I spent a lot of hours before realizing that Ubuntu 14.04 blocks tcp by default.
One can check this by
ps aux | grep X root 1127 0.1 3.4 364380 71748 tty7 Ss+ 12:08 0:00 /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
Notice the -nolisten option in output.
To enable it, add xserver-allow-tcp=true to lightdm:
sudo vi /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
[SeatDefaults] # Dump core xserver-command=X -core xserver-allow-tcp=true
Restart X (or reboot) and forward X11 desktop by launching:
# server export DISPLAY=<your-localhost-ip>:0 # client xhost +
You can test Display forwarding by launching on server
xeyes