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Re: TCP Split Handshake and Nmap
From: jah <jah () zadkiel plus com>
Date: Wed, 09 Jun 2010 00:58:38 +0100

On 08/06/2010 18:26, David Fifield wrote:
On Fri, Jun 04, 2010 at 04:22:55PM +0100, jah wrote:
  
The attached patch does very little work since the split handshake SYN
passes the existing checks for dest port matching a probe source port;
it adds a test for the SYN flag right after the test for SYN|ACK for SYN
scans.
    
+         } else if (USI->scantype == SYN_SCAN && tcp->th_flags == TH_SYN) {

Just one thing I would change here:

+         } else if (USI->scantype == SYN_SCAN && (tcp->th_flags & TH_SYN) == TH_SYN) {

That way it will work with a SYN|URG or SYN|PSH or something else silly.
Putting this test after the SYN|ACK test lets SYN|ACK continue giving
the correct reason code.
  

Hi David, All,

If we're going make the rule catch all resposes with the SYN bit set
(and optionally other bits), except for SYN|ACK, perhaps the reason
string should be changed also because "split-hanshake-syn" wouldn't then
convey the full information.  We could instead leave the reason string
as it is and print a message to the effect that a response received was
evidence of a split handshake and print the tcp flag value.  Or we could
do both and make the reason string 'syn-other' or something.

jah
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