Information
The net.ipv4.ip_forward flag is used to tell the server whether it can forward packets or not. If the server is not to be used as a router, set the flag to 0.
Rationale:
Setting the flag to 0 ensures that a server 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 net.ipv4.ip_forward parameter to 0 in /etc/sysctl.conf:
net.ipv4.ip_forward = 0
Run the following commands to set the active kernel parameters:
# /sbin/sysctl -w net.ipv4.ip_forward=0
# /sbin/sysctl -w net.ipv4.route.flush=1
Default Value:
OS Default: N/A