I have a rpi set up with a minutely cron job to update my domain name to point to home. Works pretty well. At the worst you lose connection for a minute but usually the IP address only changes when the home connection fails which can take more than a minute to reset anyway.

Isn't this what the DynDNS protocol and various daemons are for? Why write your own? :P

That's precisely how they work. You install a client that pings their server. They see if there's an IP address change and switch the A DNS record.

If your DNS provider has an API, this is probably the very first example in the docs.

Why not? It is pretty simple and very fun! My first project in golang was a program that polled for the machine's IP address and updated a AWS Route53 record.

Its not exactly "write your own" I have a single line in my crontab that just uses curl to post to a url and the remote server takes the IP address it got the request from and sets the dns to that.

Though usually on firmware like openwrt the request going out is tied to a particular interface going up (and down) as it should be, so its somewhat more robust and 'correct' than crontab would be.

Dynamic in theory, but for many people the IP is unchanged for a long time. I remember reading an article that said the average length of time between dynamic IP changes tracked by some company was something like seven months, though I can't find it now.

I have cable with a theoretically dynamic DNS but it's changed once in >4 years.

That's not really a thing. The pools are large-ish; the chances of winding back on the same IP after a change are tiny.

Not speaking for all ISPs worldwide.

Maybe running dynamic DNS client that keeps logs of IP address changes. Do DDNS clients keep logs. Maybe passive DNS would detect changes.

The point of the comment was that cannot just assume it never changed unless monitoring it contiuously.

Well if you don't notice it, for a home vpn, well... you wont notice it.