Inc. BSDI has patches available. This patch adds two networking features that can help defeat and detect some types of denial of service attacks. IBM has released AIX operating system fixes for the SYN flood vulnerability. Then, instead of just sending the synack back, it sends a syn cookie back, and waits for a response to IT before sending the synack. When it sends the cookie, it clears the syn from the queue, so while under attack, the queue will never fill up.
Cookies expire shortly after they are sent.
Basically this prevents people from filling up the queue completely. No one flooding from a spoof will be able to reply to the cookie, so nothing can be overloaded. And if they aren't flooding from a spoof, they would be getting a cookie they would have to respond to, and would have a hard time responding to all the cookies and continuing the flood. We strongly urge you to encrypt sensitive information sent by email. Introduction, paragraph 2 - added some clarification.