None of the posted solutions worked for me. My client (desktop) system is running macOS 10.12.5 (Sierra). I added -v to the options for the ssh command and it told me,

debug1: No xauth program.

which means it doesn't have a correct path to the xauth program. (On this version of macOS the path to xauth is nonstandard.) The solution was to add this line to /etc/ssh/ssh_config (may be /etc/ssh/config in some setups) or in ~/.ssh/config (if you don't have admin rights):

XAuthLocation /opt/X11/bin/xauth

Now the warning message is gone.

As noted, it seems that xauth on OS X Yosemite has regressed to an old version that doesn't work with XQuartz's $DISPLAY setting:

% xauth -V
1.0.9
% xauth generate $DISPLAY .
xauth: (argv):1: bad display name "/private/tmp/com.apple.launchd(...)/org.macosforge.xquartz:0" in "add" command

Letting Ubuntu bash on Windows 10 run ssh -X to get a GUI environment on a remote server

• First

Install all the following. On Window, install Xming. On Ubuntu bash, use sudo apt install to install ssh xauth xorg.

sudo apt install ssh xauth xorg

• Second

Go to the folder contains ssh_config file, mine is /etc/ssh.

• Third

Edit ssh_config as administrator(USE sudo). Inside ssh_config, remove the hash # in the lines ForwardAgent, ForwardX11, ForwardX11Trusted, and set the corresponding arguments to yes.

# /etc/ssh/ssh_config

Host *
 ForwardAgent yes
 ForwardX11 yes
 ForwardX11Trusted yes

• Forth

In ssh_config file, remove the front hash # before Port 22 and Protocol 2, and also append a new line at the end of the file to state the xauth file location, XauthLocation /usr/bin/xauth, remember write your own path of xauth file.

# /etc/ssh/ssh_config

# IdentifyFile ...
 Port 22
 Protocol 2
# Cipher 3des
# ...
# ...
 ...
 ...
 GSSAPIDelegateCredentials no
 XauthLocation /usr/bin/xauth

• Fifth

Now since we are done editing ssh_config file, save it when we leave the editor. Now go to folder ~ or $HOME, append export DISPLAY=localhost:0 to your .bashrc file and save it.

# ~/.bashrc
...
...
export DISPLAY=localhost:0

• Last

We are almost done. Restart your bash shell, open your Xming program and use ssh -X yourusername@yourhost. Then enjoy the GUI environment.

ssh -X yourusername@yourhost

The problem is also in Ubuntu subsystem on Windows, and the link is at

https://gist.github.com/DestinyOne/f236f71b9cdecd349507dfe90ebae776

Note: the linked text includes 2 typos (XauthLocaion instead of XauthLocation)

There is a bug in MacOS at the moment. I came across this too. The fix for me involved adding the following to my .bash_profile

dispdir=`dirname $DISPLAY`
dispfile=`basename $DISPLAY`
dispnew="$dispdir/:0"
if [ -e $DISPLAY -a "$dispfile" = "org.x:0" ]; then
 mv $DISPLAY $dispnew
fi
export DISPLAY=$dispnew

Essentially the name for the file pipe associated with your X root can't be handled correctly, and thus needs correction. :-)

Including

XAuthLocation /opt/local/bin/xauth
in ~/.ssh/config

in my macOS Sierra 10.12.6 worked for me. A small change from answer 7).

i just removed ~/.Xauthority (destination machine) from my root folder and ssh -X 192.168.123.1 again and ik worked.

In my case it was the problem of .Xauthority containing the Magic cookie
not forwarded, Fabby on http://askubuntu.com/questions/571116/ recommends
on 2014-11-14 to add this line at the end of the .bashrc or . profile
to allow forwarding of xauth keys between users when calling su:

export $(dbus-launch)

I added also previously:

export XAUTHORITY=~/.Xauthority 

to ensure remote called with ssh -X ̍@ will find it.

In my case .Xauthority is a symlink to original user /home//.Xauthority I su from...

 cd /home/<child_user>;ln -sf /home/<parent_user>/.Xauthority .xAuthority

with correct rights:

 sudo chown <parent_user> /home/<parent_user>/.profile
 chmod a+rw /home/<parent_user>/.profile 

so it is accessible to and to . will be able to trigger apps on and display X-windowed result on its local screen throughout proxy account !

TIP : Check xauth list...if reflects magic cookie on .

I would add this as a comment, but I don't have enough rep. Adding one more line to sorin's solution worked for me.

Open your ssh config file with vim ~/.ssh/config
Then add these lines to it:

Host *
 ForwardAgent yes
 ForwardX11 yes
 XAuthLocation /opt/X11/bin/xauth

You can double check your xauth location with:

which xauth