Linux Security Cookbook

2.1. Enabling Source Address Verification

Problem

You want to prevent remote hosts from spoofing incoming packets as if they had come from your local machine.

Solution

Turn on source address verification in the kernel. Place the following code into a system boot file (i.e., linked into the /etc/rc.d hierarchy) that executes before any network devices are enabled:

#!/bin/sh
echo -n "Enabling source address verification..."
echo 1 > /proc/sys/net/ipv4/conf/default/rp_filter
echo "done"

Or, to perform the same task after network devices are enabled:

#!/bin/sh
CONF_DIR=/proc/sys/net/ipv4/conf
CONF_FILE=rp_filter
if [ -e ${CONF_DIR}/all/${CONF_FILE} ]; then
        echo -n "Setting up IP spoofing protection..."
        for f in ${CONF_DIR}/*/${CONF_FILE}; do
                echo 1 > $f
        done
        echo "done"
fi

A quicker method may be to add this line to /etc/sysctl.conf:

net.ipv4.conf.all.rp_filter = 1

and run sysctl to reread the configuration immediately:

# sysctl -p

Discussion

Source address verification is a kernel-level feature that drops packets that appear to come from your internal network, but do not. Enabling this feature should be your first network-related security task. If your kernel does not support it, you can set up the same effect using firewall rules, but it takes more work. [Recipe 2.2]

See Also

sysctl(8). Source address verification is explained in the IPCHAINS-HOWTO at http://www.linux.org/docs/ldp/howto/IPCHAINS-HOWTO-5.html#ss5.7.