rule does not alert from nmap

[repniksz () aviva co hu]

Hi, 
I've written a very simply rule: 

alert tcp any any -> any 80 (msg: "SYN on 80 port" ; flags: S; ) 

I've started testing, first of all i used just a simply browser to get a 
page from there, and the rule was working. 
But after i've tried use Nmap in some way: 

nmap -sS -P0 -p 80 195.xxx.xxx.xxx 
nmap -P0 -p 80 195.xxx.xxx.xxx 
nmap -s-sacnflags syn -P0 -p 80 195.xxx.xxx.xxx 

All of them sent a SYN to the 80 port. 
I've checked with ethereal. 
But there was not any on the alert file. 
what's wrong? 


regards

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