TCP uses 32 bit Seq/Ack numbers in order to make sure that both sides of a connection can actually receive packets from each other. Additionally, these numbers make it relatively hard to spoof the source address because successful spoofing requires guessing the correct initial sequence number (ISN) which is generated by the server in a non-guessable way. It is commonly known that a 32 bit number can be brute forced in a couple of hours given a fast (gigabit) network connection. This article shows that the effort required for guessing a valid ISN can be reduced from hours to minutes if the server uses TCP SYN Cookies (a widely used defense mechanism against SYN-Flooding DOS Attacks), which are enabled by default for various Linux distributions including Ubuntu and Debian.
You can download it from the following link: https://packetstormsecurity.com/files/download/122808/blindtcpspoof.tgz
Source: https://packetstormsecurity.com/files/122808/Quick-Blind-TCP-Connection-Spoofing-With-SYN-Cookies.html