Routování paketů je nutné v kernelu nejdříve ZAPNOUT! RIP - http://bi0os.blogspot.com/2010/03/using-rip-dynamic-routing-on-debian.html

Using sysctl:

sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0

or just checking out the value in the /proc system:

cat /proc/sys/net/ipv4/ip_forward
0

sysctl -w net.ipv4.ip_forward=1

or

echo 1 > /proc/sys/net/ipv4/ip_forward

If we want to make this configuration permanent the best way to do it is using the file /etc/sysctl.conf where we can add a line containing net.ipv4.ip_forward = 1 /etc/sysctl.conf:

net.ipv4.ip_forward = 1

if you already have an entry net.ipv4.ip_forward with the value 0 you can change that 1. To enable the changes made in sysctl.conf you will need to run the command:

sysctl -p /etc/sysctl.conf

on Debian/Ubuntu systems this can be also done restarting the procps service:

/etc/init.d/procps.sh restart

http://www.ducea.com/2006/08/01/how-to-enable-ip-forwarding-in-linux/