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Syncthing

Syncthing is open-source continuous file synchronisation. Devices keep folders in sync directly with one another, without the data ever landing in a central cloud store; the relays that broker connections only ever see encrypted traffic. The honest distinction is: sync is not a backup.

Anyone who wants to keep files current between laptop, server and phone usually reaches for a third-party cloud service. That is exactly the moment the data leaves the user's control. Syncthing turns it around: it synchronises continuously between the organisation's own devices, peer-to-peer, with no waypoint at a provider. That makes it a building block of sovereign infrastructure, not a substitute for a thought-through backup and restore strategy. This page describes how the mechanism works, what it protects, and where its limit lies.

The principle

Syncthing connects two or more devices into a private synchronisation cluster. Each device holds a cryptographic identity, the device ID, derived directly from its TLS certificate. Two devices exchange data only when each has explicitly accepted the other's device ID. There is no account at a provider, no sign-in, no central data store.

  • Peer-to-peer. Changed files flow directly from one device to another. The traffic is always encrypted (TLS), not optionally.
  • No cloud server. Syncthing does not upload the data to a foreign cloud; it passes it between the devices in the cluster.
  • Helper services without insight. So that devices can find each other behind changing IP addresses or firewalls, there are optional discovery and relay servers. They only broker the connection. Relays do not see the relayed data, because the encryption holds between the devices and is not opened up at the relay.

Syncthing is open source under the MPL 2.0. The code is open, the mechanism is auditable, and no vendor lock-in to a sync service arises.

How a change travels

A changed file runs through the same sequence, whether two or ten devices are involved:

flowchart TD
    A["Change on device A<br/>file created, edited, deleted"] --> B["Index exchange<br/>metadata only between devices"]
    B --> C["Connection<br/>direct or via relay, TLS"]
    C --> D["Transfer<br/>changed blocks only"]
    D --> E["Device B takes over<br/>change is applied"]
    E --> F["Optional: versioning<br/>old copy in .stversions"]

The decisive step is the last. By default Syncthing keeps only the current state. If a file is deleted on one device, the deletion propagates to all the others. This is exactly where it differs from a backup.

Sync is not a backup

This is the honest note the project itself puts up front: Syncthing is not a backup tool, because every change, including every deletion and every accidental overwrite, is propagated to all devices. A deleted folder is, after synchronisation, deleted everywhere. Sync protects against the failure of a device, not against accidental deletion.

Syncthing does have an optional versioning feature, enabled per folder and off by default. It keeps old copies of deleted or changed files in a .stversions folder, with strategies ranging from Trash Can through Simple and Staggered to External. That softens the risk but does not replace a separate, versioned backup in a place the synchronisation cannot reach. The clean distinction between mirroring, sync and a real backup is handled by the backup and restore strategy; tools such as rsync (mirroring) and Duplicati (encrypted backup) solve the other half of the task.

Devices not fully trusted

If a location outside the organisation's control is to hold a copy, for example a rented server, Syncthing offers untrusted (encrypted) devices. When a folder is shared with a password, the remote device receives the data only in encrypted form and cannot read it. It stores encrypted blocks and passes them on without any insight into the content. This is a configurable option, not default behaviour: for trusted devices, the password field stays empty. File sizes and the folder name remain visible, the file content does not.

Where Syncthing fits and where it does not

  • Fits. Keeping files current across several devices, without a third-party cloud service. A sovereign knowledge store such as Obsidian, built on local Markdown files, can be kept in sync across devices this way.
  • Does not fit as a backup. Because of deletion propagation, Syncthing does not replace a backup. That is the job of a separate, versioned copy.
  • Does not fit as a team cloud. True concurrent multi-user editing with locking and conflict resolution is not the goal; parallel changes to the same file produce conflict copies.

Whether sovereign file synchronisation is run in-house or bought as a service is a question of digital sovereignty: for the technical enforcement of data control, from end-to-end encryption to keeping content unreadable on devices not fully under the organisation's control, Syncthing provides the building blocks.

References

Related topics

  • Disaster Recovery, the backup and restore view that sync does not cover.
  • Digital Sovereignty, control over data and infrastructure without foreign servers.
  • Obsidian, a local flat-file knowledge store that benefits from cross-device sync.

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