ddclient (Linux, NAS, Docker)

Logged in, the setup assistant fills in your own host name and token.

ddclient runs on any Linux machine, a Raspberry Pi or a NAS and keeps your host up to date.

Install it, e.g. sudo apt install ddclient (Debian/Ubuntu/Raspberry Pi OS).
Put this into /etc/ddclient.conf (replace the whole file):

daemon=300
syslog=yes
ssl=yes
protocol=dyndns2
usev4=webv4, webv4=ipify-ipv4
server=dynr.net
login=<your-host>.dynr.net
password='<your-token>'
<your-host>.dynr.net

Restart: sudo systemctl restart ddclient.

ddclient older than 3.11 does not know usev4; use use=web, web=ipify-ipv4 instead. For IPv6 add usev6=webv6, webv6=ipify-ipv6.

In Docker

The image lscr.io/linuxserver/ddclient reads the same file from /config/ddclient.conf:

docker run -d --name ddclient --restart unless-stopped \
  -v /path/to/config:/config lscr.io/linuxserver/ddclient

Check

sudo ddclient -daemon=0 -debug -verbose -noquiet runs one update and prints the answer (good or nochg).