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Communicate outside the container

In many situations, the code running in your container will need some way to communicate with outside services and devices, whether this means the host OS, the device supervisor, the network, or internal and external storage. Below, you'll find different methods for external communication, as well as some tips and tricks to keep in mind.

Host OS

Environment variables

Inside your running container, you'll have access to a number of BALENA_ namespaced environment variables, which provide information from the system outside the container:

Note: On all balenaOS versions of the OS, both RESIN_ and BALENA_ variables will be injected into the container to maintain backwards compatibility.

Variable Description
BALENA_DEVICE_UUID The unique identification number for the device. This is used to identify it on balena
BALENA_APP_ID ID number of the balena fleet the device is associated.
BALENA_APP_NAME The name of the balena fleet the device is associated with.
BALENA_DEVICE_NAME_AT_INIT The name of the device on first initialization.
BALENA_DEVICE_TYPE The type of device the fleet is running on.
BALENA The BALENA=1 variable can be used by your software to detect that it is running on a balena device.
BALENA_SUPERVISOR_API_KEY Authentication key for the supervisor API. This makes sure requests to the supervisor are only coming from containers on the device. See the Supervisor API reference for detailed usage. For multicontainer the service needs the io.balena.features.supervisor-api label set.
BALENA_SUPERVISOR_ADDRESS The network address of the supervisor API. Default: For multicontainer the service needs the io.balena.features.supervisor-api label set.
BALENA_SUPERVISOR_HOST The IP address of the supervisor API. Default: For multicontainer the service needs the io.resin.features.supervisor-api set
BALENA_SUPERVISOR_PORT The network port number for the supervisor API. Default: 48484. For multicontainer the service needs the io.balena.features.supervisor-api label set.
BALENA_API_KEY API key which can be used to authenticate requests to the balena backend. Can be used with the SDKs on the device. WARNING This API key gives the code permissions to affect the device's metadata in the balena API; refer to our security documentation for more details. For multicontainer the service needs the io.balena.features.balena-api label set.
BALENA_HOST_OS_VERSION The version of the host OS.
BALENA_DEVICE_RESTART This is an internal mechanism for restarting containers and can be ignored as it's not very useful to app code. Example: 1.13.0

Here's an example from a Raspberry Pi 3:

root@raspberrypi3-cc723d7:/# printenv | grep BALENA

D-Bus communication with host OS

In some cases it's necessary to communicate with the host OS systemd to perform actions on the host. To do this you can use dbus. In order to ensure that you are communicating to the host OS systemd and not the systemd in your container it is important to set DBUS_SYSTEM_BUS_ADDRESS for all D-Bus communication. The setting of that environment variable is different for older and newer devices (based on the balena supervisor version), choose the line that is correct for your device's OS version (can be found in your device dashboard):

Note: In multicontainer fleets, the io.balena.features.dbus label must be applied for each service that requires access to the D-Bus. If you have devices with a supervisor version lower than 7.22.0, you should use io.resin.features labeling as that will ensure backward compatibility.

# for balena supervisor versions 1.7.0 and newer (both balenaOS 1.x and 2.x) use this version:
# for balena supervisor before 1.7.0 use this version:

Below you can find a couple of examples. All of them requires either prepending the command with the above DBUS_SYSTEM_BUS_ADDRESS=... or setting the variable for all commands by running export DBUS_SYSTEM_BUS_ADDRESS=... with the correct environment variable value from above.

In multicontainer fleets, you can also set DBUS_SYSTEM_BUS_ADDRESS=unix:path=/host/run/dbus/system_bus_socket in the environment section of your service in the docker-compose.yml file:

version: '2'
      - 'DBUS_SYSTEM_BUS_ADDRESS=unix:path=/host/run/dbus/system_bus_socket'
      io.balena.features.dbus: '1'

Note: Please be aware that setting DBUS_SYSTEM_BUS_ADDRESS as a service or environment variable and enabling systemd at the same time might introduce unexpected side effects. Systemd might start to interact with the host system instead of the container. These interactions can potentially cause balenaOS devices to disconnect from balenaCloud or even fall offline. Hence, users are advised to prefer prepending the command with the variable definition.

Note: To use the dbus-send command in the example you will need to install the dbus package in your Dockerfile if you are using the Debian image, or check under what name does your chosen operating system supply the dbus-send executable.

