How do I turn on IP forwarding on Linux?

Turn on IP forwarding in Linux IP.

To turn forwarding on temporarily, use the following command as root: (this will be disabled after a reboot)

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

Or to make it permanent add the following to /etc/sysctl.conf

net.ipv4.ip_forward = 1

In older versions of Redhat / CentOS you can edit /etc/sysconfig/network and enable it there:

# CRITICAL: Enable IP forwarding since it is disabled by default since
#
# Redhat Users: you may try changing the options in
# /etc/sysconfig/network from:
#
# FORWARD_IPV4=false
# to
FORWARD_IPV4=true
#

You can also add echo "1" > /proc/sys/net/ipv4/ip_forward to /etc/rc.local although the above methods are preferred.