X11Forwarding must be set on the SSH server (in your case the Ubuntu box) in its sshd_config, and you must allow X11 to be forwarded for the SSH client (your Fedora box) by passing the -X option or editing the ssh_config file to add the ForwardX11 default.

To get X11 forwarding working over SSH, you’ll need three things in place:

1. Your client must be set up to forward X11.
2. Your server must be set up to allow X11 forwarding.
3. Your server must be able to set up X11 authentication.

If you have both #1 and #2 in place but are missing #3, then you’ll end up with an empty DISPLAY environment variable.

Soup-to-nuts, here is how to get X11 forwarding working:

1. On your server, make sure /etc/ssh/sshd_config contains:

   X11Forwarding yes
   X11DisplayOffset 10
   

   You may need to SIGHUP sshd so it picks up these changes.

   cat /var/run/sshd.pid | xargs kill -1
   

2. On your server, make sure you have xauth installed.

   belden@skretting:~$ which xauth
   /usr/bin/xauth
   

   If you do not have xauth installed, you will run into the empty DISPLAY environment variable problem.

3. On your client, connect to your server. Be certain to tell ssh to allow X11 forwarding. I prefer

   belden@skretting:~$ ssh -X blyman@the-server
   

but you may like

    belden@skretting:~$ ssh -o ForwardX11=yes blyman@the-server

or you can set this up in your ~/.ssh/config.

---

I was running into this empty DISPLAY environment variable earlier today when ssh’ing into a new server that I do not administer. Tracking down the missing xauth part was a bit fun. Here is what I did, and what you can do too.

On my local workstation, where I am an administrator, I verified that /etc/ssh/sshd_config was set up to forward X11. When I ssh -X back in to localhost, I do get my DISPLAY set correctly.

Forcing DISPLAY to get unset was not too hard. I just needed to watch what sshd and ssh were doing to get it set correctly. Here is the full output of everything I did along the way.

    blyman@skretting:~$ mkdir ~/dummy-sshd
    blyman@skretting:~$ cp -r /etc/ssh/* ~/dummy-sshd/
    cp: cannot open `/etc/ssh/ssh_host_dsa_key' for reading: Permission denied
    cp: cannot open `/etc/ssh/ssh_host_rsa_key' for reading: Permission denied

Instead of using sudo to force copying my ssh_host_{dsa,rsa}_key files into place, I used ssh-keygen to create dummy ones for myself.

    blyman@skretting:~$ ssh-keygen -t rsa -f ~/dummy-sshd/ssh_host_rsa_key
    Generating public/private rsa key pair.
    Enter passphrase (empty for no passphrase): 
    Enter same passphrase again: 
    Your identification has been saved in /home/blyman/dummy-sshd/ssh_host_rsa_key.
    Your public key has been saved in /home/blyman/dummy-sshd/ssh_host_rsa_key.pub.

Rinse-and-repeate with -t dsa:

    blyman@skretting:~$ ssh-keygen -t dsa -f ~/dummy-sshd/ssh_host_dsa_key
    # I bet you can visually copy-paste the above output down here

Edit ~/dummy-sshd/sshd_config to point to the correct new ssh_host key files.

    # before
    blyman@skretting:~$ grep ssh_host /home/blyman/dummy-sshd/sshd_config 
    HostKey /etc/ssh/ssh_host_rsa_key
    HostKey /etc/ssh/ssh_host_dsa_key

    # after
    blyman@skretting:~$ grep ssh_host /home/blyman/dummy-sshd/sshd_config 
    HostKey /home/blyman/dummy-sshd/ssh_host_rsa_key
    HostKey /home/blyman/dummy-sshd/ssh_host_dsa_key

Fire up sshd on a new port in non-detach mode:

    blyman@skretting:~$ sshd -p 50505 -f ~/dummy-sshd/sshd_config -d
    sshd re-exec requires execution with an absolute path

