[TriLUG] Detecting SYN Floods with IPTables?

[Jeff Bollinger]

I'm working on an issue where a user suspects an irregular but 
reoccurring SYN flood against a server (i.e. thousands of connections 
with only the SYN flag set -- never closed or ACKd)

I'm thinking I should be able to passively watch traffic to this host 
(mirror port) and use the IPTables 'LOG' function to note this type of 
attack to determine the source host (attacker).  Before I implement my 
sniffer, is this the proper SYNtax (sorry ;) for what I'm trying to do:

IPTABLES -A INPUT -p tcp --syn -j LOG --log-prefix "SYN FLOOD"

I've searched around, but reading the above makes me think that it will 
log *every* proper TCP connection  (i.e. all that begin with a SYN). 
Should I be using the --state flag, and if so, what attribute (NEW, 
ESTABLISHED, INVALID, RELATED)???

Thanks!
Jeff