ssh forwarding from mac to ubuntu doesn't work - macos

Question

I'm trying to use ssh forwarding feature on mac to display remote GUI application locally.
on Mac, I installed the official xserver XQaurtz, set it up as below.
$ cat ~/.ssh/config
Host *
XAuthLocation /opt/X11/bin/xauth
ForwardAgent yes
ForwardX11 yes
Then I used "ssh -v -X user#remote_machine" to login a ubuntu machine, then used xclock to test.
$ ssh -v -X user#remote_machine
OpenSSH_7.9p1, LibreSSL 2.7.3
debug1: Reading configuration data ~/.ssh/config
debug1: /Users/bwu/.ssh/config line 1: Applying options for *
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 48: Applying options for *
debug1: /etc/ssh/ssh_config line 52: Applying options for *
debug1: auto-mux: Trying existing master
On remote machine, xclock failed to launch due to $DISPLAY is empty.
$ xclock
Error: Can't open display:
I did two more tests.
From the same mac, ssh login to another centos 7 machine, it's working. $ cat /etc/ssh/sshd_config X11Forwarding yes
X11DisplayOffset 0
From a ubuntu host, ssh login to above ubuntu machine, it's working. $ cat /etc/ssh/sshd_config X11Forwarding yes X11DisplayOffset 0
So we got below results.
mac to centos, working
ubuntu to ubuntu, working
mac to ubuntu, not working
Test 1 indicates the issue might locate on remote ubuntu machine.
Test 2 indicates the issue might locate on local mac machine.
What's wrong with this? Did I miss anything?

Answer 1

Further update on this issue. I noticed x11 forwarding did not work "randomly" on centos or ubuntu (from my macbook), but after a couple hours it may work again.
I checked the sshd configuration on both centos and ubuntu, nothing special and they are same in x11 forwarding part. I don't know why.
X11Forwarding yes
X11DisplayOffset 0
#X11UseLocalhost yes

Answer 2

Here is a solution that might work. I had the same problem and this is how I solved it. Give the following solution a try.
First find the DISPLAY variable.
So in your mac if you type as a normal user
echo $DISPLAY
Then what you would get is something like the following.
/private/tmp/com.apple.launchd.0aQYNoXMFK/org.xquartz:0
Then try something like
xeyes
to see whether forwarding works. There are other apps you could try, but I like this one.
And now you know that your display is working
Now if you want to try the same as root (Please don't jump on me guys, I know some of you all are strongly against root access) echo $DISPLAY, but if does not work
then in your root prompt do the following
export DISPLAY=/private/tmp/com.apple.launchd.0aQYNoXMFK/org.xquartz:0
The same you found in your normal user account. Then copy your
.Xauthority at /Users/normal user/.Xauthority to /var/root/.
The .Xauthority file is already there, but this would over right it.
cp /Users/normal user/.Xauthority /var/root/
Of course the export might work, but there is no harm in doing the above.
Now try the following.
echo $DISPLAY
And you should see the following
/private/tmp/com.apple.launchd.0aQYNoXMFK/org.xquartz:0
If you ssh into Ubuntu from normal user prompt then you do not need to do the root part, but since, I use root to ssh into my Ubuntu systems I often have to do this.
Then when you 'ssh into Ubuntu type
echo $DISPLAY
And you would see something like the following
localhost:10.0
The above would work if you have done all those other bits like forwarding and etc.
Again, if you want to use root in your Ubuntu and if the echo $DISPLAY does not produce any response,
then try the following (Assuming you are at root prompt).
cp /home/user name/.Xauthority /root/.Xauthority
Now try
echo $DISPLAY
again and you would see something like the following
localhost:10.0
For fun try
xeyes
Of course you could try xclock or any other as well
And it works in my case. Hope this is helpful and would solve a problem like the one above or like mine that someone has come across and who spent a few hours on this problem while scratching head and trying to pull hair out like me :-)).