Syncthing with Docker Compose

What Syncthing needs

Only figures Syncthing's own documentation publishes, each linked to the page it is on. Where the project gives no number, this page does not invent one.

On top of this, Caddy (the reverse proxy in the file) is a single small container.

docker-compose.yml

This is exactly what the builder writes for Syncthing alone on example.org: the official definition with its published ports removed and Caddy added in front.

# docker-compose.yml built at dankhost.com for: Syncthing
# Each app block is its official compose definition. Only the names were prefixed with
# the app slug (services, containers, volumes, bind folders, .env variables) so any set of
# apps can share one file; other differences are listed under "Changed:" in each block.
# Only Caddy publishes ports. Secrets live in .env next to this file.

services:
  # ---- Caddy: the one reverse proxy, HTTPS certificates from Let's Encrypt ----
  # Source: https://caddyserver.com/docs/running (checked 2026-09-23)
  # Source: https://github.com/dani-garcia/vaultwarden/wiki/Using-Docker-Compose (checked 2026-09-23)
  # Caddyfile goes in ./caddy/Caddyfile. Fixed address 172.30.0.2 so apps can trust it exactly.
  caddy:
    image: "caddy:2"
    container_name: "caddy"
    restart: "unless-stopped"
    ports:
      - "80:80"
      - "443:443"
      - "443:443/udp"
    volumes:
      - "./caddy:/etc/caddy"
      - "caddy-data:/data"
      - "caddy-config:/config"
    networks:
      proxy:
        ipv4_address: "172.30.0.2"

  # ---- Syncthing (sync.example.org) ----
  # Source: https://github.com/syncthing/syncthing/blob/main/README-Docker.md (checked 2026-09-23)
  # Source: https://docs.syncthing.net/users/reverseproxy.html (checked 2026-09-23)
  # Source: https://docs.syncthing.net/users/relaying.html (checked 2026-09-23)
  # Source: https://docs.syncthing.net/users/firewall.html (checked 2026-09-23)
  # Changed: Removed the documented 'network_mode: host' and the empty STGUIADDRESS, so the web GUI listens on 0.0.0.0:8384 inside the container (the image's default, which the README says is for running without host networking) and Caddy reaches it on the proxy network.
  #          The documented /wherever/st-sync becomes ./syncthing/st-sync; the obsolete 'version: "3"' line is dropped.
  syncthing:
    image: "syncthing/syncthing"
    container_name: "syncthing"
    hostname: "my-syncthing"
    environment:
      PUID: "1000"
      PGID: "1000"
    volumes:
      - "./syncthing/st-sync:/var/syncthing"
    restart: "unless-stopped"
    healthcheck:
      test: "curl -fkLsS -m 2 127.0.0.1:8384/rest/noauth/health | grep -o --color=never OK || exit 1"
      interval: "1m"
      timeout: "10s"
      retries: 3
    networks:
      proxy: {}

networks:
  # Pinned so the trusted-proxy address given to each app (172.30.0.2) is exact.
  # If "docker compose up" reports "Pool overlaps", change 172.30.0 here, in Caddy's
  # ipv4_address and in every trusted-proxy value to a free 172.x.0 range.
  proxy:
    ipam:
      config:
        - subnet: "172.30.0.0/24"
          ip_range: "172.30.0.128/25"
          gateway: "172.30.0.1"

volumes:
  caddy-data:
  caddy-config:

Caddyfile save as caddy/Caddyfile

Caddy gets and renews the HTTPS certificate for sync.example.org by itself once the DNS record points at your server and ports 80 and 443 reach it.

# Caddyfile built at dankhost.com. Save it as ./caddy/Caddyfile next to docker-compose.yml.
# Caddy gets and renews a Let's Encrypt certificate for each name below by itself,
# once the DNS records point at this server and ports 80 and 443 reach it.

# Syncthing
sync.example.org {
	reverse_proxy syncthing:8384 {
		header_up Host {upstream_hostport}
	}
}

.env

Syncthing needs no secret here; the builder adds nothing you have to fill in by hand.