Change the Device hostname

Changing the device hostname via a dbus-send method invocation of org.freedesktop.hostname1.SetHostname is no longer possible, due to the fact that this would attempt to write to /etc/hostname, which on the host OS is stored in the read-only root partition. To change the device hostname, use the balena supervisor API

Rebooting the Device

DBUS_SYSTEM_BUS_ADDRESS=unix:path=/host/run/dbus/system_bus_socket \
  dbus-send \
  --system \
  --print-reply \
  --dest=org.freedesktop.systemd1 \
  /org/freedesktop/systemd1 \

Stopping a systemd service

At times, you may wish to stop a running service on the host OS, such as bluetooth.service, in order to run your own instance of bluez containerized.

DBUS_SYSTEM_BUS_ADDRESS=unix:path=/host/run/dbus/system_bus_socket \
  dbus-send \
  --system \
  --print-reply \
  --dest=org.freedesktop.systemd1 \
  /org/freedesktop/systemd1 \
  org.freedesktop.systemd1.Manager.StopUnit \
  string:bluetooth.service string:replace

Checking if device time is NTP synchronized

DBUS_SYSTEM_BUS_ADDRESS=unix:path=/host/run/dbus/system_bus_socket \
  dbus-send \
  --system \
  --print-reply \
  --reply-timeout=2000 \
  --type=method_call \
  --dest=org.freedesktop.timedate1 \
  /org/freedesktop/timedate1  \
  org.freedesktop.DBus.Properties.GetAll \

The reply would look like this:

method return time=1474008856.507103 sender=:1.12 -> destination=:1.11 serial=4 reply_serial=2
   array [
      dict entry(
         string "Timezone"
         variant             string "UTC"
      dict entry(
         string "LocalRTC"
         variant             boolean false
      dict entry(
         string "CanNTP"
         variant             boolean true
      dict entry(
         string "NTP"
         variant             boolean true
      dict entry(
         string "NTPSynchronized"
         variant             boolean true
      dict entry(
         string "TimeUSec"
         variant             uint64 1474008856505839
      dict entry(
         string "RTCTimeUSec"
         variant             uint64 1474008857000000

The entry NTPSynchronized shows true, so the device is NTP synchronized. (The key NTP only shows whether the device is using the systemd service systemd-timesyncd; starting from balenaOS 2.13.1, the chrony service is used for time management.)

Note: For additional D-Bus examples see the balenaOS masterclass

Blacklisting kernel modules won't work

Since the /etc/modules you see in your container belongs to the container's filesystem and is not the same as /etc/modules in the host OS, adding kernel modules to the modules blacklist in the container will have no effect. So in order to remove a module, you need to explicitly do a rmmod.


Note: In multicontainer fleets, the io.balena.features.supervisor-api label must be applied for each service that requires access to the Supervisor API. If you have devices with a supervisor version lower than 7.22.0, you should use io.resin.features labeling as that will ensure backward compatibility

Reboot from Inside the Container

You may notice that if you issue a reboot, halt, or shutdown your container either gets into a weird zombie state or doesn't do anything. The reason for this is that these commands do not propagate down to the host OS system. If you need to issue a reboot from your container you should use the supervisor API as shown:

curl -X POST --header "Content-Type:application/json" \

Read more about the supervisor API

Note: BALENA_SUPERVISOR_API_KEY and BALENA_SUPERVISOR_ADDRESS should already be in your environment by default for single containers, but for multicontainer devices the service needs the io.resin.features.supervisor-api set . You will also need curl installed in your container.

Alternatively, it is possible to reboot the device via the D-Bus interface as described above.

Writing to logs on the Dashboard

Anything written to stdout and stderr should appear on the device's dashboard logs. Have a look at some of our example projects on GitHub to get an idea of how to do this.


BalenaEngine supports host and bridge network modes:

  • Host mode allows a service to use all host network interfaces.
  • Bridge mode uses a user-defined bridge network interface, to which service containers are connected.

Any service that uses host networking does not have to define ports for traffic ingress, and a service can bind to all interfaces of the host. Single container releases always use host networking.

In contrast to host networking, bridge networks isolate all services from the host, requiring services to explicitly define open ports to allow traffic from the host to be passed to them (all outgoing traffic is permitted). By default, multicontainer releases use a bridge network.

Single container

Single container releases always use host networking, allowing them to bind to any of the host's network interfaces. If security and sandboxing are required for either privilege level or to ensure self-contained networking, then a multicontainer release should be created, even if only a single service is required.


