Well hello, this is issue four! I have the Man Flu™, so not feeling super 100%, but the newsletters won’t send out themselves.

This week I learned about, more DNS things, typewriters, RADIUS servers, trains, buttons, and standards!

Split horizon domains are cool

The task I wanted to achieve is to have proper TLS (https://) working for my internal devices: pihole, NAS, router, whatever that has a local admin interface.

I can do self signed certificates, but computers won’t trust those unless the signing certificate is trusted by all of them, which brings a whole load of device management I don’t want to do.

Split horizon domains are real domains that you verify using DNS-01 via acme, get the certs for the wildcard subdomain, and use those certs on the devices, and set the DNS internal only to resolve to the devices.

So you’d get *.home.yourdomain.com and home.yourdomain.com verfied using DNS-01, get the certs, keys, and in your local DNS:

• 192.168.1.1 → gateway.home.yourdomain.com

• 192.168.1.99 → nas.home.yourdomain.com

• 192.168.1.255 → pihole.home.yourdomain.com

• and so on…

What makes this split horizon? These sub-subdomains will only resolve from inside of your home / corporate network. From the internet, because you don’t add an A, AAAA, or CNAME record, everyone will get a lookup error.

Royal typewriters are insanely cool

I got a Model H from 1934 for £45. I’ll clean it and restore it, and I bought the Definitive Guide to Restoring Typewriters, because why wouldn’t I start yet another incredibly niche side quest?!

In case you want to look at the sheer number of typewriter repair manuals: https://twdb.sellfy.store/.

And to look at the typewriters: https://typewriterdatabase.com/royal.72.typewriter-serial-number-database, scroll down to 1934, “"H" and "KH" in 1664000 to 1850000 ("K" means Keyset)”.

The little chime as you return the carriage is fantastic!

Here’s a post by someone else about the same kind of typewriter: https://typewriterdatabase.com/1934-royal-h.24268.typewriter.

The plasterboard on the ceiling of houses in the UK might actually have void behind it

According to Claude, the ceilings in most modern houses in the UK are timber joists and plasterboard is fixed to them, rather than them being fixed to concrete, or whatever the floors are made out of.

This came up because I need to fix a mounting plate into the ceiling, and plasterboards are not particularly strong.

I can’t find definitive guidance, though it’s also 2:35am. I am going to check for studs though.

Stud finders use relative density of things to find studs

As you’re sliding the stud finder along a wall, if there’s a stud, made of wood, or pipes, or something other than a void or cavity, the relative density reading is going to change, and that’s the signal.

This is different than the inductive scanners that detect metal, or antennae that look at live wires behind the surface. Magnets.

Just don’t forget to point that stud finder at yourself and loudly exclaim “found one!” It is law.

Gotta talk about inductive and electric field sensing stud finders

To detect metals, the stud finders generate alternating magnetic fields to induce eddy currents in the metal which it will then picks up.

Live wires are alternating current, so they generate their own alternating (electro)magnetic fields, that the stud finder picks up.

Though electric field, electromagentic field, and magnetic field are all their own things, interconnected, but different.

The relative density measurement for finding timber is done by capacitors!

Capacitance is determined by (and let’s see if Buttondown can do math correctly):

Where:

• εr is the relative permittivity (dielectric constant) of the material — this is what changes between cavity and timber

• ε0 is the permittivity of free space (~8.854 × 10⁻¹² F/m), a physical constant

• A is the plate area of the capacitor plates in the stud finder in m², the machine knows this

• d is the plate separation in m, again something they would have coded into the machine

• C is capacitance in Farads, this is the signal that generates the beeping

Extruded aluminium has a Velcro phenomenon

Velcro is a brand name that became so widely used, the word ended up becoming the thing we call the loop and hook sticky systems, regardless of who made it.

Similarly, BR, short for Bosch Rexroth, a heavy industrial company, subsidiary of the Bosch Group (you probably have a drill or a lawnmower, maybe an oven from them), designed an aluminium extrusion profile for T-slot applications that became so widely used that everyone else went “yeah that’s actually good, we’re gonna adopt it.”

Aluminium extrusion is a tiny part of what Bosch Rexroth does.

RADIUS needs things on the network device you’re connecting to directly

It’s the 802.1x auth protocol. That means the switch you plug your computer into, or the access point your laptop connects to needs to have a RADIUS authenticator running in order to let you onto the network. The RADIUS server can live somewhere else, but the network device needs to support 802.1x auth to let you in.

Which is extra fun when you’re designing enterprise network systems.

Why you have to pay lots of money to access standards

Besides the obvious sociopolitical commentary this statement has, I’m specifically talking about industrial standards, like EN 14752. That one costs a whopping £488, or £244 if you’re a member (annual cost of £220 for private individuals, sole traders, private companies).

The reason being is that some standards bodies, like the W3C and IETF, operate open access models, where they get funding from donations, grants, sponsorships, memberships, while others, like ISO, and CEN, the authors of EN 14752, operate a different model.

Putting together those standards takes enormous work: multiple experts, infinite rounds of revisions, admin, lawmaking, consultations, all of those take time, work, and therefore money.

Why you need to wait for the button to light up when the train comes to a stop at a station in the UK to open the door

That is governed by EN 14752, but the extremely short and not at all precise tldr version is that the button is a state machine, and for accessibility reasons when you do a thing with a button, it needs to respond in an unambiguous way.

You press the unlit button, it flashes red to tell you it rejected whatever you asked of it. You pressed the button when it lit up as green, it opened the door for you. You press the button when it’s lit up with red, it will do nothing.

Hold to open breaks this. Buttons are now digital, which means when you press the button, an event is fired. It does not retry. So you pressing the button once, and holding it are the same thing for the controller.

Why not press to request door opening before we get to the station? Sure, you press the button, it gets registered, and you’re safe in the knowledge that the door will open. Except you’re at the end of the train, the station is too short, and the door would open into air, because there’s no platform. The door remains closed, and you’re annoyed.

Okay, why not open ALL the doors then? Sure, except it’s the dead of winter, and it’s raining at the same time, there’s like 5 of you on the train, and each stop all the lukewarm air gets replaced by a really cold and wet draught. Or in the heat, same thing.

Okay, it’s 3 am, I collected all the things from the last week. Time to go to sleep. And remember, send this to a friend, ask questions, and go forth and research things!

Loveyoubye! ❤️

By the way, Buttondown keeps bugging me with wanting to add the subscribe form to the email. I have no idea what that looks like in the email you already get because you’re subscribed, but now I’m curious, so please enjoy this subscriber form template tag below which you’ll no doubt see rendered. On the web apparently. Below here, and above the P.S..