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Microsoft KB Archive/166846

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Knowledge Base

Cannot Reconnect to TN3270 Server with Close Listen Sockets

Article ID: 166846

Article Last Modified on 11/1/2006


  • Microsoft Windows NT Server 4.0, Terminal Server Edition
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition

This article was previously published under Q166846


When you configure the Microsoft SNA Server 2.11 or SNA Server 3.0 TN3270 Server to use the Close Listen Socket option, TN3270 clients are unable to reconnect to the TN3270 server even though LUA sessions are available.

When the Close Listen Socket option is enabled, the TN3270 service performs a CloseSocket() call when all SNA sessions are in use. The TN3270 service indicates this by logging the following Windows NT event:

   Event ID: 207
   Source: TN3270 Server
   The TN3270E Server has closed the socket which listens for clients

While existing TN3270 client sessions continue to work normally, new TN3270 client connection requests are rejected immediately with a TCP/IP Reset. This allows the TN3270 client emulator to be notified of a connection failure so it can try a different TN3270 service for a session.

When a TN3270 client disconnects causing an SNA session to become available, the TN3270 service reopens the listen socket again and logs the following event:

   Event ID: 206
   Source: TN3270 Server
   The TN3270E Server has opened a socket to listen for clients

However, TN3270 clients are still unable to reconnect until the TN3270 server is stopped and restarted or if all users disconnect from the TN3270 server.


When the TN3270 server calls the CloseSocket() function, the Windows NT TCP/IP address object connect handler address is set to NULL, which causes TCP/IP to send a TCP/IP Reset when clients attempt to connect to the TN3270 port number.

When an SNA session becomes available, the TN3270 Server reopens the listen socket by calling the socket() function. 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.


To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

152734 How to Obtain the Latest Windows NT 4.0 Service Pack

A hotfix for Windows NT 3.51 is available to correct this problem.


Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.

Microsoft has confirmed this to be a problem in Windows NT version 3.51. A supported fix is now available, but has not been fully regression-tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.

Additional query words: closelistensocket prodsna snatn3270

Keywords: kbbug kbfix KB166846