enable : echo 1 > /proc/sys/net/ipv4/ip_forward
enable : sysctl -w net.ipv4.ip_forward=1

disable : echo 0 > /proc/sys/net/ipv4/ip_forward
disable : sysctl -w net.ipv4.ip_forward=0

note : without rebooting the system

nano /etc/sysctl.conf
uncomment net.ipv4.ip_forward and change 0 to 1 for enable ip forwarding
reboot system or :
sysctl -p /etc/sysctl.conf

تست شده بر روی debian 9.3

Linux debian 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2+deb9u2 (2017-06-26) x86_64 GNU/Linux

لغات انگلیسی :

/etc/sysctl.conf
uncomment
without
rebooting
sysctl
/proc/sys/net/ipv4/ip_forward
net.ipv4.ip_forward
forwarding

منبع