CentOS Stream 8: Issue with hang of non-X11 command via SSH X11 Forwarding

• When openssh-server, xorg-x11-xauth and x11-dbus are installed in server machine, running non-X11 command with ssh -X/-Y on client machine will be hanged because SSH connection will be not closed.

$ ssh -Y lxd-centos-8-ssh-x11.hiroom2.com -- uname -a
Warning: No xauth data; using fake authentication data for X11 forwarding.
Linux lxd-centos-8-ssh-x11 5.10.52-1-lts #1 SMP Tue, 20 Jul 2021 16:46:09
+0000 x86_64 x86_64 x86_64 GNU/Linux
(hang)

• CentOS Stream 8's dbus-x11 provides /etc/profile.d/ssh-x-forwarding.sh. This script will be done after SSH connection is established.

$ rpm -ql dbus-x11
/etc/X11/xinit/xinitrc.d/00-start-message-bus.sh
/etc/profile.d/ssh-x-forwarding.csh
/etc/profile.d/ssh-x-forwarding.sh
/usr/bin/dbus-launch
/usr/lib/.build-id
/usr/lib/.build-id/4b
/usr/lib/.build-id/4b/439e4019de8f19a66c1b6676d47dd121f684f3
/usr/share/man/man1/dbus-launch.1.gz

• When specifying -X/-Y option of ssh command, /etc/profile.d/ssh-x-forwarding.sh will run dbus-launch –exit-with-x11.
• dbus-launch –exit-with-x11 will be done after x11 command is done. But dbus-launch –exit-with-x11 will be NOT done after non-x11 command is done. This is the reason why ssh command is hanged.

$ cat /etc/profile.d/ssh-x-forwarding.sh
# DBus session bus over SSH with X11 forwarding
[ -z "$SSH_CONNECTION" ] && return
[ -z "$DISPLAY" ] && return
[ "$SHLVL" -gt 1 ] && return

GDK_BACKEND=x11; export GDK_BACKEND
eval $(dbus-launch --sh-syntax --exit-with-x11)

• Disable /etc/profile.d/ssh-x-forwarding.sh.

#!/bin/sh

sudo dnf install -y openssh-server xorg-x11-xauth dbus-x11

sudo systemctl enable sshd
sudo systemctl start sshd
firewall-cmd --add-service=ssh --permanent
firewall-cmd --reload

sudo mv /etc/profile.d/ssh-x-forwarding.sh \
     /etc/profile.d/ssh-x-forwarding.sh.disabled

• If you need D-Bus, running dbus-launch in ssh command like the following.

$ ssh -X server '. /etc/profile.d/ssh-x-forwardning.sh.disabled; command'

• With dbus 1.12.8-12.el8_3 ssh session executing single command hang
• dbus-x11 could start per-connect session bus when user does ssh X11 DISPLAY forwarding