Comment by 1vuio0pswjnm7

IIRC Dropbox was originally created using librsync

Dropbox has modified it

https://github.com/dropbox/librsync

This is why I prefer open source software. I can modify it

One person can use librsync to create a Dropbox company. Another person can use librsync for noncommercial purposes, e.g., to transfer and sync their own files

Either way, it's librsync