# .env for Syncthing, from dankhost.com/app/syncthing/. Nothing here is secret.
# Keep this file private (chmod 600).
# None of the chosen apps needs a variable here.

Changes from the official file

Besides prefixing every service, container, volume, folder and variable with syncthing (so any set of apps can share one file), these are all the differences from what the project documents:

• Removed the documented 'network_mode: host' and the empty STGUIADDRESS, so the web GUI listens on 0.0.0.0:8384 inside the container (the image's default, which the README says is for running without host networking) and Caddy reaches it on the proxy network.
• The documented /wherever/st-sync becomes ./syncthing/st-sync; the obsolete 'version: "3"' line is dropped.

Notes for Syncthing behind the proxy

• Set a GUI user name and password at once (Actions > Settings > GUI). Until you do, anyone who finds the address controls this Syncthing and every folder it can reach.
• No sync port is opened: the file publishes only Caddy's ports, so Syncthing's port 22000 (TCP and UDP) cannot be reached from outside. Syncing still works: this server connects out to your other devices, and when neither side can reach the other directly the traffic goes through Syncthing's public relays, which are on by default, end-to-end encrypted and, the docs say, much slower than a direct connection.
• Opening 22000 is faster but is not done by this file. To do it, add '22000:22000/tcp' and '22000:22000/udp' under the syncthing service yourself and forward both on your router.
• Local discovery does not work from Docker's bridge network (the README explains why it recommends host networking), so add this server on your other devices by its device ID (Actions > Show ID).
• The Caddyfile's 'header_up Host {upstream_hostport}' line is from Syncthing's Caddy v2 example; it lets the GUI accept requests that arrive through the proxy.

Back it up

Syncthing publishes no backup procedure, and its FAQ says it is not a backup tool: changes and deletions spread to every device. The part only this server has is the config directory, whose cert.pem and key.pem are the device's identity (its device ID). The copy below follows Docker's generic method.

Below is the BACKUP.md the builder writes next to the compose file above: which of its folders and volumes hold data, what to copy with which container stopped, and a restore check. Pick more apps in the builder and it covers them all in one file.

# BACKUP.md built at dankhost.com for: Syncthing

What to copy from the docker-compose.yml built with this file, the database dump each project documents (rewritten for the service names in that file), what to copy with which container stopped, and how to check the result. Run every command in the folder that holds docker-compose.yml.

The folders are owned by the containers' users: put sudo in front of tar if it says "Permission denied". Named volumes live under /var/lib/docker/volumes, not next to the compose file, so they are copied through a throwaway ubuntu container (Docker's documented --volumes-from method). tar makes a full copy each run; for large folders an incremental tool (restic, borg, rsync) saves time and space.

Never run "docker compose down -v" on this file: it deletes the named volumes of every app in it.

## Before you start

    BACKUP=/mnt/backup/selfhost   # change this: a disk or machine other than this server's system disk
    mkdir -p "$BACKUP"

## Every time: the three stack files

    tar czf "$BACKUP/stack-files.tar.gz" docker-compose.yml .env caddy

.env holds the database passwords and secret keys the apps were set up with; keep this archive as private as the data.

## Caddy (the reverse proxy)

Caddy's docs: 'The data directory must not be treated as a cache.' It holds the TLS certificates and their private keys; without it Caddy has to ask Let's Encrypt for every certificate again.

- caddy-data (named volume, /data in caddy)
  BACK UP: TLS certificates, private keys, OCSP staples.
- caddy-config (named volume, /config in caddy)
  SKIP: the last active configuration, kept for 'caddy run --resume', which this setup does not use: it starts from the Caddyfile.
- ./caddy (/etc/caddy in caddy)
  IN stack-files.tar.gz: the Caddyfile, copied with the stack files above.

