FIX: Both sides of a TCP connection are closed when the client or remote application half-closes the TCP connection in a Forefront Threat Management Gateway 2010 environment

Symptoms

Consider the following scenario:

• You have internal clients who are trying to access non-web-based application servers in a Microsoft Forefront Threat Management Gateway (TMG) 2010 environment.

• The client traffic passes through an application filter in Forefront TMG that data-flows the traffic in user mode.

• The client or the remote application server half-closes the TCP connection by sending a FIN/ACK packet. In doing this, the client or the remote application server expects the other half of the TCP connection to stay active.

In this scenario, you may notice that both sides of the TCP connection are closed immediately by the FIN/ACK packet.

Cause

When traffic is passed through a user-mode data flow, both sockets are closed after a FIN packet is received on one of them. This prevents Forefront TMG from supporting half-closed TCP connections as supported in the kernel-mode data flow.

Resolution

To resolve this issue, install the hotfix package that is described in the following Microsoft Knowledge Base article:

2649961 Rollup 1 for Forefront Threat Management Gateway (TMG) 2010 Service Pack 2To enable this fix, follow these steps:

1. Start Notepad. To do this, click Start, click Run, type notepad.exe in the Open box, and then click OK.

2. Paste the following script to a new file in Notepad:

   set curArray = CreateObject("FPC.Root").GetContainingArray()
   Const SE_VPS_GUID = "{143F5698-103B-12D4-FF34-1F34767DEabc}"
   Const SE_VPS_NAME = "AllowHalfClosedConnection"
   Const SE_VPS_VALUE = True
   Sub SetValue()
       ' Create the root obect.
       Dim root  ' The FPCLib.FPC root object
       Set root = CreateObject("FPC.Root")
   
       'Declare the other objects needed.
       Dim array       ' An FPCArray object
       Dim VendorSets  ' An FPCVendorParametersSets collection
       Dim VendorSet   ' An FPCVendorParametersSet object
   
       ' Get references to the array object
       ' and the network rules collection.
       Set array = curArray 
       Set VendorSets = array.VendorParametersSets
   
       On Error Resume Next
       Set VendorSet = VendorSets.Item( SE_VPS_GUID )
   
       If Err.Number <> 0 Then
           Err.Clear
   
           ' Add the item
           Set VendorSet = VendorSets.Add( SE_VPS_GUID )
           CheckError
           WScript.Echo "New VendorSet added... " & VendorSet.Name
   
       Else
           WScript.Echo "Existing VendorSet found... value- " &  VendorSet.Value(SE_VPS_NAME)
       End If
   
       if VendorSet.Value(SE_VPS_NAME) <> SE_VPS_VALUE Then
   
           Err.Clear
           VendorSet.Value(SE_VPS_NAME) = SE_VPS_VALUE
   
           If Err.Number <> 0 Then
               CheckError
           Else
               VendorSets.Save false, true
               CheckError
   
               If Err.Number = 0 Then
                   WScript.Echo "Done with " & SE_VPS_NAME & ", saved!"
               End If
           End If
       Else
          WScript.Echo "Done with " & SE_VPS_NAME & ", no change!"
       End If
   
   End Sub
   
   Sub CheckError()
   
       If Err.Number <> 0 Then
           WScript.Echo "An error occurred: 0x" & Hex(Err.Number) & " " & Err.Description
           Err.Clear
       End If
   
   End Sub
   SetValue

3. Save the file as AllowHalfClosedConnection.vbs, and then exit Notepad.

4. On a Forefront TMG server, open an administrative command prompt.

5. Move to the location to which you saved the AllowHalfClosedConnection.vbs script, and then type the following command:

   Cscript.exe AllowHalfClosedConnection.vbs

This fix lets Forefront TMG enable half-closed TCP connections in user-mode data flows.

Note To change the functionality back to the default behavior, change the following line in the script by changing the value from True to False, and then run the script:

Const SE_VPS_VALUE = True

Status

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

References

For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:

824684 Description of the standard terminology that is used to describe Microsoft software updates