There are dozens of free and commercial programs available for all sorts of platforms that can help you easily and conveniently sync your files from one device to another. But they all suffer from one big problem – your files are routed through their servers even if they are encrypted first on your local computer. This means that you have to show some level of trust to these developers or the servers they are using. In case of encrypted connections, you can trust them for most of your files but when it comes to top-secret information, you cannot trust someone running a server in their kitchen. For these highly sensitive files, you can use an open-source application called Syncthing.
Syncthing is an open-source utility for syncing your files across a number of devices without having to route your files through third-party servers. It supports Windows, Mac, Linux, Android, and many BSD variants. It comes without any native GUI but as soon as you launch it, it starts a local server and displays the server interface in the default web browser window.
You can now connect to this device by manually clicking on Add Device and entering the device ID for the remote device. If you are using Android smartphone, then you can just scan the QR code that can be displayed by selecting Action → Show ID. Once one or more devices are connected, you can start syncing files across the remote devices and the local device . All you have to do is add the default shared folders (the folders that will be synced) and click on the Resume button under these shared folders. An Up to Date status will be displayed for the synced devices and folders.
All of the files are transmitted over a highly encrypted TLS connection, none of the files are stored on any server except on your devices, and only devices with authenticated nodes are allowed to connect to your device. This makes Syncthing a very secure and fast file syncing software for all of your devices.
You can download Syncthing from https://syncthing.net/.