Multicontainer releases use a user-defined bridge network by default. No ports are exposed to the host and must be explicitly enabled through the ports keyword. Services on the same bridge network have access to all other services' ports.

The following sample multicontainer docker-compose.yml file allows incoming traffic on port 80 to the proxy service, but the frontend and data services are isolated from the host and only accessible via the bridge network, which all services are connected to.

version: '2'
    build: ./frontend
      - "80"
    build: ./haproxy
      - frontend
      - data
      - "80:80"
    build: ./data
      - "8080"

Note: Exposing ports via the expose keyword is optional and a way of documenting which ports are used, but does not map or open any ports. By default, services on the same bridge network have access to all other services' ports.

For multicontainer releases, setting the service network_mode to host in docker-compose.yml allows the container to share the same network namespace as the host OS.

Balena docker-compose.yml files support the creation of multiple bridge networks allowing you to compartmentalize further, so that some services exist in only one defined network, whereas others may be able to communicate in many. The aliases keyword for providing alias names for services (including FQDNs) and IPAM bridge networks are also supported.

Note: For more information on networking with balena, see the balena services masterclass.

Public device URLS

Balena currently exposes port 80 for web forwarding. To enable web forwarding on a specific device, navigate to the device's actions tab on the balenaCloud dashboard and select the Enable a public URL for this device button. For more information about device URLs see the Device Management Page

Enable public device URL

Running a server listening on port 80 with public device URL enabled will allow you to serve content from the device to the world. Here is an example of an express.js server which will serve to the devices URL.

var express = require('express')
var app = express()

app.get('/', function (req, res) {
  res.send('Hello World!')

var server = app.listen(80, function () {

  var host = server.address().address
  var port = server.address().port

  console.log('Example app listening at http://%s:%s', host, port)


Using DNS resolvers in your container

In the balena host OS dnsmasq is used to manage DNS since balenaOS 1.1.2. This means that if you have dnsmasq or other DNS resolvers such as bind9 running in your container, it can potentially cause problems because they usually try to bind to, which interferes with the host dnsmasq. To get around this, you need to add bind-interfaces to your dnsmasq configuration in your container or make sure your server only binds to external IPs, and there shouldn't be conflicts anymore.


Persistent Storage

If you have data or configurations that you would like to persist through application and host OS updates, you have the option to keep them in persistent storage. Persistent storage is a good place to write system logs and other application data that should remain untouched even as your code changes.

Before balenaOS v2.12.0

On devices running OS versions before 2.12.0, the /data folder in the container is automatically linked to a directory on the host OS and guaranteed to persist across updates. The contents of the /data folder can be accessed via the host OS at /mnt/data/resin-data/<APP ID>.

The /data folder is not synced between devices in your fleet. In addition, the folder is unique to a specific fleet, so if you transfer your device to a new fleet the /data folder from the previous fleet will not be accessible in the container. It will, however, still be available via the host OS and if the device is moved back to the original fleet.

Note that the /data folder is not mounted when your project is building on our build servers, so you can't access it from your Dockerfile. The /data volume only exists when the container is running on the deployed devices.

balenaOS v2.12.0 and above

Beginning with balenaOS v2.12.0, persistent storage is handled through named volumes. The behavior is much the same as persistent storage on older host OS versions. In fact, for single-container fleets, the default docker-compose.yml sets up a resin-data named volume that links to a /data directory in the container. The only difference between this and earlier versions is that accessing this data via the host OS is done at /var/lib/docker/volumes/<APP ID>_resin-data/_data, rather than the /mnt/data/resin-data/<APP ID> location used with earlier host OS versions.

Named volumes can be given arbitrary names and can be linked to a directory in one or more containers. As long as every release includes a docker-compose.yml and the volume name does not change, the data in the volume will persist between updates.

When using named volumes, note that:

  • If a device is moved to a new fleet, the old /data folder will be automatically purged.
  • During the build process, data added to a container directory that is configured to link to a named volume will be copied to the volume the first time it's created on the device.

Using a Supervisor with a version >= v10.0.0

Since balena-supervisor v10.0.0, volumes are no longer automatically removed from disk when references to them are removed from a fleet's docker-compose file. This means that it's no longer possible for data to be lost due to the accidental rename of a volume.

If you change volume names regularly, your device will now continue to retain all previous volumes including their contents. To avoid this the supervisor API now provides an endpoint to cleanup unreferenced volumes. Additionally, it is possible to perform this action from the dashboard via the Purge Data action, found on the Actions tab for a device.

