Linux Iptables Avoid IP Spoofing And Bad Addresses Attacks

Spoofing and bad address attack tries to fool the server and try to claim that packets had come from local address/network.

Incoming source IP address is your servers IP address

Bad incoming address from following ranges:

• 0.0.0.0/8
• 127.0.0.0/8
• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
• 192.168.0.0/16
• 224.0.0.0/3
• Your own internal server/network ip address/ranges.

Following small shell script tries to prevent this kind of attacks:

#!/bin/bash
 
INT_IF="eth1" # connected to internet 
SERVER_IP="202.54.10.20" # server IP
LAN_RANGE="192.168.1.0/24" # your LAN IP range 
 
# Add your spoofed IP range/IPs here
SPOOF_IPS="0.0.0.0/8 127.0.0.0/8 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 224.0.0.0/3"
 
IPT="/sbin/iptables" # path to iptables
 
# default action, can be DROP or REJECT 
ACTION="DROP"
 
# Drop packet that claiming from our own server on WAN port
$IPT -A INPUT -i $INT_IF -s $SERVER_IP -j $ACTION
$IPT -A OUTPUT -o $INT_IF -s $SERVER_IP -j $ACTION
 
# Drop packet that claiming from our own internal LAN on WAN port
$IPT -A INPUT -i $INT_IF -s $LAN_RANGE -j $ACTION
$IPT -A OUTPUT -o $INT_IF -s $LAN_RANGE -j $ACTION
 
## Drop all spoofed 
for ip in $SPOOF_IPS
do
 $IPT -A INPUT -i $INT_IF -s $ip -j $ACTION
 $IPT -A OUTPUT -o $INT_IF -s $ip -j $ACTION
done
## add or call your rest of script below to customize iptables ##
 

Save and close the file. Call above script from your own iptables script. Add following line to your /etc/sysctl.conf file

net.ipv4.conf.all.rp_filter=1
net.ipv4.conf.all.log_martians=1
net.ipv4.conf.default.log_martians=1

The net.ipv4.conf.all.rp_filter=1 entry enables source address verification which is inbuilt into Linux kernel itself and last two lines logs all such spoofed packets in log file.