IP Forwarding

Ubuntu:

cat >> /etc/network/options
ip_forward=yes

Other linux:

cat >> /proc/sys/net/ipv4/ip_forward
sysctl net.ipv4.ip_forward = 1

instantaneous change in values:

sysctl -w net.ipv4.ip_forward=1

or

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