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Initiating a TCP session with Windows Server 2003 using a non-standard flag combination may succeed

Support for Windows Server 2003 ended on July 14, 2015

Microsoft ended support for Windows Server 2003 on July 14, 2015. This change has affected your software updates and security options. Learn what this means for you and how to stay protected.

A TCP 3-way handshake is initiated with a SYN packet sent to the remote peer (a packet with only the SYN flag set). If a TCP handshake is initiated with other TCP flag values set in addition to the SYN flag, Windows Server 2003 will accept the connection.
This behavior is by design.
The behavior is changed with Windows Vista and later platforms. A SYN packet will not be accepted if it's set in combination with either FIN, RST, or ACK flags and will only be accepted if it's either the only flag set or if that flag is set in combination with PSH, URG, ECE, or CWR.
Note This is a "FAST PUBLISH" article created directly from within the Microsoft support organization. The information contained herein is provided as-is in response to emerging issues. As a result of the speed in making it available, the materials may include typographical errors and may be revised at any time without notice. See Terms of Use for other considerations.

Article ID: 2551021 - Last Review: 05/17/2011 20:45:00 - Revision: 1.0

  • Microsoft Windows Server 2003, Standard Edition (32-bit x86)
  • Microsoft Windows Server 2003, Standard x64 Edition
  • Microsoft Windows Server 2003 Service Pack 2
  • Microsoft Windows Server 2003 Service Pack 1
  • Microsoft Windows Server 2003 R2 Enterprise x64 Edition
  • Microsoft Windows Server 2003 R2 Enterprise Edition (32-Bit x86)
  • KB2551021