Note: volumes will continue to be removed automatically when moving a device between fleets.

Transfer large files

If you have large files you would like your containers to have access to, you can transfer them from your computer directly to your device's SD card. First insert the SD card in your computer and find the resin-data partition. Then look for the folder associated with your application, which will either be at /resin-data/<APP ID> or /docker/volumes/<APP ID>_<VOLUME NAME>/_<CONTAINER DIRECTORY>, depending on your host OS version. Note that these directories will only exist after your application has been started at least once.

Temporary directories

Note that the /tmp and /var/tmp directories in a container are not true tmpfs volumes by default, and they are treated like any other ephemeral container layers.

As a result, you can expect that data in these directories will persist over a device reboot, but will not persist when a services restart is triggered.

If you would like these directories to act more like tmpfs volumes and write to volatile memory, you can use tmpfs mounts.

    image: foo/bar
      - /tmp
      - /var/tmp

Mounting external storage media

Since the release of multicontainer on the balena platform we no longer recommend the use of an initsystem in the container. This affects the way we deal with external storage since previously we relied on systemd/OpenRC and /etc/fstab.

The recommended way for mounting external storage media (SD cards, USB sticks, external drives, etc) into a container is now through the use of mount. Here we include a set of recommendations that will help you get started.

balenaOS kernel support

Before you start it's a good idea to check if the balenaOS kernel you are running was compiled with support for the filesystem you want to use. To do so, you can run this command on the host which will produce a list of supported filesystems: cat /proc/filesystems.

If your filesystem is not supported you can contact us through our forums and we will be glad to help.

Preparing the container

In order to be able to detect external media dynamically you will need to run the container in privileged mode and enable udevd on it. This can be easily done if you are using balena base images by:

  • Adding privileged: true to your container's service definition on the docker-compose.yml file.
  • Adding ENV UDEV=on to your container's Dockerfile.
  • Running the entrypoint as the root user in the container namespace. This is often the default but can be set in your container's Dockerfile with USER root in the target build stage, or in your docker-compose.yml file with user: root.

This will ensure that the host propagates udev events into the container, enabling us to manipulate the device from within it.

General tips for external media

Devices can be selected in many ways, for example by its device name (/dev entry), label, or UUID. From a practical point of view, we recommend using labels (LABEL=... entries). Labels can easily be made the same across multiple cards or thumb drives, while you can still identify each device by their UUID. Also, /dev entries are not static on some platforms, and their value depends on which order the system brings up the devices. Device names or UUIDs are a good choice when you can easily identify or predict their values, for example within the context of a udev rule.

Note: You can get a list of device names, labels and filesystem types by running lsblk -f (both on the host or container).


To mount an external drive you can use Linux's mount command. Again, any selection method is supported:

mount -t <fstype> -o rw <device-name> <mount-point>
mount -t <fstype> -o rw -L <device-label> <mount-point>
mount -t <fstype> -o rw -U <device-uuid> <mount-point>

Note: The mount point folder needs to exist for the mount to be successful.

For more information about the mount command see the mount man page.


To unmount an external drive you can use Linux's umount command:

umount <mount-point>

For more information about the umount command see the umount man page.

Automounting/unmounting with udev rules

The previous sections show how to manually mount or unmount external media. You probably want to automate this and have your media automatically mount/unmount when you plug it or unplug it. Fortunately this can be easily achieved by using udev rules.

First we create a rules file usb.rules:

ACTION=="add", SUBSYSTEM=="block", ENV{DEVTYPE}=="partition", RUN+="/bin/sh -c '/usr/src/scripts/'"
ACTION=="remove", SUBSYSTEM=="block", ENV{DEVTYPE}=="partition", RUN+="/bin/sh -c '/usr/src/scripts/'"

These rules will trigger every time we plug or unplug a block partition device and run the scripts we provide (/usr/src/ or /usr/src/

Copy both the rules and scripts to your container:

COPY usb.rules /etc/udev/rules.d/usb.rules
COPY scripts /usr/src/scripts

Finally we need to write the and scripts. These scripts will use mount and umount commands in the same way we described on the Mounting and Unmounting sections above.

You can find a fully working example of automounting/unmounting devices with udev rules in this project.

Sharing mounted devices across containers

Note that currently it's not possible to share a mounted device across multiple containers. This is a feature that we are currently working on. This documentation will be updated once we add support for this feature.