Necessary software

• In the computer:

   sudo apt install x11vnc
  

• In the tablet:

  • bVNC Free

1. Get some information

Firt we need to know available displays and output ports.

xrandr

The output should be something like this (I removed some entries because the output was very long):

android@myos:~$ xrandr
Screen 0: minimum 320 x 200, current 1366 x 768, maximum 16384 x 16384
LVDS-1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
   1366x768      59.98*+  39.98  
   1360x768      59.80    59.96  
   1280x720      60.00    59.99    59.86    59.74  

   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
VGA-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)

As you can see, my current display is LVDS-1 with the resolution 1366x768 59.98.00Hz. The unused output ports are HDMI-1, DP-1- and VGA-1. You may have a different output ports and resolutions depending on your graphic card.

2. Create a new virtual monitor

We need to know the tablet's resolution. In my case is 1024x768.

If you don't know your tablet's resolution, visit this site from your tablet's web browser and it will show you your screen resolution. If it is different from mine you will have to replace it in all the commands.

Generate modeline for the resolution of the VPN screen. Run in terminal:

gtf 1024 768 60

60 is the screen refresh rate for the tablet

The modeline generated by this command is:

android@myos:~$ gtf 1024 768 60

  # 1024x768 @ 60.00 Hz (GTF) hsync: 47.70 kHz; pclk: 64.11 MHz
  Modeline "1024x768_60.00"  64.11  1024 1080 1184 1344  768 769 772 795  -HSync +Vsync

Copy everything after the word Modeline (exclude it) into the next command. Now let's add a new mode for our Android device:

xrandr --newmode "1024x768_60.00"  64.11  1024 1080 1184 1344  768 769 772 795  -HSync +Vsync

Note: xrandr means X Resize and Rotate (just a curiosity)

Add this new mode to an unused output port. HDMI-1 in this case.

You may have a different output ports so you need to try the output ports that work for you. Find the available ports with the command xrandr like in the first section

xrandr --addmode HDMI-1 1024x768_60.00

Now let’s enable HDMI-1 and move it to the left of the default display (LVDS-1). After this mouse cursor can be moved to the left side more than your default display allows. It is because we are adding 1280×800 to the left side of LVDS-1.

xrandr --output HDMI-1 --mode 1024x768_60.00 --left-of LVDS-1

If you want to put the second screen in the right side of the default display just change the option --left-of by --right-of.

The final step is to start the VNC server:

x11vnc -clip 1280x800+0+0

x11vnc -clip xinerama0  #to auto calculate the screen size needed

Now you need to connect your Android device using a VNC client. To do so you need an IP and port. To find your computer's IP run:

ip a

In my case it is 192.168.1.67. The default port in VNC is 5900 unless you changed it.

Screenshot of the bVNC Free app

3. Disable the second screen

After stopping the VNC server you need to disable the second screen HDMI-1-1:

xrandr --output HDMI-1-1 --off

Summary

Every time you reboot the computer you will have to add the modeline again. Then I will make a summary of all the commands used in this tutorial:

# Create the virtual monitor (every time after rebooting the computer)
xrandr --newmode "1280x800_60.00"  83.46  1280 1344 1480 1680  800 801 804 828  -HSync +Vsync
xrandr --addmode HDMI-1-1 1280x800_60.00

# Enable the second display
xrandr --output HDMI-1-1 --mode 1280x800_60.00 --left-of eDP-1-1
x11vnc -clip 1280x800+0+0

# Disable
xrandr --output HDMI-1-1 --off

Clean modes

To remove the mode:

xrandr --output HDMI-1-1 --off
xrandr --delmode HDMI-1-1 "1280x800_60.00"
xrandr --rmmode "1280x800_60.00"

Check displays and modes

xrandr -q

ADB Support

You can connect an Android device to the computer with an USB cable to have access to the VNC server. It is done through the ADB platform.

First, you need to install the proper tools:

sudo apt install adb android-tools-adb android-tools-fastboot

And now follow this steps:

• connect your device to the PC with an USB cable
• turn on USB debugging on your device
• run the command adb reverse tcp:5900 tcp:5900
• enable the second display with the commands of the tutorial
• in the app bVNC Free set 127.0.0.1 as server address and connect to the server

After stopping the VNC server stop the ADB daemon:

adb kill-server

Notices

• Data is unencrypted! (Relevant for public network connections)
• Any network you are connected that can reach port 5900 can connect to your monitor! (Not a problem if using a USB connection)

Due to this you should use some security options when using x11vnc.

Add more security to the connection

Change the port of the connection

By default x11vnc use the port 5900. We can change it by running the following command:

x11vnc -rfbport <port_number>

If something else is using that port x11vnc will exit immediately.

Establish a password for the connection

You can use one of the following commands:

x11vnc -storepasswd password /path/to/passfile
x11vnc -storepasswd /path/to/passfile
x11vnc -storepasswd
# The last one will save the password in ~/.vnc/passwd

And then start x11vnc via:

x11vnc -rfbauth /path/to/passfile

Or run

x11vnc -usepw

It automatically use your ~/.vnc/passwd or ~/.vnc/passwdfile password files. Under -usepw, x11vnc will exit if it cannot find a password to use.

WARNING: Even with a password, the subsequent VNC traffic is sent in the clear. Solution => SSL connection.

Use a SSL connection or Tunneling

Consider tunnelling via ssh: http://www.karlrunge.com/x11vnc/#tunnelling

Or using the x11vnc SSL options: -ssl and -stunnel

The -ssl mode requires an SSL certificate and key (i.e. .pem file). These are usually created via the openssl program. In fact when you run the -ssl option (same as "-ssl SAVE") it will run openssl for you automatically. It will prompt you if you want to protect it with a passphrase. In general, the PEM file contains both the Certificate (i.e. public key) and the Private Key, so it should be protected from being read by untrusted users. The best way to do this is to encrypt the key with a passphrase (note however this requires supplying the passphrase each time x11vnc is started up).

Example:

x11vnc -ssl SAVE ...

This way it will be saved in the default directory ~/.vnc/certs/ as server.crt (the certificate only) and server.pem (both certificate and private key.) This opens up the possibility of copying the server.crt to machines where the VNC Viewer will be run to enable authenticating the x11vnc SSL VNC server to the clients.

If you are using the bVNC Free app in the Android device, go to the Connection Type menu and select the Secure VNC over SSL Tunnel option.

Limit which machines can connect to the VNC server

With the -allow option we can limit connections by hostname or IP address. Example:

x11vnc -allow 192.168.0.1,192.168.0.2

Or use -localhost, that achieves the same thing as -allow 127.0.0.1

Known issues

• Often having a Firewall/Router sitting between the vncviewer and x11vnc will make it impossible for the viewer to connect to x11vnc. Try to use a USB connection to solve this problem. (+info)

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Resources I used to create this tutorial

• https://sangams.com.np/using-android-pc-as-a-second-monitor-in-linux
• https://askubuntu.com/a/750497
• https://github.com/brunodles/linux-second-screen/blob/master/tutorial.md
• https://github.com/mrenrich84/vnc_virtual_display_linker