How to forward X over SSH to run graphics applications remotely?

X11 forwarding needs to be enabled on both the client side and the server side.

On the client side, the -X (capital X) option to ssh enables X11 forwarding, and you can make this the default (for all connections or for a specific conection) with ForwardX11 yes in ~/.ssh/config.

On the server side, X11Forwarding yes must specified in /etc/ssh/sshd_config. Note that the default is no forwarding (some distributions turn it on in their default /etc/ssh/sshd_config), and that the user cannot override this setting.

The xauth program must be installed on the server side. If there are any X11 programs there, it's very likely that xauth will be there. In the unlikely case xauth was installed in a nonstandard location, it can be called through ~/.ssh/rc (on the server!).

Note that you do not need to set any environment variables on the server. DISPLAY and XAUTHORITY will automatically be set to their proper values. If you run ssh and DISPLAY is not set, it means ssh is not forwarding the X11 connection.

To confirm that ssh is forwarding X11, check for a line containing Requesting X11 forwarding in the ssh -v -X output. Note that the server won't reply either way, a security precaution of hiding details from potential attackers.

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.

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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.

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).

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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

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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?}

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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

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

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