Io ho risolto con
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
echo 0 > /proc/sys/net/ipv4/tcp_window_scaling
echo 0 > /proc/sys/net/ipv4/tcp_ecn
o inserendo in /etc/sysctl.conf
net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_ecn = 0
Ciao
Stefano