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7.5. Viruses 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 services such as trojans, worms, and other client/server viruses from contacting their server. For example, 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
There is a distinction between the
REJECTtargets when dealing with appended rules. The
REJECTtarget denies access and returns a
connection refusederror to users who attempt to connect to the service. The
DROPtarget, as the name implies, drops the packet without any warning. Administrators can use their own discretion when using these targets. However, to avoid user confusion and attempts to continue connecting, the
REJECTtarget is recommended.