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Many screen lockers (mine is i3lock) do not block access to other Virtual Terminals. This means that, if I leave a session opened in some VT, then even when the desktop is locked (for example when resuming), a malicious person can switch to the VT and do anything.

This is an actual issue for me, as I occasionally switch to a VT, then switch back to the graphical environment and forget to log out from the VT.

The question then is: how to add VT-locking on top of an existing screen locker?

The Arch Linux wiki suggests to simply disable VTs from Xorg, with this piece of configuration for the X server:

Section "ServerFlags"
    # disable VT switching:
    Option "DontVTSwitch" "True"
    # disable “zapping”, ie. killing the X server with Ctrl-Alt-Bksp:
    Option "DontZap"      "True"
EndSection

This is not an option since I use VTs, as already explained above. Maybe one solution would be to set and reset those options dynamically, but I found nothing to change X server options at runtime, at least in general (there are things like setxkbmap for keyboard layouts, or xset for misc stuff). Is this possible?

I also found the command vlock -a which, when called from a text-based VT, locks the session and disable VT switching. However, it does not work from the graphical environment, and would anyway be redundant with the graphical screen locker.

How can I solve this problem?

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Using vlock with a delay worked for me. I had X server already running, switched to VT-1 with Ctrl-Alt-F1, ran sleep 3; vlock -a, then switched back to VT-7 with Alt-F7. The first time, I didn't switch back to VT-7 fast enough and had to unlock and try again. I got it the second try, and X appears and works like normal. Attempting to switch back to VT-1 presents a locked terminal which you can't switch out of until you unlock it. Don't try to background vlock - I got an endless loop of messages until I exited that session. Maybe putting just vlock -a in custom startx script which executes /usr/bin/startx after would work for automating it. I'm thinking including the delay in the script wouldn't help unless it could run in parallel or backgrounded (not sure I would want to try that again with vlock).

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