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What I’d do is set up a simple website that uses a little JavaScript to rewrite the date and time into the page and periodically refresh an image under/next to it. Size the image to fit the remaining free space of however you set up the iPad, and then you can stick anything you want there (pictures/reminder text/whatever) with your favorite image editor. Upload a new image to the server when you want to change the note. The idea with an image is that it’s just really easy to do and keeps the amount of effort to redo layout to a minimum – just drag stuff around in your image editor and you’ll know it’ll all fit as expected as long as you don’t change the resolution (instead of needing to muck around with CSS and maybe breaking something if you can’t see the device to check that it displays correctly).

There’s a couple issues to watch out for – e.g. what happens if the internet connection/server goes down, screen burn-in, keeping the browser from being closed/switched to another page, keeping it powered, etc. that might or might not matter depending on your particular circumstances. If you need to fix all that for your circumstances, it might be more trouble than just buying something purpose built… but getting a first pass DIY version working is trivial if you’re comfortable hosting a website.

Edit: If some sample code that you can use as a starting point would be helpful, let me know.

Do you have systemd-initiald configured correctly? :-)

I’ve used Wireshark when I want to inspect the traffic going through my computer. I’ve found it particularly handy for debugging my own networking code. I’ve also used netstat to see active connections and programs listening for traffic when I don’t care about the packet contents specifically.

With that said, can I possibly only allow traffic to and fro from the proxy through my firewall?

Yes. That is what I suggested. If you configure the firewall to only allow traffic to/from the specific IP and port combination of your proxy, other traffic will be blocked.

I should be able to (in theory) inspect traffic too, although I don’t know how far that will take me.

You can do content filtering via a proxy like that, yes. A similar sort of configuration is used on school computers to do things like block adult content, with varying degrees of success. Some ad-blocking techniques work on similar principles.

If I understood your question correctly, you’d run the proxy application (which might be Squid or Apache or some other program) either on the host computer outside the VM or elsewhere on your network. (I’m not well versed on all the ins and outs of setting Firefox up to communicate through a proxy; I just know it can be done.) The proxy would listen for incoming traffic on a specific port you configure. You then tell Firefox (in its network settings) to communicate with the specific IP and port of the proxy instead of talking to web servers directly.

To prevent other programs from communicating, you’d firewall off the VM with iptables (or maybe ufw or something else depending on what you use on your system). You’d set it to drop all traffic going to/from the VM’s network except packets going to or coming from the specific IP/port combinations you want to allow.

This isn’t a bulletproof way to block other apps from talking to the internet – anything that knows about the proxy (or which can hijack/manipulate a program like Firefox that you’ve told about the proxy) could communicate with web servers via the proxy, but depending on your specific concerns it may be good enough.

You could try configuring Firefox to access the internet through a proxy and then block the VM off from everything except the proxy and your network mount with a firewall (outside the VM).