tun0 doesn't forwards network

(wlan0 -> tun0) Ubuntu desktop "A" (eth0) -> (eth0) other Ubuntu desktop "B"

I have an OpenVPN connection on "A". OpenVPN server is from a Mikrotik RouterBoard.

I need to forward the OpenVPNs network to "B". So that "B" would get an IP from the OpenVPN server via DHCP.

"A" originally got the internet connection from wlan0. The OpenVPN goes through wlan0 to get to the OpenVPN server.

I am physically connecting the two ethX'es. I bring up IP on both ethX in the same subnet.

So on "A" I am doing:

pkill nm-applet
service iptables stop
iptables -t nat -I POSTROUTING -o tun0 -j MASQUERADE
echo '1' > /proc/sys/net/ipv4/ip_forward

But even from tcpdump I cannot see any traffic from "B" to "A" even if I do pings. Why?