Enable SSH X11Forwarding to access Ubuntu from the local Mac OS X

X11Forwarding config in ssh can enable the remote desktop access from the local client. This is a set up of access between Ubuntu server and local Mac Client.

1. Ubuntu Server

• make sure the X11Forwarding option in /etc/ssh/sshd_config is set to yes. If not, set it to yes, and restart the sshd service by "sudo service ssh restart"
• test X11Forwarding in server
• Open a terminal in Ubuntu, and "ssh -v -X localhost" to establish the ssh connection with X11Forwarding. 
• Check if there's a line with "Requesting X11 Forward" shown in the debug output
• Check DISPLAY environment variables by "echo $DISPLAY" and it should be localhost:10.0

2. Mac OS X client
Reference: http://ruepprich.wordpress.com/2013/01/08/x11-forwarding-on-os-x-10-8-2-mountain-lion/

•  Install  XQuarz for Mac OS
• Open iTerm and add remote server to access list by "sudo xhost + 192.168.56.101"
• There's an error about 'xhost: unable to open display ""' 
• And ssh -X user@192.168.56.101 also failed to take X running

• Open XQuarz, and open "Terminal" in Application Menu
• bash-3.2$ xhost + 192.168.56.101
  192.168.56.101 being added to access control list
• bash-3.2$ ssh -X rye@192.168.56.101
• rye@rye-desktop:~$ echo $DISPLAY
  localhost:11.0
• xclock works