Enable IP forwarding
If we are setting up a Linux router/gateway or maybe a VPN server (pptp or ipsec) then we will need to enable forwarding. This can be done in several ways that I will present bellow.
First we check if IP forwarding is already enabled. This can be done through using the sysctl util or checking the ip_forward file:
sysctl net.ipv4.ip_forward
Will output something like:
net.ipv4.ip_forward = 0
Or
cat /proc/sys/net/ipv4/ip_forward
0
To enable IP forwarding (without rebooting the system) on the fly we do :
sysctl -w net.ipv4.ip_forward=1
or
echo 1 > /proc/sys/net/ipv4/ip_forward
the setting is changed instantly; the result will not be preserved after rebooting the system.
To make it permanent we set it on the /etc/sysctl.conf file
/etc/sysctl.conf: net.ipv4.ip_forward = 1
To apply it we do:
sysctl -p /etc/sysctl.conf