Copy (Caddy keeps serving while you copy):

    docker run --rm --volumes-from caddy -v "$BACKUP":/backup ubuntu tar cvf /backup/caddy-data.tar /data

Restore check:

    tar tf "$BACKUP/caddy-data.tar" > /dev/null && echo OK
    docker run --rm --volumes-from caddy -v "$BACKUP":/backup ubuntu bash -c "cd /data && tar xvf /backup/caddy-data.tar --strip 1"

Sources:
- Caddy docs: Conventions, Data directory: https://caddyserver.com/docs/conventions#data-directory (checked 2026-09-24)
- Docker docs: Volumes, Back up, restore, or migrate data volumes: https://docs.docker.com/engine/storage/volumes/#back-up-restore-or-migrate-data-volumes (checked 2026-09-24)

## Syncthing

Syncthing publishes no backup procedure, and its FAQ says it is not a backup tool: changes and deletions spread to every device. The part only this server has is the config directory, whose cert.pem and key.pem are the device's identity (its device ID). The copy below follows Docker's generic method.

What this compose mounts:
- ./syncthing/st-sync (/var/syncthing in syncthing)
  BACK UP: the synced folders plus config/ (Syncthing's home in this image, STHOMEDIR=/var/syncthing/config): config.xml, the cert.pem and key.pem device identity, and the index database.

1. Stop syncthing. Stopping it keeps the index database from changing mid-copy.

    docker compose stop syncthing

2. Copy while syncthing is stopped:

    tar czf "$BACKUP/syncthing-st-sync.tar.gz" ./syncthing/st-sync

3. Start it again:

    docker compose start syncthing

Restore check:

    tar tzf "$BACKUP/syncthing-st-sync.tar.gz" > /dev/null && echo OK

Put a copy back (with syncthing stopped):

    tar xzf "$BACKUP/syncthing-st-sync.tar.gz"   # recreates ./syncthing/st-sync; move the old folder aside first

Stop it, put ./syncthing/st-sync back, start it. With the same key.pem and cert.pem it comes back with the same device ID, so the other devices know it.

Sources:
- Syncthing FAQ: Is Syncthing my ideal backup application?: https://docs.syncthing.net/users/faq.html (checked 2026-09-24)
- Syncthing docs: Configuration file locations: https://docs.syncthing.net/users/config.html (checked 2026-09-24)
- Syncthing Dockerfile (STHOMEDIR=/var/syncthing/config): https://github.com/syncthing/syncthing/blob/main/Dockerfile (checked 2026-09-24)
- Docker docs: Volumes, Back up, restore, or migrate data volumes: https://docs.docker.com/engine/storage/volumes/#back-up-restore-or-migrate-data-volumes (checked 2026-09-24)

Sources

Every page below was fetched on the date shown.

• syncthing.net front page checked 2026-09-23 (what it does)
• Syncthing: README-Docker.md (Docker compose, Discovery, GUI Security) checked 2026-09-23 (compose definition and proxy settings)
• Syncthing docs: Reverse Proxy Setup (Caddy v2) checked 2026-09-23 (compose definition and proxy settings)
• Syncthing docs: Relaying checked 2026-09-23 (compose definition and proxy settings)
• Syncthing docs: Firewall Setup (port 22000 TCP and UDP) checked 2026-09-23 (compose definition and proxy settings)
• Caddy docs: Keep Caddy running (Docker Compose section) checked 2026-09-23 (Caddy)
• Vaultwarden wiki: Caddy with HTTP challenge (uses the caddy:2 image) checked 2026-09-23 (Caddy)
• Syncthing FAQ: Is Syncthing my ideal backup application? checked 2026-09-24 (backup plan)
• Syncthing docs: Configuration file locations checked 2026-09-24 (backup plan)
• Syncthing Dockerfile (STHOMEDIR=/var/syncthing/config) checked 2026-09-24 (backup plan)
• Docker docs: Volumes, Back up, restore, or migrate data volumes checked 2026-09-24 (backup plan)
• Caddy docs: Conventions, Data directory checked 2026-09-24 (backup plan)

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