Ah, got you!

I do something similar using restic to encrypt, deduplicate, and backup my photos to backblaze every now and then. Out of curiosity, any particular reason you choose zip over something like restic?

It does some UDP fuckery to bypass NAT and firewalls

I wouldn’t be surprised if they use hole punching. It’s an old but effective technique which Skype famously used back in its heyday.