelevant. The inherited handle keeps the socket connection alive via the additional reference.
 * 3) Terminate the server/parent process, recreate the server process while leaving the child process open.
 * 4) Attempt to connect a client to the server, it should be refused.