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I'm planning a system upgrade from Eoan to Focal on a remote machine where I only have SSH access. I've previously dealt with broken packages causing the system to boot into rescue mode, so I want to ensure whatever happens I can keep my access to the server. To further secure the process, I'm doing a rehearsal using a local virtual machine running the target system.

With my knowledge in Ubuntu, I did the following tasks beforehand:

  • systemctl edit ssh.service and insert DefaultDependencies=no to [Unit] section and WantedBy=rescue.target to [Install] section, then systemctl enable ssh.service. Systemd reports "created symbolic link".
  • systemctl edit rescue-ssh.target and insert WantedBy=rescue.target to [Install] section, then systemctl enable rescue-ssh.target.

Then I edited grub to enable boot menu, update-grub then systemctl reboot, and use e to edit the default entry. I scrolled down to the first linux line and appended rescue and booted this entry with Ctrl-X, which led to this screen:

rescue mode

Everything seems fine here as I can SSH into this VM right at this point.

My question begins:

I tried again in a different way to test SSH availability in rescue mode. First I booted the system normally into graphical mode (with GNOME and whatever), started a terminal and ran systemctl isolate rescue.target. I see the same prompt text again, but when trying to SSH into the VM, I got the following result:

ubuntu@WSL:~ $ ssh root@vmwares
Warning: Permanently added '192.168.2.5' (ECDSA) to the list of known hosts.
Timeout, server 192.168.2.5 not responding.
255|ubuntu@WSL:~ $

Running systemctl status ssh.service highlighted the following text:

pam_systemd(sshd:session): Failed to create session: Failed to activate service 'org.freedesktop.login1': timed out (service_start_timeout=25000ms)

Restarting systemd-logind.service makes no difference. Going back to graphical interface with systemctl isolate default.target brings everything back to normal again.

Question: Why can't I login to SSH when the rescue target is isolated from graphical.target? How to I resolve this?

I think there's something to do with "isolating targets", but apparently things have gone beyond my knowledge.

iBug
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