Syncthing: Sync Files Between Devices – Without the Cloud

Dropbox without Dropbox: Syncthing syncs folders directly between your devices – encrypted, without the files ever sitting on someone else’s server. On an always-reachable VPS your server becomes a reliable sync node that mediates even when your laptop and phone are never online at the same time.

What are we building?

An always-on Syncthing node with Syncthing v2.1.2 behind Traefik. By the end you reach the web interface encrypted under your domain, have secured it with a password and can pair your devices via their device IDs so that selected folders sync automatically. Unlike Nextcloud, Syncthing isn’t central storage but a peer-to-peer sync: every device holds a full copy, and the server is just a particularly reliable participant.

Prerequisites

• A server with Traefik running and Docker Compose
• A subdomain pointing at the server – YOUR_DOMAIN below
• The ability to open port 22000 (TCP+UDP) in the firewall – that’s where the actual sync runs
• Syncthing on at least one other device (desktop, or the Android app)

Step by step

Step 1: Create the directories – and set the permissions right away

Syncthing stores its configuration and the synced data in two folders. The container runs as a normal user (UID 1000), so the folders must belong to that user – this is the most common startup trap:

mkdir -p /opt/syncthing/config /opt/syncthing/data
chown -R 1000:1000 /opt/syncthing/config /opt/syncthing/data
cd /opt/syncthing

Step 2: The Compose file

Replace YOUR_DOMAIN. The important thing is the difference between the web interface and the sync traffic:

services:
  syncthing:
    image: syncthing/syncthing:2.1.5
    restart: unless-stopped
    hostname: koch-server
    environment:
      PUID: '1000'
      PGID: '1000'
    volumes:
      - ./config:/var/syncthing/config
      - ./data:/var/syncthing/Sync
    ports:
      - "22000:22000/tcp"   # sync protocol (direct, not via Traefik)
      - "22000:22000/udp"   # sync over QUIC
      - "21027:21027/udp"   # local device discovery
    networks: [proxy]
    labels:
      - "traefik.enable=true"
      - "traefik.http.routers.syncthing.rule=Host(`YOUR_DOMAIN`)"
      - "traefik.http.routers.syncthing.entrypoints=websecure"
      - "traefik.http.routers.syncthing.tls.certresolver=le"
      - "traefik.http.services.syncthing.loadbalancer.server.port=8384"

networks:
  proxy:
    external: true

Only the web interface (port 8384) runs through Traefik – that’s what the labels handle. The actual sync traffic (port 22000) is its own protocol, not HTTP, so it’s published directly via ports:, not routed through Traefik. This separation is exactly what Understanding Docker networks explains.

Step 3: Open the firewall and start

Open the sync port in the firewall (otherwise the devices won’t find each other):

ufw allow 22000/tcp
ufw allow 22000/udp

Then start:

docker compose up -d

Once the status is healthy, the interface responds with valid HTTPS:

curl -sI https://YOUR_DOMAIN/ | head -1

HTTP/2 200

Step 4: The web interface and the crucial security step

Open https://YOUR_DOMAIN/. The interface is in English by default; you see your device (“This Device”) with its device ID, version and status:

On a fresh install Syncthing warns at the top with a red banner that no GUI password is set yet – and that’s serious: without a password, anyone who reaches the interface can access your files. So under Actions → Settings → GUI immediately set a username and a strong password. After that Syncthing asks for a login on every access – on top of Traefik’s HTTPS.

Step 5: Pair a second device

Syncthing connects devices via their device ID – a long, unique key. Pairing is mutual: each device has to know the other. Click Add Remote Device:

You find the other device’s ID there under Actions → Show ID (as text and QR code). Enter it here, give it a name and save. On the other device you repeat this with your server’s ID. Once both sides know each other, the device shows as “connected”.

Step 6: Share a folder

Now you define what gets synced. Via Add Folder you choose a path (on the server e.g. /var/syncthing/Sync/documents, which corresponds to the mounted ./data folder) and give it a folder ID. Under the Sharing tab you tick the devices to sync the folder with. On the other device you confirm the incoming share – from then on Syncthing keeps both sides identical automatically.

When things go wrong

Maintenance & backups

• Updates. Occasionally bump the image tag (syncthing/syncthing:2.1.5) to the current version and docker compose up -d. Syncthing stays compatible within a major version; when jumping to a new major (like v2 here) read the release notes first. Your normal update process handles the rest.
• Backup is mandatory – sync is not a backup. This is the most important point: Syncthing mirrors changes, including accidental deletion and ransomware encryption. So back up the sync folder additionally with a real, versioned backup that the sync can’t reach – e.g. Restic on the server. The config folder (device IDs, folder shares) belongs in the backup too.
• Honest effort. Once set up, Syncthing runs with very little maintenance. Keep an eye on disk space (df -h) when syncing large folders with versioning, and prune old .stversions folders occasionally.