MobaXTerm X11 forwarding issue with UNIX

Question

Ran below commands on MobaXTerm:

Using MobaXTerm terminal on windows laptop:

> xhost +ulv78.abc.com
ulv78.abc.com being added to access control list

> ssh -l someuser ulv78.abc.com # RHEL 7.x
SECURITY NOTICE:
Unauthorized use is prohibited. Use of this private computer system is your consent to being recorded and monitored. We reserve the right to seek all remedies for unauthorized use. Evidence of suspected illegal use may be given to law enforcement.
X11 forwarding request failed on channel 0
Last login: Thu Sep 20 12:06:57 2018 from win_host_name
$ bash
$ echo DISPLAY=win_host_name:0.0; export DISPLAY >> .bashrc
$ sudo su - # as root sudoer
$ echo DISPLAY=win_host_name:0.0; export DISPLAY >> .bashrc
$ cd /install_path
$ ./setup
Error: Can't open display: win_host_name:0.0

Above session has X11 forwarding request failed on channel 0 error

Edit:

Set DISPLAY entry in .profile & .bashrc. As soon as I connect from MobaXterm terminal, below is the output:

> ssh -l someuser ulv78.abc.com
SECURITY NOTICE:
Unauthorized use is prohibited. Use of this private computer system is your consent to being recorded and monitored. We reserve the right to seek all remedies for unauthorized use. Evidence of suspected illegal use may be given to law enforcement.
X11 forwarding request failed on channel 0
Last login: Thu Sep 20 12:36:54 2018 from win_host_name
$ echo $DISPLAY
win_host_name:0.0
$ xterm
xterm: Xt error: Can't open display: win_host_name:0.0
$

---

1) How to resolve X11 forwarding error?

2) I see wrong domain name showing in the last login msg of ssh client.

Solution

X11 forwarding (as the initial user)

MobaXTerm supports X11 forwarding, enabled by default. If X11 forwarding is enabled on the linux server (ulv78), then DISPLAY is set to an apparently local address, and your ssh client forwards those requests back to your X11 server (running on your Windows machine). See How to forward X over SSH to run graphics applications remotely? for more details about how this works.

When this is setup correctly, you should not change the DISPLAY environment variable yourself; if you've added lines to your login files (.bashrc etc) to set it, you'll have to delete them again. Your should see results like this:

ssh -l user ulv78.domain.com

(Now running as user on remote machine)

user@ulv78$ echo $DISPLAY
localhost:10
user@ulv78$ xterm

xterm is used as a test app here. It should appear on your Windows machine, despite being started by the remote Linux machine. Its prompt will indicate that you are user@ulv78, rather than MobaXTerm's initial prompt.

Congratulations, that's the first step. X11 forwarding is working. Now on to the advanced part:

Accessing the X11 server as another user

When you switch to another user (such as root) via su - and try to run graphical applications on the same X server, you'll discover that your environment variables have all disappeared. This is when you do have to change DISPLAY manually, as well as copying the X credentials with the xauth(1) command (instructions based on https://blog.mobatek.net/post/how-to-keep-X11-display-after-su-or-sudo/ ):

ssh -l user ulv78.domain.com

(Now running as user on remote machine)

user@ulv78$ echo $DISPLAY
localhost:10
user@ulv78$  xauth list | tail -n 1
ulv78/unix:10  MIT-MAGIC-COOKIE-1  4fa72fbe2b05ebe3f047a1b0430ecf6a
user@ulv78$ sudo su -

(Now running as root)

root@ulv78$ export DISPLAY=localhost:10    # <- copied from above
root@ulv78$ xauth add ulv78/unix:10  MIT-MAGIC-COOKIE-1  4fa72fbe2b05ebe3f047a1b0430ecf6a # <- copied from above
root@ulv78$ cd /install_path
root@ulv78$ ./setup

The ./setup application will now appear on your Windows machine, just like the xterm did earlier.

Comments

[VocalFan]

MobaXTerm implements its own X11 forwarding, so you shouldn't need to use xhost. What was the value of DISPLAY as soon as you connected, before you changed it?

[JdeBP]

A related question is unix.stackexchange.com/questions/111519 .

[overexchange]

@JigglyNaga DISPLAY value is win_host_name:0.0 as soon as connected. I have added entries in .profile and .bashrc.

[VocalFan]

Sorry, I mean: what was it before you added those lines to .bashrc and .profile (you might need to delete those lines again in order to find out)? And could you run an X client (such as xterm) as the normal user, before you sudo?

[overexchange]

@JigglyNaga I have edited query with an answer for your query

[VocalFan]

Did you remove the "DISPLAY=..." lines from .profile and .bashrc before doing that?

[overexchange]

@JigglyNaga No, i did not remove. If I remove.. then DISPLAY variable will not be set..

[VocalFan]

Then please remove them and try again. MobaXTerm should have set up DISPLAY automatically.

[overexchange]

@JigglyNaga After removing them, as soon as I connect I see DISPLAY not set. why would MobaXterm set it?

[VocalFan]

What is the output of grep X11 /etc/ssh/sshd_config, on the server?

[overexchange]

@JigglyNaga Where is it mentioend that MobaXTerm will set display variable?

[VocalFan]

Then X11 Forwarding is correctly set up on the server. Are you able to connect to it from another Unix/Linux machine and run X11 applications, as described in unix.stackexchange.com/questions/12755/… ?

[VocalFan]

From the accepted answer on the linked question: "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."

[overexchange]

But this documentation says... you need to set DISPLAY variable explicily

[VocalFan]

From the section 'DISPLAY settings': "When you use a SSH connection, if the remote server supports X11-Forwarding, your DISPLAY is automatically set and it will be sent through the SSH connection to your Windows desktop."

[overexchange]

@JigglyNaga Firstly... why ssh client is showing wrong domain of windows name in last login msg?

[VocalFan]

Let us continue this discussion in chat.

[overexchange]

DISPLAY is showing localhost:10.0

[overexchange]

What next? to forward X11

[overexchange]

xterm is also working

[VocalFan]

If xterm opens on your Windows machine, then forwarding is working. Are you still trying to run another X11 application (./setup) as root, after sudo? If so, the set the two variables while in the root shell, as shown in the second part of the answer.

[overexchange]

yes xterm opens on my windows laptop. Now, how do I launch installer from linux machine? Your second part of the answer does not help me., because display should be set to windows machine. Isn't it?

[VocalFan]

If you've run the commands in the order I wrote them, then the xterm that appeared on your Windows laptop was started from inside the ssh session - ie. from the linux machine, over X11 Forwarding. It uses DISPLAY=localhost:10 to make that connection, and you don't need to change it. But su - clears your environment, including DISPLAY and XAUTHORITY, so you will have to set those variables manually before running an X11 program as root.

[overexchange]

But as per this answer... unix.stackexchange.com/a/469669/62659 variable DISPLAY should be set to windows machine... am not sure.. why you are recommend ing to set to local host?

[VocalFan]

The first half of that question is about connecting directly to a server without X11 forwarding, which mean accessing port 6000-something, and messing with xhost, and relies on there not being a firewall. Please scroll down to the second half, "The modern, secure way of using X11 is SSH-based X11 forwarding...". Or read the answers to "how to forward X" question I linked earlier.