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Windows Server



    

Overview of SMB Signing    

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Overview of SMB Signing



Apr
16

Overview of SMB Signing

 

How to determine whether SMB signing is enabled in a network monitor trace

To determine whether SMB signing is enabled, required at the server, or both, view the Negotiate Dialect Response from the server:

 

SMB: R negotiate, Dialect # = 5
  SMB: Command = R negotiate
      SMB: Security Mode Summary (NT) = [a value of 3, 7 or 15]
        SMB: .......1 = User level security
          SMB: ......1. = Encrypt passwords

 

 

In this Response the "Security Mode Summary (NT) =" field represents the configured options on the Server. This value will be either 3, 7 or 15.

 

For additional information about how to use Network Monitor, click the following article number to view the article in the Microsoft Knowledge Base:

812953 (http://support.microsoft.com/kb/812953/) How to use Network Monitor to capture network traffic

The following information helps explain what the Negotiate Dialect Response numbers represent:

UCHAR SecurityMode; Security mode:

bit 0: 0 = share

bit 0: 1 = user

bit 1: 1 = encrypt passwords

bit 2: 1 = Security Signatures (SMB sequence numbers) enabled

bit 3: 1 = Security Signatures (SMB sequence numbers) required

 

 

If SMB signing is disabled at the server, the value is 3.

"SMB: Security Mode Summary (NT) = 3 (0x3)"

 

If SMB signing is enabled and not required at the server, the value is 7.

"SMB: Security Mode Summary (NT) = 7 (0x7)"

 

If SMB signing is enabled and required at the server, the value is 15.

"SMB: Security Mode Summary (NT) = 15 (0xF)"

For additional information about CIFS, visit the following Microsoft Web site:

http://msdn2.microsoft.com/en-us/library/Aa302188.aspx (http://msdn2.microsoft.com/en-us/library/Aa302188.aspx)

 

SMB signing scenarios

The behavior of the SMB session after the Dialect Negotiation shows the client configuration.

 

If SMB Signing is enabled and required at both the client and the server, or if SMB signing is disabled at both the client and the server, the connection is successful.

 

If SMB signing is enabled and required at the client and disabled at the server, the connection to the TCP session is gracefully closed after the Dialect Negotiation, and the client receives the following "1240 (ERROR_LOGIN_WKSTA_RESTRICTION)" error message:

System error 1240 has occurred. The account is not authorized to log in from this station.

If SMB signing is disabled at the client and enabled and required at the server, the client receives the "STATUS_ACCESS_DENIED" error message when it receives a response to a Tree Connect or Transact2 for DFS referrals. 



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