How do I fix a "cannot open display" error when opening an X program after ssh'ing with X11 forwarding enabled?

From xhost+ : How to Fix “Cannot Open Display” Error While Launching GUI on Remote Server:

Answer: You can fix the “cannot open display” error by following the xhost procedure mentioned in this article.

Allow clients to connect from any host using xhost+

Execute the following command to disable the access control, by which you can allow clients to connect from any host.

$ xhost +

access control disabled, clients can connect from any host

Enable X11 forwarding

While doing ssh use the option -X to enable X11 forwarding.

$ ssh username@hostname -X

Enable trusted X11 forwarding, by using the -Y option,

$ ssh username@hostname -Y

Open GUI applications in that host

After opening ssh connection to the remote host as explained above, you can open any GUI application which will open it without any issue.

If you still get the “cannot open display” error, set the DISPLAY variable as shown below.

$ export DISPLAY='IP:0.0'

Note: IP is the local workstation’s IP where you want the GUI application to be displayed.

Check the server's sshd_config (normally /etc/ssh/sshd_config), and make sure X11Forwarding option is enabled with the line

X11Forwarding yes

If X11Forwarding is not specified, the default is no on the Debian machines I have available to check.

I've had this problem when logging into a Ubuntu VM from Mac OS X as well -- it doesn't seem to like 'localhost' in the display variable for some reason. So set the IP manually, as harrymc suggests:

export DISPLAY="127.0.0.1:10.0"

Then X11 programs should be fine. It doesn't seem like it should be necessary to tell the OS that localhost and 127.0.0.1 are equivalent, but it works, at least.