TCP sinful cookies?

[Jeff Mings]

I've been looking through the various man pages, HOW-TOs, and other 
documentation, and haven't been able to figure out what TCP SynCookies 
are and what they do.  I assume they relate to defeating Syn-based DOS 
attacks.  I've been using MonMotha's excellent iptables firewall script 
(thanks for the tip, Deven) to figure out how to use iptables, and 
SynCookies are enabled in the same way that forwarding is turned on in 
ipchains or iptables.   Script snippet:

# Enable TCP Syncookies
echo -n "Checking IP SynCookies..."
if [ -e /proc/sys/net/ipv4/tcp_syncookies ] ; then
	echo 1 > /proc/sys/net/ipv4/tcp_syncookies
	echo "enabled."
else
	echo "support not found, but that's OK."
fi


Can anyone give a quick summary of SynCookies?

Thanks,
-Jeff Mings