Updating my DNS records to point to my home LAN, and other GleSYS DNS stuff.
This is a reimplementation of an ancient perl script I wrote years and years ago. Since I'm doing everything else in Go these days, I might as well do this in Go.
The main task is to figure out what my public IP is, and then update the GleSYS DNS records as needed. This means I can access my home network by a valid hostname even when the IP changes. It almost never does, but it's a pain in the smurf when it does.
Fixed by code, because that's how I think.
There is also a "Certbot mode"
Certbot, from Let's Encrypt, can call a manual_auth_hook program/script. This used to call a perl script for me, to set the domain's _acme-challenge TXT record. As is now my custom, this is replaced with Go code.
Since I do this by calling the Glesys DNS API, it made all manner of sense to include it here, even if the two "modes" aren't even needed in the same context.