Making your edirectory servers invisible.
echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_all and to make it stick after a reboot you can add following line to /etc/sysctl.conf file:
- vi /etc/sysctl.conf
Append following line: net.ipv4.icmp_echo_ignore_all = 1
Use filtcfg to create a filter to not allow icmp (make a note in the autoexec.ncf that this has been done).