Linux as GATEWAY iptables policy

/proc/sys/net/ipv4/ip_forward = 1
iptables -t nat -A POSTROUTING -o enp3s0 -j MASQUARADE
iptables -A FORWARD -i enp1s0 -o enp3s0 -j ACCEPT
iptables -A FORWARD -i enp3s0 -o enp1s0 -m state –state RELATED,ESTABLISHED \
-j ACCEPT