You talk about the net.inet.tcp.syncookies=1 knob, how about an description on what it does and why you are recommending using it.
The net.inet.tcp.syncookies 'knob', if set to 1, enables syn cookies. Syn cookies were invented specifically for syn flood protection. A brief description of syncookies idea can be read here:
How would one go about mirroring back the attackers syn packets to port 80 or 22? Please describe this easy method of yours.
Mirroring back packets to the attacker is, first of all, a nasty thing. Secondly, it is only possible if the attacker's IP is known. If it is not known, then obviously it's not possible.
Knowing the attacker's IP does not necessarly mean that he is performing the current attacks from that IP.
Packet redirection with ipfw is done using divert sockets. One needs to have it compiled into the kernel. Divert sockets are also used by ipfw nat redirection. It's all in the man pages of ipfw.
If the flood is severly intense (from the point of stack memory exhaution), it might be a good improvement to drop 5% of incoming SYN packets. This can also be done with ipfw, and is described in the manual pages. However, I don't think one would ever come to this.
Asking the ISP to put the server behind a decent cisco router, and implement syn cookies on hardware devices, is the best protection.
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