Linux Iptables Avoid IP Spoofing And Bad Addresses Attacks

Posted on in Categories Debian Linux, Gentoo Linux, Iptables, Linux, Networking, RedHat/Fedora Linux, Security, Shell scripting, Suse Linux, Ubuntu Linux last updated June 28, 2005

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

Following IP/netwok address are know to open this kind of attack:

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.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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21 comment

  1. Hi,
    How to configure squid as a direct connection?
    I would like to following option enable through squid
    1. Username password security with individual site Blocking security.
    2. FTP, SMTP, POP3 etc.. protocol and port configure.
    3. Cache configuration
    4. log maintenance of internet usage, bad site request, downloading, uploading with user name.
    5. firewall configuration to block intruders.(Apart from local user nobody outside the LAN can use our port and connection to the internet
    6. Antivirus : Clamav

    Reply Report comment

  6. I think that the following code:,

    # Original code – Start
    SERVER_IP=”202.54.10.20”
    # Add your 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”
    iptables -A INPUT -s $SERVER_IP -j DROP
    for ip in $SPOOF_IPS
    do
    iptables -A INPUT -s $ip -j DROP
    done
    # Original Code – End

    , can be re-written as :-

    # Suggested Code – Start
    PUBLICIF=eth0
    SERVER_IP=202.54.10.20
    # Add your IP range/IPs here,
    SPOOF_IPS=”$SERVER_IP 0.0.0.0/8 127.0.0.0/8 10.0.0.0/8 172.16.0.0/16 192.168.0.0/16 224.0.0.0/3″
    for ip in $SPOOF_IPS
    do
    iptables -A INPUT -i $PUBLICIF -s $ip -j DROP
    done

    # Suggested code – Stop

    Please correct me if I am wrong, but IMHO, if we DROP packets without mentioning the interface then legitimate connections originating from the same server on loopback (lo) will also get dropped.

    Thanks.

    Reply Report comment

    1. lo is a different interface and is not related to outgoing packets. That’s why it is called the loopback interface. lo should always be accepted for INPUT and OUTPUT. That’s regardless of state; indeed, it is for local connections only (and by local I mean 127.0.0.0/8). Not allowing lo is going to break a lot of services and otherwise cause unexpected results.

      Reply Report comment

  10. It is impossible to spoof your address if your Linux computer is behind NAT enabled router such as Cisco.

    This script is more useful on *Linux based router* as packet filtering is one defense against IP spoofing attacks. The Linux gateway to a network usually performs ingress filtering, which is blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally the Linux gateway would also perform egress filtering on outgoing packets, which is blocking of packets from inside the network with a source address that is not inside. This prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines.

    I hope this will help someone.

    Reply Report comment

  17. hi, my Linux Email Server(centos 5.7) is giving IP Address Spoofing in N/W.
    am not running any proxy server in that. kept firewall off. now port 25 smtp is giving ip address spoofing with server’s public ip.

    please advice.

    Reply Report comment

  18. Something comes to mind (and I admit I’ve been awake since 2 this morning, but…):

    /sbin/iptables -A OUTPUT -o eth1 -s 202.54.10.20 -j DROP

    seems highly suspect to me. If eth1 is the interface connected to the Internet that also has the IP 202.54.10.20 bound to it, then you’re blocking outgoing traffic. Note the OUTPUT chain. The point of blocking spoofed packets and in particular the specific public IP(s) the server (or depending on network: firewall, …) is incoming packets. Outgoing packets would naturally have your node’s IP address, after all (unless we’re talking spoofing and/or not wanting your traffic forwarded by routers, potentially even your ISP’s router [like your default gateway]). It is hosts that are NOT you that are claiming to be you. Your server SHOULD send outbound as its IP (NOT to itself) but should not be sending packets TO itself FROM its IP (that should be blocked).

    Reply Report comment

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