Re: nmap udp scan time

[jpecou () gmail com]

Kevin,
    I believe UDP scans can take such a long time because UNLIKE TCP scans UDP does not rely on a handshake and 
therefore their is no STATE assigned to a connection.  If your scanning a firewalled machine that is not replying to 
the UDP packets that are sent to CLOSED ports then I believe your scan will take quite sometime to complete. With TCP 
you will be sending out a SYN packet that will only wait a short time frame before determining that that port is either 
Open, Closed, or Filtered.  UDP is a less reliable scan which I believe is based on correlating results. It is hard for 
a UDP scan to determine if a port is Open|Filtered unless it discovers a true Closed port. So your scanner may be 
sitting around waiting for responses from ports that will never respond. Next time you scan try using a protocol 
analyzer like Wireshark/Ethereal to see if you are getting responses from the majority of ports you have scanned.  This 
is just my understanding of differences in scan types an
 d I apologize if any of my statements are incorrect.


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