3.1.1 Ensure IP forwarding is disabled - sysctl.conf ipv4

Information

The net.ipv4.ip_forward and net.ipv6.conf.all.forwarding flags are used to tell the
system whether it can forward packets or not.

Rationale:

Setting the flags to 0 ensures that a system with multiple interfaces (for example, a hard
proxy), will never be able to forward packets, and therefore, never serve as a router.

Solution

Set the following parameter in /etc/sysctl.conf or a /etc/sysctl.d/* file:

net.ipv4.ip_forward = 0
net.ipv6.conf.all.forwarding = 0

Run the following commands to set the active kernel parameters:

# sysctl -w net.ipv4.ip_forward=0
# sysctl -w net.ipv6.conf.all.forwarding=0
# sysctl -w net.ipv4.route.flush=1
# sysctl -w net.ipv6.route.flush=1

See Also

https://workbench.cisecurity.org/files/2420