so, it should be this

PostUp     = `iptables -A FORWARD -i <INTERFACE> -j ACCEPT; iptables -A FORWARD -o <INTERFACE> -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE`
PostDown   = `iptables -D FORWARD -i <INTERFACE> -j ACCEPT; iptables -A FORWARD -o <INTERFACE> -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE`