2

I have an Ubuntu Desktop machine. My scope is to take a screenshot of this machine via SSH. This machine is playing a video, and I would like to take a screenshot of this video via SSH. I've tried as root user:

root@user-desktop:~# export DISPLAY=:0
root@user-desktop:~# echo $DISPLAY
:0
root@user-desktop:~# scrot screenshot.png
No protocol specified
Can't open X display. It *is* running, yeah?root@user-desktop:~#

Then I've tried with DISPLAY=:0.0. instead of DISPLAY:=0 (I don't know if this matters).

root@user-desktop:~# export DISPLAY=:0.0
root@user-desktop:~# echo $DISPLAY
:0.0
root@user-desktop:~# scrot screenshot.png
No protocol specified
Can't open X display. It *is* running, yeah?root@user-desktop:~#

Then I've done the same attempts as non-root user:

user@user-desktop:~$ export DISPLAY=:0
user@user-desktop:~$ echo $DISPLAY
:0
user@user-desktop:~$ scrot screenshot.png
Invalid MIT-MAGIC-COOKIE-1 keyCan't open X display. It *is* running, yeah?user@user-desktop:~$

user@user-desktop:~$ export DISPLAY=:0.0
user@user-desktop:~$ echo $DISPLAY
:0.0
user@user-desktop:~$ scrot screenshot.png
Invalid MIT-MAGIC-COOKIE-1 keyCan't open X display. It is running, yeah?user@user-desktop:~$
LearningC
  • 21
  • Hm, first question: you're running on X, and not on Wayland, right? – Marcus Müller Nov 16 '22 at 09:25
  • Maybe your ssh block X connections (frequent, for security reasons: most people do not want that a remote program can access so many thing from the local computer). I recommend you to use some screenshot program on your local machine – Giacomo Catenazzi Nov 16 '22 at 09:41
  • Ssh has no chance to block any x connection here, @GiacomoCatenazzi, I think. The X connection doesn't go across ssh – Marcus Müller Nov 16 '22 at 09:49
  • @MarcusMüller What about sshd configuration: X11Forwarding, AllowTcpForwarding, etc.? -- ssh builds a tunnel (which eventually several channels). terminal is just one, but we uses a lot ssh to tunnel TCP traffic. – Giacomo Catenazzi Nov 16 '22 at 09:50
  • All irrelevant, since the asker runs the screenshotting program on the machine that is running X, not on a different machine. – Marcus Müller Nov 16 '22 at 09:52
  • So, the x machine requires a xhost +, so that external console programs can access X (programs not opened by X) – Giacomo Catenazzi Nov 16 '22 at 09:55
  • Uff, that would tear down all access control and might not be a good idea on a machine where people can log on via ssh – Marcus Müller Nov 16 '22 at 10:10
  • @GiacomoCatenazzi, since he runs scrot on the same machine that runs the X11 and using the local display (he doesn't need to actually display anything on the screen, he uses the DISPLAY just so scrot could take it's screenshot), he doesn't need xhost +. It would only be needed if you tried to run scrot from a remote machine using the DISPLAY of the host he wants to take the screenshot at. – aviro Nov 16 '22 at 13:10

2 Answers2

0

Could you try this:

ssh -x user@user-desktop<<'EOF'
export XAUTHORITY=$(
    ps -u $(id -u) -o pid= |
    xargs -I{} cat /proc/{}/environ 2>/dev/null |
    tr '\0' '\n' |
    grep -m1 '^XAUTHORITY='
)
export DISPLAY=$(
   ps -u $(id -u) -o pid= |
   xargs -I{} cat /proc/{}/environ 2>/dev/null |
   tr '\0' '\n' |
   grep -m1 '^DISPLAY='
)

scrot screenshot.png
EOF

Based on this

Gilles Quénot
  • 33,867
  • 2
    You have few issues here: 1. The result looking inside the environ files will be DISPLAY=xxx and XAUTHORITY=xxx, which means that DISPLAY would actually contain DISPLAY=xxx (instead of the actual of value of the DISPLAY it found) and XAUTHORITY will have XAUTHORITY=xxx. 2. If the first process it catches with a DISPLAY environment variable is sshd (or one of its children), it might use the port forwarded by ssh instead of the actual local display. – aviro Nov 16 '22 at 12:57
  • 1
  • The first ssh won't work because scrot will try to take a screenshot of the X11 tunnel created by ssh instead of the actual local X11 on the destination host. 4. You don't need to use the -X flag on your second ssh, because you don't want to enable X11 forwarding, since you will change the DISPLAY environment variable anyway. It might be even better to disable it with -x. 4.
    • – aviro Nov 16 '22 at 13:05