2.8.6. Malicious Software and Spoofed IP Addresses

More elaborate rules can be created that control access to specific subnets, or even specific nodes, within a LAN. You can also restrict certain dubious applications or programs such as Trojans, worms, and other client/server viruses from contacting their server.
For example, some Trojans 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 them can effectively diminish the chances that potentially infected nodes on your network independently communicate with their remote master servers.
The following rules drop all TCP traffic that attempts to use port 31337:
~]# 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 range, you can design a rule that instructs 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 -i eth0 -j DROP


There is a distinction between the DROP and REJECT targets when dealing with appended rules.
The REJECT target denies access and returns a connection refused error to users who attempt to connect to the service. The DROP target, as the name implies, drops the packet without any warning.
Administrators can use their own discretion when using these targets.