If your attack seems unsuccessful, one thing that you can investigate is whether the SYN cookie mechanism is turned on. SYN cookie is a defense mechanism to counter the SYN flooding attack. The mechanism will kick in if the machine detects that it is under the SYN flooding attack. You can use the sysctl command to turn on/off the SYN cookie mechanism: $ sudo sysctl -a | grep cookie (Display the SYN cookie flag) $ sudo sysctl -w net. ipv4.tcp_syncookies