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Well recently I've been reading about different Denial of Service methods. One method that kind of stuck out was SYN flooding. I'm a member of some not-so-nice forums, and someone was selling a python script that would DoS a server using SYN packets with a spoofed IP address.

However, if you sent a SYN packet to a server, with a spoofed IP address, the target server would return the SYN/ACK packet to the host that was spoofed. In which case, wouldn't the spoofed host return an RST packet, thus negating the 75 second long-wait, and ultimately failing in its attempt to DoS the server?

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possible duplicate of Is SYN flooding still a threat? –  pQd May 16 '10 at 9:53

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up vote 1 down vote accepted

There are several cases in which this might not happen.

  • The spoofed host might not exist.
  • It might exist, but be configured to silently drop a SYN/ACK that does not match a SYN packet that the host sent out.
  • It might exist and respond, but not have enough bandwidth to handle all incoming SYN/ACKs from the host under attack.

SYN cookies can be used as a defense against SYN floods, by removing all server-side state during the handshake. (Check out how they work if you don't know, it's quite a brilliant hack that does not break the TCP spec.) SYN cookies are enabled on my Ubuntu Lucid system by default, so I'd expect that most servers use them nowadays.

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I read somewhere last night that if the spoofed host doesn't exist, the target will receive an ICMP packet telling him so. –  Rob May 14 '10 at 17:27
    
Not that I know of. Who would send that packet? –  Thomas May 15 '10 at 9:12
    
"However, the target machine is likely to receive an ICMP host unreachable message if the attacker’s fake IP address does not belong to any particular host." cobweb.ecn.purdue.edu/~kak/compsec/NewLectures/Lecture16.pdf Page 65 –  Rob May 15 '10 at 14:17
    
I think that enabling them when needed is better than enabling them by default. They might be spec compliant, but they also reduce the amount of the spec you can use... –  Vinko Vrsalovic Jul 26 '11 at 9:54

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