XMAS Scanning

[Bhargav Bhikkaji <bbhikkaji () yahoo co in>]

Hi,

I am trying to understand the working of few port scanning methods using TCP. There are few methods like SYN 
Scan,FIN/NULL Scan, XMAS Scan and Zombie Scan. 

Based on my understanding. "If a packet is sent to a closed port with any control bit except RST, then the port will 
send a RST packet". Now my question is, why does XMAS scanning sets control bit to FIN/URG/PSH. Is it not enough to set 
any one of the three control bit ?

Thanks
Bhargav

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