Extending your X11 Desktop using VNC

I love multihead setups and I am a thrifty grad student. Today, this combination lead to an interesting and fun Saturday morning project. I figured out how to add an additional monitor to my dual-monitor set-up, which is already a little convoluted.

Let me clear things up: My main setup currently consists of a Thinkpad with its lid closed connected to two monitors (one over VGA, one over DisplayPort). Since my laptop has no third physical display connector, I had to get creative in order to add a third physical monitor to my set-up.

I use a cheap DisplayLink adapter at home for exactly this reason, but unfortunately the official dirvers are buggy and the whole set-up is generally very unreliable. So, with me being the thrifty grad student that I am, I did not want to shell out another $30 for a piece of hardware that doesn't even really work.

What I ended up doing is pretty neat.

The high-level description of my set-up is the following: I have a desktop machine that I was using headless until today. It is now connected to my third monitor and runs the TightVNC client, which is being served by x11vnc. In other words, I'm using xrandr to create a virtual output device on my laptop, I make it available for "remote" viewing using x11vnc, and use the TightVNC client on my desktop machine to view it over the network. My desktop machine and my laptop are connected via a small switch behind my desk, which means that the connection is fast enough to even watch YouTube videos on my networked monitor.

I was heavily inspired by this post on the arch forums and added some tricks of my own to make this set-up comfortable.

For this blog post, I will follow along the lines of the forum post above and describe how this can be replicated. I have most of the code below sitting in a bash script, which I invoke whenever I get to the office in the morning.

Step 1: Create a temporary directory to hold PIDs and logs

This is mostly for debugging purposes and will hold most of the relevant information for my SSH tunnel and the VNC server.

if ! [[ -d $TMPD ]]; then
    mkdir -p $TMPD

Step 2: Force XRandR to connect the VIRTUAL1 device to our X11 desktop

The below part is still a little messy, but it gets the job done for now. I'm using the new monitor in a portrait orientation, so I'm creating a output of dimension 1024x1280. I'm also telling xrandr to put this output to the right of the output DP2.

# the local virtual xrandr device

# get the modeline from
# gtf 1024 1280 60 | sed -n 's/.*Modeline "\([^" ]\+\)" \(.*\)/\1 \2/p'
MODELINE="1024x1280_60.00 110.66 1024 1096 1208 1392 1280 1281 1284 1325 -HSync +Vsync"

# FIXME get rid of error messages
xrandr --delmode "$DEVICE" "${NAME}"
xrandr --rmmode "${NAME}"
xrandr --newmode ${MODELINE}
xrandr --addmode "$DEVICE" "${NAME}"
xrandr --output $DEVICE --mode $NAME --right-of DP2

The above commands extend my desktop over a—as of right now—invisible area of 1024x1280 pixels. We could theoretically drag windows in this area. Next, I'm using VNC to make this area available to a client on my desktop machine.

Step 3: Generate a random VNC password and store it on the desktop machine

From now on, any reference to $REMOTE should be replaced with SSH username and hostname, e.g. foo@bar.baz.

PW=$(openssl rand -hex 50 | vncpasswd -f | tee $TMPD/vncpw)
scp $TMPD/vncpw $REMOTE:~/.ava_vncpw

Step 4: Create a reverse SSH tunnel to the desktop machine

Since this is the 21st century, we want to encrypt any traffic that we send through a network. Here, we set up a reverse SSH Tunnel, which forwards port 5900 on my laptop to port 5900 on my desktop machine through an encrypted pipe. I'm also using the nohup command to send the process in the background and have it persist even after I close my terminal.

nohup ssh -2tnNv -R $PORT:localhost:$PORT $REMOTE > $TMPD/tunnel_log 2>&1 &
echo -n $! > $TMPD/tunnel_pid # write the pid of the sshtunnel to kill later

Step 5: Launch x11vnc, clipped to the invisible region

x11vnc has a bunch of configuration options and I spent a significant amount of time reading tweaking its configuration to get the best performance.

CLIP=$(xrandr | grep "^$DEVICE.*$" | grep -o '[0-9]*x[0-9]*+[0-9]*+[0-9]*')
nohup x11vnc -clip $CLIP -noxinerama -noxrandr \
      -repeat -localhost -nevershared -forever \
      -rfbauth $TMPD/vncpw \
      -nowf -noncache -wait 1 -defer 1 > $TMPD/x11vnc_log.log 2>&1 &
echo -n $! > $TMPD/x11vnc_pid # write the pid of x11vnc to kill later
  • -clip defines the area which is exported. This coincides with the area of the output created using the mode line above
  • I had to set -noxinerama to get rid of some nasty letterboxing
  • Without setting -repeat, I couldn't enter a letter more than once by long-pressing a key on the keyboard. That was annoying
  • -localhost lets x11vnc only accept connections from localhost, which is fine since we're using a SSH tunnel.
  • -forever makes x11vnc not quit after the first client quits
  • without the -noncache switch, x11vnc exported a much larger area filled with mostly black pixels

Step 4: Be lazy and write a script to kill x11vnc and the SSH tunnel

This saves a few keystrokes during debugging.

echo "#!/bin/sh" > $KILL
echo -n "kill $(cat $TMPD/tunnel_pid) && " >> $KILL
echo -n "kill $(cat $TMPD/x11vnc_pid) && " >> $KILL
echo "echo \"Killed x11vnc and SSH.\"" >> $KILL
chmod +x $KILL

Step 5: Start a VNC client on the desktop machine

Since I'm not doing anything else on this machine other than running the VNC client, I have an infinite loop in my .xinitrc file which attempts to restart the client if it is not running.

# rotate the display
xrandr --output DVI-0 --rotate left
# disable screen sleep
xset s off
xset -dpms
# unclutter removes the local mouse pointer
unclutter &
force reloading of the vnc viewer to simulate plug & play
while true; do
    if ! ps aux | grep -v grep | grep -q vncviewer; then
        vncviewer localhost:0 -passwd ~/.ava_vncpw -viewonly -fullscreen -encodings "copyrect tight"
      sleep 0.5

Step 6: Celebrate!

This was fun. I hope this inspires someone to do their own thrifty grad student multihead set-up.


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