Whoops, better correct that path:

    blyman@skretting:~$ /usr/sbin/sshd -p 50505 -f ~/dummy-sshd/sshd_config -d
    debug1: sshd version OpenSSH_5.5p1 Debian-4ubuntu6
    debug1: read PEM private key done: type RSA
    debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-2048
    debug1: Checking blacklist file /etc/ssh/blacklist.RSA-2048
    debug1: private host key: #0 type 1 RSA
    debug1: read PEM private key done: type DSA
    debug1: Checking blacklist file /usr/share/ssh/blacklist.DSA-1024
    debug1: Checking blacklist file /etc/ssh/blacklist.DSA-1024
    debug1: private host key: #1 type 2 DSA
    debug1: setgroups() failed: Operation not permitted
    debug1: rexec_argv[0]='/usr/sbin/sshd'
    debug1: rexec_argv[1]='-p'
    debug1: rexec_argv[2]='50505'
    debug1: rexec_argv[3]='-f'
    debug1: rexec_argv[4]='/home/blyman/dummy-sshd/sshd_config'
    debug1: rexec_argv[5]='-d'
    Set /proc/self/oom_adj from 0 to -17
    debug1: Bind to port 50505 on 0.0.0.0.
    Server listening on 0.0.0.0 port 50505.
    debug1: Bind to port 50505 on ::.
    Server listening on :: port 50505.

Pop a new terminal and ssh into localhost on port 50505:

    blyman@skretting:~$ ssh -p 50505 localhost
    The authenticity of host '[localhost]:50505 ([::1]:50505)' can't be established.
    RSA key fingerprint is 81:36:a5:ff:a3:5a:45:a6:90:d3:cc:54:6b:52:d0:61.
    Are you sure you want to continue connecting (yes/no)? yes
    Warning: Permanently added '[localhost]:50505' (RSA) to the list of known hosts.
    Linux skretting 2.6.35-32-generic #67-Ubuntu SMP Mon Mar 5 19:39:49 UTC 2012 x86_64 GNU/Linux
    Ubuntu 10.10
    
    Welcome to Ubuntu!
     * Documentation:  https://help.ubuntu.com/
    
    1 package can be updated.
    0 updates are security updates.
    
    Last login: Thu Aug 16 15:41:58 2012 from 10.0.65.153
    Environment:
      LANG=en_US.UTF-8
      USER=blyman
      LOGNAME=blyman
      HOME=/home/blyman
      PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
      MAIL=/var/mail/blyman
      SHELL=/bin/bash
      SSH_CLIENT=::1 43599 50505
      SSH_CONNECTION=::1 43599 ::1 50505
      SSH_TTY=/dev/pts/16
      TERM=xterm
      DISPLAY=localhost:10.0
    Running /usr/bin/xauth remove unix:10.0
    /usr/bin/xauth add unix:10.0 MIT-MAGIC-COOKIE-1 79aa9275ced418dd445d9798b115d393

Look at the last three lines there. I fortuitously had DISPLAY set, and had those two nice-looking lines from /usr/bin/xauth.

From there it was child’s play to move aside my /usr/bin/xauth to /usr/bin/xauth.old, disconnect from ssh and stop the sshd, then launch sshd and ssh back in to localhost.

When /usr/bin/xauth was gone, I did not see DISPLAY reflected in my environment.

---

There is nothing brilliant going on here. Mostly I got lucky in choosing a sane approach to try reproducing this on my local machine.

The fix is to add this line to your /etc/ssh/sshd_config (on the server side):

X11UseLocalhost no

https://joshua.hoblitt.com/rtfm/2013/04/how_to_fix_x11_forwarding_request_failed_on_channel_0/

For me the problem was in nodev mount option for /tmp filesystem.
X11 needs a special file to be created in there.

So check what are the mount options for /tmp filesystem if you use a separate partition or disk for that.

Add X11UseLocalhost no to /etc/ssh/sshd_config and restart the SSH server.

If you get no DISPLAY, check if xauth is installed correctly and then try it again.

RHE/CEntos doesn’t have this issue, this is a Ubuntu thing!

Make sure that:

• You’ve xauth installed on the server (see: xauth info/xauth list).

• On the server your /etc/ssh/sshd_config file have these lines:

  X11Forwarding yes
  X11DisplayOffset 10
  X11UseLocalhost no
  

• On the client side your ~/.ssh/config file have these lines:

  Host *
    ForwardAgent yes
    ForwardX11 yes
  

• On the client side, you’ve X server installed (e.g. macOS: XQuartz; Windows: Xming).

