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Expand All Sending Any Message

Unless otherwise stated, all SMB messages sent by the client and the server MUST comply with the following rules:

  • SMB messages MUST be composed of three parts:

    • An SMB_Header, as specified in section

    • An SMB_Parameters block, as specified in section

    • An SMB_Data block, as specified in section

  • The SMB_Header MUST be included in full.

  • At minimum, the WordCount field of the SMB Parameters block MUST be included. The remainder of the SMB_Parameters block MUST be two times WordCount bytes in length. If WordCount is 0x00, then zero parameter bytes MUST be included in the SMB_Parameters block.

  • At minimum, the ByteCount field of the SMB_Data block MUST be included. The remainder of the SMB_Data block MUST be ByteCount bytes in length. If ByteCount is 0x0000, then zero data bytes MUST be included in the SMB_Data block.

Thus, the minimum size of an SMB message is 35 bytes. Section lists required values for some SMB Header fields. See the individual command descriptions for specific per-message requirements.

If a message is sent and IsSigningActive is TRUE, the message MUST be signed.

This logic MUST be applied for messages sent in response to any of the higher-layer actions and in compliance with the message sequencing rules.

  • The client or server that sends the message MUST provide the 32-bit sequence number for this message, as specified in sections and

  • The SMB_FLAGS2_SMB_SECURITY_SIGNATURE flag in the header MUST be set.

  • To generate the signature, a 32-bit sequence number is copied into the least significant 32 bits of the SecuritySignature field and the remaining 4 bytes are set to 0x00.

  • The MD5 algorithm, as specified in [RFC1321], MUST be used to generate a hash of the SMB message from the start of the SMB Header, which is defined as follows.

  •  CALL MD5Init( md5context )
     CALL MD5Update( md5context, Connection.SigningSessionKey )
     CALL MD5Update( md5context, Connection.SigningChallengeResponse )
     CALL MD5Update( md5context, SMB message )
     CALL MD5Final( digest, md5context )
     SET signature TO the first 8 bytes of the digest

The resulting 8-byte signature MUST be copied into the SecuritySignature field of the SMB Header, after which the message can be transmitted.

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