Syncthing is a decentralized continuous file synchronization software with many security features, like end-to-end encryption, device authentication, permission management and more. Both the protocol specification and the Syncthing software are free and open source.
With Syncthing files can be easily kept in sync across multiple devices, e.g. one desktop PC, one notebook and several mobile devices, *without* relying on third-party infrastructure (optional). File syncing can be limited to a local Wifi network, avoiding sending files to the outside world altogether.

Installation on Ubuntu

For Ubuntu and other Debian-like Linux distros the package repository for Syncthing should be added to the list of apt sources.

# Add the release PGP keys:
sudo curl -o /usr/share/keyrings/syncthing-archive-keyring.gpg https://syncthing.net/release-key.gpg

# Add the "stable" channel to your APT sources:
echo "deb [signed-by=/usr/share/keyrings/syncthing-archive-keyring.gpg] https://apt.syncthing.net/ syncthing stable" | sudo tee /etc/apt/sources.list.d/syncthing.list

# Update and install syncthing:
sudo apt-get update
sudo apt-get install syncthing

This way new versions of Syncthing become available automatically every time apt update is run and updating is as simple as apt upgrade synthing.

You can also run Syncthing manually whenever needed by executing syncthing serve.

Configuration of shared folders is basically the same across all OS, in Ubuntu the web interface at localhost:8384 is used.

Installation on Windows 10

For Windows devices install Syncthing using the official msi-installer.
When compiling Syncthing from it’s sources, make sure to verify the signature of the source code package.

To auto-run Syncthing when Windows boots hit Win+R and type shell:startup, hit enter and wait for the file explorer to open.
Create a shortcut to the location of syncthing.exe (wherever you installed/extracted it). Right-click on the shortcut, select properties, and append -no-console and -no-browser to the target of the shortcut.

You can also run Syncthing manually whenever needed by executing syncthing.exe.

Configuration of shared folders is basically the same across all OS, in Windows the web interface at localhost:8384 is used.

Installation on Android

For Android devices install the Syncthing app from Google Playstore or from F-Droid. Start the Syncthing app, set the appropriate permissions and get going. Configuration of shared folders is basically the same across all OS, in Android the app is used instead of a web interface.

Configuration

When working with a GUI the Syncthing configuration interface can be accessed by connection to localhost:8384 in a browser.

On a headless system, like a server or NAS, the configuration of Syncthing requires a few more steps.

Normally connecting to the server with ssh and launching syncthing would be sufficient.

ssh user@server

user@server> syncthing serve

This time however, connect to the server via ssh and forward the port on which syncthing exposes it’s web interface to your local machine.

# Connect to hostname and forward localhost:8384 from hostname to 8385 on local machine.
# This port forwarding allows to connect to hostname's syncthing web interface in a browser on the local machine using port 8385.
ssh -L 8385:localhost:8384 user@hostname

It should be possible now to connect to hostname’s Syncthing web interface on port 8385 on the local machine. Note that I used a different port to avoid a port collision in case syncthing is already running on the local machine.

Alternatively, and not-recommended, the Syncthing web interface can be exposed completely to the local network. This is achieved by editing the config file under ~/.config/syncthing and setting the IP address to 0.0.0.0 and optionally changing the port as well. Note that the firewall must allow the selected port to pass through, so additional steps may be needed in this case.

<gui enabled="true" tls="false" debugging="false">
    <address>0.0.0.0:8384</address>
...

Anyway, use the web interface to set up Syncthing initially. Add at least one folder for syncing and allow at least one other device to access it (if a device has been set up).

If that worked, enable Syncthing to run as a service, which ensures that it will start up on boot and execute continuously in the background.

# Replace "user" with the user as which syncthing should run.
sudo systemctl enable syncthing@user.service
sudo systemctl start syncthing@user.service

That’s is from the “server” side. Note that Syncthing is a peer-to-peer (P2P) protocol, so there is not actual server. What I call the server is just another node.

Device Setup

The rest of the setup is pretty straight forward.

On each device, tell Syncthing which folders should be synced between the current device and the other devices. The mapping of shared folders to local folders is quite flexible. Each shared folder gets an unique identifier which must be the same across all devices. Where the content of a shared folder is stored can be controlled for each device individually. Also if syncing for a folder should be done at all or if it should be paused.

Another useful feature for shared folders is the “Send only” and “Receive only” option. These allow setting up a shared folder to serve as a simple backup system, e.g. have a notebook send files to a desktop PC, but not the other way around.

Take your time and test the different options to get a feel for it.

Note that the initial sync can take a long time, e.g. when syncing a folder with pictures from a mobile phone to a NAS.

That’s it.


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