---

Then to do X11 forwarding using SSH, you need to add -X to your ssh command, e.g.

ssh -v -X user@host

then verify that your DISPLAY is not empty by:

echo $DISPLAY

If it is, then having verbose parameter for ssh (-v), check for any warnings, e.g.

debug1: No xauth program.
Warning: untrusted X11 forwarding setup failed: xauth key data not generated

---

In case you’ve got untrusted X11 as shown above, then try -Y flag instead (if you trust the host):

ssh -v -Y user@host

^{See: What does “Warning: untrusted X11 forwarding setup failed: xauth key data not generated” mean when ssh’ing with -X?}

---

In case you’ve warning: No xauth data, you may try to generate a new .Xauthority file, e.g.

xauth generate :0 . trusted
xauth list

See: Create/rebuild a new .Xauthority file

---

If you’ve got a different warnings than above, follow the further clues.

---

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 in the terminal, 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

To add to the previous excellent answers (setting up ~/.ssh/config and checking to see if the DISPLAY environment variable is set on the client, setting up /etc/ssh/sshd_config and installing xauth on the server), also make sure xterm is installed on the client, e.g.

sudo apt-get install xterm

xauth can get locked.

   -b      This  option  indicates  that  xauth  should  attempt to break any authority file locks before proceeding.  Use this
           option only to clean up stale locks.

Using

xauth -b

On the machine that I was trying to ssh into broke the lock on xauth. Logging out of the ssh session after issuing xauth -b then logging back in finally allowed me to successfully echo $DISPLAY. Definitely try this before re-creating .Xauthority

If you are doing ssh from Windows (10) to a Linux system – do yourself a favour and use a SSH client like Putty ( regular ssh -X etc does not work).

Step 1: Install a XServer in Windows : Example XMing Server (listens on localhost:0.0)

Step 2: In putty enable X11 forwarding

Step 3: Connect to remote Linux server

Make sure all conditions here are met in Linux server – that is X11Forwarding is yes and xauth is present as explained in the answer https://unix.stackexchange.com/a/12772/121634

Fire up XClock and wait a minute for the display to appear in your Windows machine

Note – If from this Linux server you are connecting to another server and want to forward X11 back to your Windows, you just need to connect to the next in chain with ssh -X.

That is

[Windows] Putty (with X11forwarding) –> [Server1] (xclock works) –> ssh -X [Server 2] (xclock works)

• Follow this steps on both server and client sides
  • For the server I have a unix-like OS
    • make sure on file /etc/ssh/sshd_config you have yes line

      X11Forwarding yes
      

    • if was just enabled, restart ssh. In my case

      sudo systemctl restart ssh
      

  • On the client side I am on a macOS
    • Provide xterm

      brew install --cask xquartz
      

    • Restart the computer was necessary before the first connect to server

      ssh -Y user@host
      

    • Check if it’s working

      chromium http://google.com
      

If you’re using connection sharing, you need to close all existing connections before any changes to your client ssh options will take effect.

I was using a script to connect to a remote machine with these options set.

-oControlPath=${control_path}
-oControlMaster=auto

I was trying to change my client ssh options to enable X11 forwarding and couldn’t figure out why they weren’t taking effect. I had to close the existing connections and reconnect with the new client ssh options for enabling X11 forwarding for it to work. You could also just open a new connection by removing any options for setting the control path.

Took me awhile to figure this out. Hope this helps someone.

If all of the above fails…

Consider that perhaps DISPLAY is being set somewhere along the way from creation of the process to the time the user can type a command by something other than the ssh mechanism.

This could be the case either from a user level script such as ~/.bashrc, ~/.bash_profile, or from a system level one, typically in /etc/profile.d (at least for RedHat based systems).

In my experience, this is the case in environments that either use currently or have used non-ssh-based X forwarding, typically those that have been around for a good while, especially before such use was considered by some to be a security risk. (Such use has always been a potential security risk, but whether it is or not depends on details and the modern view is apparently "just don’t.")

That said, mixed environments can add to the spot where they set DISPLAY a check to see if DISPLAY is already set before setting it.