I'd really like to know how your Mac update client determines my IP address.

If I'm using a proxy or a VPN, your update client updates my domain with the address of the proxy/VPN. Maybe that's what you want, but it's not very helpful.

For example:

Here's my real IP address:

MrMuscle:~ mnewman$ /usr/bin/dig +short myip.opendns.com @resolver1.opendns.com
101.51.3.84

Here's the IP address that your client used to update my domain:

2021-07-08 12:17:14 IP address update initiated.
2021-07-08 12:17:14 Updating IP address for username Buadhai.
2021-07-08 12:17:29 Server response: Good
2021-07-08 12:17:29 [good] IP address updated.
2021-07-08 12:17:29 Current IPv4 Address: 51.75.215.63

And here's the result:

MrMuscle:~ mnewman$ dig +short korat.myddns.rocks
51.75.215.63

This is not useful because the IP address is not routable. If I try to connect to my home machine from outside the LAN it fails:

pi@raspcondo:~/webcam $ ssh -p 10000 mnewman@korat.myddns.rocks
ssh: connect to host korat.myddns.rocks port 10000: Network is unreachable

pi@raspcondo:~/webcam $ dig +short korat.myddns.rocks
51.75.215.63

But, if I use what I know is my real IP address, I can connect:

pi@raspcondo:~/webcam $ ssh -p 10000 mnewman@101.51.3.84
Last login: Thu Jul  8 12:12:58 2021
MrMuscle:~ mnewman$

How can I stop your update client from updating my domain using a proxy/VPN IP adderess?