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2615Re: Segments with no flags set

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  • David S. Miller
    May 30, 2001
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      bryan@... writes:
      > I recently encountered an interoperability problem between a firewall and a
      > Voice-over-IP hardware device (with embedded TCP stack), where the VoIP box
      > would begin to send segments with no flags set (including ACK) over an
      > established session. Although I've never observed such use of TCP before,
      > I can't find hard evidence that indicates it is a violation of spec. Can
      > anyone confirm if this is truly against the law?

      I don't see how it could be illegal, if the ACK flag bit is clear this
      simply means to ignore the ACK field.

      I suppose a firewall vendor could claim that this provides a 32-bit
      field in which to create a covert channel of information, but the URG
      field can already be (and honestly, by hardcore crackers probably is)
      used for this purpose :-)

      Later,
      David S. Miller
      davem@...
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