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Network Security Snort-Signatures
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Re: [Snort-sigs] FP for sid 2000538

from [Chich Thierry] Network Security [Permanent Link]
Subject: Re: [Snort-sigs] FP for sid 2000538
Date: Wed, 23 Nov 2005 11:19:28 +0100 
Le Mercredi 23 Novembre 2005 09:52, Chich Thierry a écrit :

I have a lot of false positives with the rule that detect Nmap scan.
It come frequently from web sites

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg: "BLEEDING-EDGE SCAN NMAP
-sA (1)"; fragbits: !D; dsize: 0; flags: A,
12; window: 1024; reference: arachnids,162; classtype: attempted-recon;
sid: 2000538; rev:3; ):

Sample  (xxx.xxx.xxx.xxx is a web site , and  yyy.yyy.yyy.yyy is a local
host) :
16:51:08.196181 IP xxx.xxx.xxx.xxx.80 > yyy.yyy.yyy.yyy.46490: . ack
1899022551 win 1024 <mss 536>




Furthermore, I just have done a scan with nmap. It seems that the window size 
can be 1024, and also 2048, 3072 or 4096. 

I have tried to specify that the packet with the Ack flag  is not included in 
a tcp stream, but i have not been successfull with the "flow: no_stream".

Thierry 


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