tutorial - Protection from Viruses and Spoofed IP Addresses

There are some trojans that scan networks for services on ports from 31337 to 31340 (called the elite ports in cracking terminology). Since there are no legitimate services that communicate via these non-standard ports, blocking it can effectively diminish the chances that potentially infected nodes on your network independently communicate with their remote master servers.

iptables -A OUTPUT -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
iptables -A FORWARD -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP

You can also block outside connections that attempt to spoof private IP address ranges to infiltrate your LAN. For example, if your LAN uses the 192.168.1.0/24 range, a rule can set the Internet facing network device (for example, eth0) to drop any packets to that device with an address in your LAN IP range. Because it is recommended to reject forwarded packets as a default policy, any other spoofed IP address to the external-facing device (eth0) is rejected automatically.

iptables -A FORWARD -s 192.168.1.0/24 -i eth0 -j DROP

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source:http://linuxpoison.blogspot.com/2008/02/1357817580668.html