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Port of the Week: Presenting Syncthing

Written by Solène, on 04 October 2023.
Tags: #syncthing #privacy #security #networking

Comments on Fediverse/Mastodon

1. Introduction §

Today's "port of the week" article is featuring Syncthing, a file synchronization software.

Syncthing official project website

Related blog posts:

Blog post about the complementary Relay server

Blog post about the complementary discovery server

2. Quick intro §

As stated earlier, Syncthing is a network daemon that synchronize files between computers/phones. Each Syncthing instance must know the other instance ID to trust them and find them over the network. The transfer are encrypted and efficient, the storage itself can be encrypted.

Some Syncthing vocabulary:

  • a folder: a local directory that is shared with a remote device,
  • a remote device: a remote computer running Syncthing, each of them have a unique ID and a user-defined name, you can choose which shared folders you want to synchronize with them
  • an item: this word appears when syncing two remotes, an item can be either a directory or a file that isn't synchronized yet
  • a discovery server: a server which helps remotes finding known remotes over the Internet, or in the worst case scenario, relays data from a remote to another if they can't communicate directly

3. Interesting features §

I gathered a list of interesting features that you may find interesting in Syncthing.

3.1. Security: authentication and encryption §

When you need to add a new remote, you need to add the remote's ID on a Syncthing and trust that one on the remote one. The ID is a human representation of the Syncthing instance certificate fingerprint. When you exchange ID, you are basically asked to review each certificate and allow each instance to trust the other.

All network transfers occurring between two Syncthing are encrypted using TLS, as the remote certificate can be checked, the incoming data integrity can be verified and authenticated.

Syncthing official documentation about security principles in the software

3.2. Relaying §

I guess this is Syncthing killer feature. Connecting two remotes is very easy and file transfer between them can bypass firewalls and NATs.

This works because the Syncthing offers a default discovery server which has two purposes:

  • if the two servers could potentially communicate to each other but are behind NATs, it does what we call "hole punching" to establish a connection between the two remotes and allow them to transfer directly from one to the other
  • if the two servers can't communicate to each other, the discovery server acts as a relay for the data

The file transfer is still encrypted, but having a third party server involved may rise privacy issues, and security risks if a vulnerability can be exploited.

My next blog post will show how to self-host your own Syncthing relay, for better privacy and even more complicated setups!

Note that the discovery server or the relaying can be disabled! You could also build a mesh VPN and run Syncthing on each node without using any relay or discovery server.

3.3. Built-in file versioning §

This may be my preferred feature in Syncthing!

On a given Syncthing instance, you can enable per shared folder a retention policy, aka file versioning in the interface.

Basically, if a file is modified / removed in the share by a remote, the local instance can keep a hidden copy for a while.

There are different versioning modes, from a simple "trash bin" style keeping the files for n days, or more elaborated policies like you could have in backup tools.

Syncthing official documentation about file versioning

3.4. Partial share synchronization §

For each share, it's possible to write an exclusion filter, this allows you to either discard sync changes for some pattern (like excluding vim swap files) or entire directories if you don't want to retrieve all the shared folder.

The filter works in both way, if you accept a remote, you could write a filter before starting the synchronization and remove some huge directories you may not want locally. But this could also allow preventing a directory to be sent to the remotes, like a temporary directory for instance.

This is a topic I covered with a very specific use case, only sync a single file in a directory.

Earlier blog post: Configure Syncthing to sync a single file

3.5. Encrypted remotes §

A pretty cool feature I found recently was the support for encrypted shared folders per remote. I'm using syncthing to keep my KeepassXC databases synchronized between my computers.

As I don't always have at least two of my computers turned ON at the same time, they can't always synchronize directly with each other, so I use a remote dedicated server as a buffer to hold the files, Syncthing encryption is activated for this remote, both my computers can exchange data with it, but on the server itself you can't get my KeepassXC databases.

This is also pretty cool as it doesn't leave any readable data on the storage drive if you use 3rd party systems.

Taking the opportunity here, KeepassXC has a cool feature that allows you to add a binary file as a key in addition to a password / FIDO key. If this binary file isn't part of the synchronized directory, even someone who could access your KeepassXC database and steal your password shouldn't be able to use it.

3.6. Data chunk based §

When Syncthing scans a directory, it will hash all the file into chunks and synchronize all these chunks to other remotes, this is basically how BitTorrent work too.

This may sound boring, but basically, this allows Syncthing to move or rename files on a remote instead of transferring the data again when you rename / move files in a local shared directory. Indeed, only the changed paths list is sent, and the chunks used in the files, as the files already exist on the remote, the data chunks don't have to be retrieved.

Note that this doesn't work for encrypted remotes as the chunks contain some path information, once encrypted, the same file with different paths will look as two different encrypted chunks.

3.7. Bandwidth control §

Syncthing GUI allows you to define inbound or outbound bandwidth limitation, either globally or per remote. If like me, you have a slow ADSL line with slow upload, you may want to limit the bandwidth used to send data to set the non-local remotes.

3.8. Support for all attributes synchronization §

This may sound more niche, but it's important for some users: Syncthing can synchronize file permissions, ownership or even extended attributes. This is not enabled by default as Syncthing requires elevated privileges (typically running as root) to make it work.

3.9. Runs everywhere §

Syncthing is a Go program, it's a small binary with no dependencies, it's quite portable and runs on Linux, all the BSD, Android, Windows, macOS etc... There is nothing worse than a synchronization utility that can't be installed on a specific computer...

4. Conclusion §

I really love this software, especially since I figured the file versioning and the encrypted remotes, now I don't fear conflicts or lost files anymore when syncing my files between computers.

My computers also use a local discovery server that allows my Qubes OS to be kept in sync together over the LAN.

5. Note for SystemD users §

When you install Syncthing on your system, you can enable the service as your user, this will make Syncthing start properly when you log in with your user:

systemctl enable --user syncthing.service

6. Note for OpenBSD users §

Syncthing has to listen for each file change, you will need to increase the maximum opened files limit for your user, and maybe the limit in the kernel using the according sysctl.

You can find more detailed information about using Syncthing on OpenBSD in the file /usr/local/share/doc/pkg-readmes/syncthing.