Syncthing

Syncthing is a free, open-source continuous file synchronization tool for anyone who wants folders kept identical across PCs and servers without a cloud provider in the middle. Devices talk to each other directly over encrypted connections, so it suits sysadmins syncing scripts and tools between workstations, small offices replicating a share to a second site, and homelabs.

There is no central server and no account: every device has a cryptographic device ID, and you decide which devices share which folders. It runs as a background process with a local web interface on Windows, macOS, Linux and BSD, and in Docker.

Syncthing at a glance

What it does

• Keeps folders in sync continuously between any number of devices, detecting changes with a filesystem watcher plus periodic rescans.
• Encrypts all traffic with TLS and authenticates devices by certificate-based device IDs.
• Finds devices on the LAN by local discovery and over the internet through global discovery servers; falls back to relays when a direct connection is impossible.
• Folder types for different roles: Send & Receive, Send Only (master copy), Receive Only (replica) and Receive Encrypted (data stored encrypted on an untrusted device).
• File versioning (trash can, simple, staggered or an external script) keeps old copies of changed or deleted files.
• Ignore patterns in a .stignore file to skip caches, temp files and build output.
• Since 2.1: grouping of devices and folders in the GUI, HTTP/HTTPS proxy support, and an option to turn off block indexing for very large folders.

How sysadmins use it

Pair two machines and share a folder

1. Start Syncthing on both machines and open the web GUI at http://127.0.0.1:8384.
2. On machine A, open Actions > Show ID and copy the device ID.
3. On machine B, click Add Remote Device, paste the ID and save. Accept the prompt that appears on machine A.
4. On A, click Add Folder, set the path, and tick machine B on the Sharing tab. Accept the folder on B and choose where to store it.

Run it headless on a Linux server

1. Install from the project’s APT repository (apt.syncthing.net) or your distribution, then enable it as a system service for a dedicated user:

sudo systemctl enable --now syncthing@svc-sync.service

2. The GUI listens only on localhost. Reach it through an SSH tunnel instead of exposing it:

ssh -L 8385:127.0.0.1:8384 admin@fileserver01
# then open http://127.0.0.1:8385 on your PC

3. Set a GUI user name and password under Actions > Settings > GUI anyway.

Replicate a branch-office share one way

1. On the branch server, set the folder type to Send Only.
2. On the head-office server, set the same folder to Receive Only and enable Staggered File Versioning so deletions are recoverable.
3. Add a .stignore in the folder root:

// skip Office lock files and temp files
~$*
*.tmp
Thumbs.db

Run it in Docker

docker run -d --name syncthing --restart unless-stopped 
  -p 8384:8384 -p 22000:22000/tcp -p 22000:22000/udp -p 21027:21027/udp 
  -v /srv/syncthing:/var/syncthing 
  syncthing/syncthing:2

The :2 tag follows the current 2.x release; pin an exact version such as 2.1.5 if you need predictable upgrades.

Install and first run

Windows: the official download is a ZIP with syncthing.exe; it has no installer or tray icon of its own. Most Windows users install one of the community wrappers listed on the official site, such as Syncthing Windows Setup (an installer that also sets up automatic start) or SyncTrayzor v2 (tray icon and launcher). Without one, start it at logon with Task Scheduler.

macOS: the syncthing-macos app bundle adds a menu-bar icon; the plain binary also works.

Linux: APT repository, distribution packages or the binary; run as a systemd user unit (systemctl --user enable --now syncthing.service) or the syncthing@user system unit shown above.

Ports to allow through firewalls: 22000/TCP and 22000/UDP (sync, QUIC) and 21027/UDP (local discovery). The GUI uses 8384/TCP on localhost. On Ubuntu, sudo ufw allow syncthing; on firewalld, sudo firewall-cmd --zone=public --add-service=syncthing --permanent followed by --reload.

Upgrading from 1.x: version 2.0 moved the database from LevelDB to SQLite. The one-time migration can take a long time on large setups, old single-dash command-line options such as -home no longer work (use --home), and deleted-file records are now forgotten after six months by default.

Limitations

• It is sync, not backup: deletions and ransomware-encrypted files replicate to every device unless versioning or Receive Only folders protect you.
• Both ends must be online at the same time for changes to move; there is no cloud copy in between.
• No official mobile apps: the official Android app was retired after its December 2024 release. Android users rely on community forks, and iOS on third-party apps.
• Very large folders with millions of files need RAM and disk for the database and take time to scan.
• Simultaneous edits on two devices produce .sync-conflict-... copies that you resolve by hand.

Syncthing vs alternatives

FreeFileSync compares and syncs folders on demand or on a schedule, with a preview before changes, which is better for backups to a USB disk or NAS; Syncthing syncs continuously between live devices. Resilio Sync is also peer-to-peer but proprietary. Nextcloud and Seafile give you a central server with web access, sharing links and user accounts, at the cost of running and securing that server. For one-way copies to a server over SSH, rsync is simpler.

FAQ

Is Syncthing free?

Yes. Syncthing is free and open source under MPL-2.0, with no paid tier or account; it is funded by donations.

Is Syncthing safe?

Traffic is TLS-encrypted and only devices you approve can connect. Set a GUI password, keep the GUI on localhost, and use Receive Encrypted folders for devices you do not trust with your data.

Is there Syncthing for Android or iOS?

Not from the project any more. On Android, the community “Syncthing-Fork” continues the app; on iOS, Möbius Sync is a third-party app.

How do I install Syncthing on Windows?

Unpack the official ZIP and run syncthing.exe, or use Syncthing Windows Setup or SyncTrayzor for an installer, autostart and tray icon. Allow the firewall prompt for private networks.

What ports does Syncthing use?

22000 TCP and UDP for sync, 21027 UDP for local discovery, and 8384 TCP for the web GUI on localhost.

Syncthing vs Nextcloud: which should I use?

Syncthing if you only need folders kept in sync between your own devices. Nextcloud if you need web access, sharing with other users or a server-side copy.

How do I fix “Out of Sync” items?

Click the out-of-sync count on the folder to see the failing files; the usual causes are permissions, files locked by another program, or paths too long or invalid on the receiving OS. A Rescan after fixing the cause clears the list.

Last checked against official sources: 30 September 2026 (developer website: syncthing.net). Versions and licence terms change — confirm on the developer's site before deploying in production.