# Docker Container for Syncthing

Use the Dockerfile in this repo, or pull the `syncthing/syncthing` image
from Docker Hub.

Use the `/var/syncthing` volume to have the synchronized files available on the
host. You can add more folders and map them as you prefer.

Note that Syncthing runs as UID 1000 and GID 1000 by default. These may be
altered with the `PUID` and `PGID` environment variables. In addition
the name of the Syncthing instance can be optionally defined by using
`--hostname=syncthing` parameter.

To grant Syncthing additional capabilities without running as root, use the
`PCAP` environment variable with the same syntax as that for `setcap(8)`.
For example, `PCAP=cap_chown,cap_fowner+ep`.

To set a different umask value, use the `UMASK` environment variable. For
example `UMASK=002`.

## Example Usage

**Docker cli**
```
$ docker pull syncthing/syncthing
$ docker run -p 8384:8384 -p 22000:22000/tcp -p 22000:22000/udp -p 21027:21027/udp \
    -v /wherever/st-sync:/var/syncthing \
    --hostname=my-syncthing \
    syncthing/syncthing:latest
```

**Docker compose**
```yml
---
version: "3"
services:
  syncthing:
    image: syncthing/syncthing
    container_name: syncthing
    hostname: my-syncthing
    environment:
      - PUID=1000
      - PGID=1000
    volumes:
      - /wherever/st-sync:/var/syncthing
    ports:
      - 8384:8384 # Web UI
      - 22000:22000/tcp # TCP file transfers
      - 22000:22000/udp # QUIC file transfers
      - 21027:21027/udp # Receive local discovery broadcasts
    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
```

## Discovery

Note that Docker's default network mode prevents local IP addresses from
being discovered, as Syncthing is only able to see the internal IP of the
container on the `172.17.0.0/16` subnet. This will result in poor transfer rates
if local device addresses are not manually configured.

It is therefore advisable to use the [host network mode](https://docs.docker.com/network/host/) instead:

**Docker cli**
```
$ docker pull syncthing/syncthing
$ docker run --network=host \
    -v /wherever/st-sync:/var/syncthing \
    syncthing/syncthing:latest
```

**Docker compose**
```yml
---
version: "3"
services:
  syncthing:
    image: syncthing/syncthing
    container_name: syncthing
    hostname: my-syncthing
    environment:
      - PUID=1000
      - PGID=1000
    volumes:
      - /wherever/st-sync:/var/syncthing
    network_mode: host
    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
```

Be aware that syncthing alone is now in control of what interfaces and ports it
listens on. You can edit the syncthing configuration to change the defaults if
there are conflicts.

## GUI Security

By default Syncthing inside the Docker image listens on 0.0.0.0:8384 to
allow GUI connections via the Docker proxy. This is set by the
`STGUIADDRESS` environment variable in the Dockerfile, as it differs from
what Syncthing would otherwise use by default. This means you should set up
authentication in the GUI, like for any other externally reachable Syncthing
instance. If you do not require the GUI, or you use host networking, you can
unset the `STGUIADDRESS` variable to have Syncthing fall back to listening
on 127.0.0.1:

```
$ docker pull syncthing/syncthing
$ docker run -e STGUIADDRESS= \
    -v /wherever/st-sync:/var/syncthing \
    syncthing/syncthing:latest
```

With the environment variable unset Syncthing will follow what is set in the
configuration file / GUI settings dialog.