it maybe preventable writing rule in this way if jim-san use iptables on 2ch server.

!
# ***Do with SYNFLOOD attack ***
iptables -N drop_synflood
iptables -A drop_synflood -m limit --limit 10/s --limit-burst 20 -j RETURN
iptables -A drop_synflood -j LOG --log-prefix "SYNFLOOD: " -m limit --limit 1/s --limit-burst 10
iptables -A drop_synflood -j DROP

# SYN FLOOD attack check
iptables -A INPUT -p tcp --syn -j drop_synflood