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From a currently running X11 session, I would like to provide/run a VNC server such that it appears to my system as a second, “virtual” monitor – i.e. so that I can position it using xrandr and drag/position windows onto it.

How, if at all, could I achieve that?

Edit: More info from OP in comments: "Also asked here, without an answer. "

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tl;dr: Force a "virtual" output of your graphics card to a display mode, and export that with x11vnc.

You can achieve this, but there are a few prerequisites:

  • A graphics card with multi-head capabilities (= can render several "desktop" surfaces). Which is most cards these days.
  • x11vnc, a mature software () to export X11 surfaces (among others) to VNC clients.

Most consumer cards these days can render several different outputs. Mine can do 3, out of the 5 that xrandr shows (eDP1,HDMI[12],DP[12]).

  • Pick an unused output from , in my example HDMI2.

  • Pick a resolution for the screen of the vnc client, and generate a mode:

    $ cvt 1920 1080
    # 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz
    Modeline "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync

  • Add that mode to xrandr

    xrandr --newmode "1920x1080_60" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync

  • Put e.g. HDMI2 in that mode, and attach to the right of eDP1 (Main screen)

    xrandr --addmode HDMI2 1920x1080_60 --output HDMI2 --mode 1920x1080_60 --right-of eDP1

  • Now export that with x11vnc, choosing the appropriate offset:

    x11vnc -display :0 -clip 1920x1080+1600+0 <other options>

    Note: Add desired encryption/authentication/other options to that command.

  • Now connect with a VNC client to your "virtual monitor". (or modify above command to connect to a "listening" VNC-client.

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    With the notebook screen and a VGA monitor it fails saying xrandr: Configure crtc 2 failed. If I use only one screen (either VGA or embedded one) it works. How can I make it work with both screens?
    – Zhigalin - Reinstate CMs
    Oct 13, 2017 at 18:50
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See this answer to be able to use VIRTUAL1 instead of a real output like HDMI2, and be able to manage it (select resolution, arrange desktop geometry, clone or extend) in the desktop environment screen settings like any other physical monitor.

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