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Sharing my screen to the web (from Oracle free tier compute instance) using tmux and gotty

Sharing my screen to the web (from Oracle free tier compute instance) using tmux and gotty

For a long time, I use tmux for my live demos because I can multiplex my screen between my laptop and the beamer, I can show several panes, and I can script my commands using send-keys. I also use tmux to keep a stateful work environment which I run on my Oracle Cloud always free tier Compute Instance, which is 100% free and 100% available, and 100% accessible

  • 100% because even if you need to create a trial account with your credit card it is never charged and the free tier remains after the 30 days trial
  • 100% available because you never have to shut it down and it will never be terminated as long as it was used in the last 3 months (there was a bug recently but was quickly solved even if the free tier has no support)
  • 100% accessible because you can ssh from the internet, and your private key gives you access to the opc user that can sudo to root.

Here I’ll describe what I did to get further in sharing my screen. TMUX can share by attaching clients, which means open a new tty, ssh to the host, and run “tmux attach”. But in case there is any problem with the beamer, or if the screen is too large, I want a simple solution for attendees to see my screen on their laptop or tablet, in a web browser. I need two thins for that: open the port for http access and use gotty to run a tmux session in read-only.

Open a TCP port to the internet

I’ll share my screen on the port 12345 and I need first to open in on the host:

sudo iptables -I INPUT 5 -i ens3 -p tcp --dport 12345 -m state --state NEW,ESTABLISHED -j ACCEPT

Then on my network, I add this port in addition to SSH access from public network:

I follow this path: Compute Instance -> VCN Details -> Security List -> Default Security List -> Add Ingress Rule -> Source CDR = 0.0.0.0/0 and destination port = 12345

Compute Instance -> VCN Details -> Security List -> Default Security List -> Add Ingress Rule -> Source CDR and destination port

Here it is, the world wide web has access to my network on port 12345 and now I have to listen to it. I’ll use:

gotty

yudai/gotty

I download the latest version to my opc home dir:

wget -O- https://github.com/yudai/gotty/releases/download/v1.0.1/gotty_linux_amd64.tar.gz | tar -C ~ -zxvf -

I define a basic configuration file:

cat > ~/.gotty <<-CAT
preferences {
font_size = 14
background_color = "rgb(42, 42, 42)"
}
CAT

I generate a certificate if I want to enable TLS later:

openssl req -x509 -nodes -days 9999 -newkey rsa:2048 -keyout ~/.gotty.key -out ~/.gotty.crt <CH
Vaud
Lausanne
pachot.net
franck
$(hostname)
hello@pachot.net
stdin

And I can now run something to test it:

./gotty --port 12345 top -c 

which logs the following

I connect to my host:port (the IP address from the public network is the same used to ssh — not the one displayed by gotty here) from my browser and can see a terminal with “top -c” running:

That’s perfect. gotty has logged my connection:


tmux

As I use this to share my tmux screen, I add the following in .tmux.conf to start a tmux attach to the current tmux session through gotty, binding this to ^T here:

grep "gotty" ~/.tmux.conf || cat >> ~/.tmux.conf <<'CAT'
# gotty https://github.com/yudai/gotty
bind-key C-t new-window -P -F "screen shared on: http://12345.pachot.net" "TMUX= ~/gotty --port 12345 --title-format "@FranckPachot" --width $(( 0 + `tmux display -p '#{window_width}'` )) --height $(( 4 + `tmux display -p '#{window_height}'` )) --reconnect tmux attach -r -t `tmux display -p '#S'`"
CAT
  • --port defines the port to listen to (opened in iptables and in VCN)
  • I use tmux variables #{window_width} and #{window_height} to get the same number of columns and lines (width and height) because tmux resizes in all clients to fit the smallest one.
  • --reconnect tries to reconnect every 10 seconds if connection is lost
  • tmux attach -r (read-only as additional security but gotty starts in read-only by default). I unset TMUX variable as it is not a nested session.

Now when I am on TMUX, I can C-B C-T to open a window that runs gotty and attach to my current session. Then I just have to share the url and people can see my screen on their screen.

Here is my browser and my tty: