This daemon automatically propagates the current brightness level from a laptop screen to attached monitors via DDC. It doesn't support any configuration and has no way of knowing the brightness curves of your various monitors, so I make no guarantees. However, it works great for my laptop/monitor setup.
If you're using Arch Linux, you should install it from the AUR via the backlight-sync-git package.
Otherwise, you can manually install it by running make followed by sudo make install. If you plan to use the built-in backlight-syncd.service unmodified and your /dev/i2c-* devices are not readable/writable by an i2c group, you'll need to run sudo make install-udev-rules then reboot.
Run systemctl enable --now backlight-syncd.service to enable and start the service.
If you want to run the service without systemd, you'll need to make sure to run it as a user/group with read/write access to /dev/i2c-*. It needs no other permissions.
This project is licensed under the GPL-v3, copyright Steven Allen 2024-2025.