How does BitTorrent synchronization work

"BitTorrent Sync 1.4" - Dropbox alternative in the test

The tool "Bit Torrent Sync" (Btsync) takes a slightly different approach. It comes from the creators of the bit torrent protocol µTP, the peer-2-peer architecture that has fallen into disrepute and is often used for illegal file sharing. But neither the tool nor its manufacturer can do anything to prevent this technology from being used illegally. Many legal tasks, such as the distribution of open source Linux distributions, are also done. With Bittorrent, two or more systems connect directly via the Internet and synchronize their databases. So-called trackers are used for this. These are central servers on the Internet on which the peer-2-peer clients register in order to find other clients. However, the trackers only act as intermediaries for all participants. The remote stations connect directly to each other for the actual data transmission. Btsync uses the P2P protocol to synchronize private data over the Internet. The user first releases a folder on a system. To do this, it generates a 20-byte hash code as a unique identification key for this folder. If the user assigns this key to a different folder on another computer, Btsync begins to synchronize these two folders. Almost any number of remote stations can be included in the synchronization.

Btsync uses a small web server for the GUI and synchronizes shared folders with high performance. Btsync does not need firewall rules as there are no incoming connections. The connection between two peers is established via UDP, NAT traversal and UPNP mapping. In the same LAN segment, the clients find and connect to each other via broadcast announcements. Btsync does not recognize user / password authentication. Whoever has the key can synchronize. Optionally, additional keys can be generated for shared directories that only allow read access or that allow access to a folder for a limited period of time. The actual data synchronization is AES encrypted. The practical thing about Btsync is that the user can share any number of folders with different keys and synchronize their contents independently of one another with other clients. The manufacturer provides the Btsync tool free of charge, but the software is not based on open source code.

Install Btsync

As with all P2P tools, Btsync does not have a dedicated server. Bittorrent offers the Btsync client free of charge for a large number of platforms. In addition to the usual suspicious versions for Windows, Mac and Linux, there are apps for iOS and Android, as well as ARM Linux variants for the Rasberry Pi or ARM-based NAS systems (e.g. Qnap, Synology). The Linux version doesn't even need an installer or root rights.

Btsync clients

In the German version, the btsync Android client compares its data with the connected »colleagues«. The Btsync client is delivered as a binary package that can be started directly. Instead of its own user interface, the Btsync client integrates a small web server on port 8.888. This allows the user to manage the shared folders in the browser and get an overview of the current synchronization processes.

For the test, we set up Btsync on various Linux desktops, Windows 7 and a Linux server at an Internet host. There are also Btsync services on an Android mobile phone and a Synology NAS. After the basic configuration of several folders, the tool starts immediately. The synchronization performance is impressive. Even large files such as DVD images can be sent over the line in just a few minutes. The transfer rates go up to ten Mbytes / s in the local LAN and around 80 percent of the Internet line capacity in the desktop systems.

To prevent accidental deletions, Btsync creates a hidden directory called »SyncArchive« in every synchronized folder. If the user deletes a file in a shared folder, the peers connected via Btsync store this file in the sync archive. They remain there for 14 days until they are finally deleted, so that accidental deletions can be undone. However, Btsync does not offer versioning like Owncloud. The Android client only works in the basic configuration if the tablet or mobile phone is connected to a WLAN. For example, the photos from the cell phone camera can be automatically saved to other systems.

Conclusion: Fast with an unexplained future

Btsync is the fastest tool in the test and is very easy to set up and use. However, its range of functions is limited to the synchronization of the data. There is no web or WebDAV access to a central server via which individual files can be uploaded or downloaded. The tool is suitable for all users who primarily want to quickly synchronize file folders.

On the negative side, it should be noted that Btsync is proprietary code and you have to rely on the manufacturer's information for the security features. In the end, there is also no guarantee in the long term that users will be able to receive the software for free.


Download:Sync, client

Price: Open source


  • UPnP port mapping
  • Peer exchange
  • AES-256 encryption

+ easy setup and configuration
+ quick file synchronization
+ encrypted data transmission
+ no foreign server

- No web / WebDAC access to the central server
- no web interface