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 <<stdin
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: