This protection must be enabled if you use your Linux server as a gateway to masquerade internal traffic to the Internet IP Masquerading.

            [root@deep] /#echo 1 > /proc/sys/net/ipv4/ip_always_defrag
            

Add the above commands to the /etc/rc.d/rc.local script file and you'll not have to type it again the next time you reboot your system.

Edit the /etc/sysctl.conf file and add the following line:

            # Enable always defragging Protection
            net.ipv4.ip_always_defrag = 1
            

You must restart your network for the change to take effect. The command to manually restart the network is the following:

            [root@deep] /# /etc/rc.d/init.d/network restart
            Setting network parameters	  [  OK  ]
            Bringing up interface lo	  [  OK  ]
            Bringing up interface eth0	  [  OK  ]
            Bringing up interface eth1	  [  OK  ]