NervesHubLink

The Device-side client library for NervesHub.

This is the 2.x version NervesHubLink. If you have been using NervesHub prior to around April, 2023 and have not updated to 2.0 or newer see the maint-v1 branch.

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Overview

NervesHub is an open-source IoT fleet management server that is built specifically for Nerves-based devices.

Devices connect to the server by joining a long-lived Phoenix channel (for HTTP polling, see nerves_hub_link_http). If a firmware update is available, NervesHub will provide a URL to the device and the device can update immediately or when convenient.

NervesHub does impose some requirements on devices and firmware that may require changes to your Nerves projects:

• Firmware images are cryptographically signed (both NervesHub and devices validate signatures)
• Devices are identified by a unique serial number

When using client certificate authentication, each device will also require its own SSL certificate for authentication with NervesHub.

These changes enable NervesHub to provide assurances that the firmware you intend to install on a set of devices make it to those devices unaltered.

Getting started

The following sections will walk you through updating your Nerves project to work with a NervesHub server.

Many of the steps below can be automated by NervesHub users to set up automatic firmware updates from CI and to manufacture large numbers of devices.

Adding NervesHubLink to your project

The first step is to add nerves_hub_link to your target dependencies in your project's mix.exs. For example:

  defp deps(target) do
    [
      {:nerves_runtime, "~> 0.13"},
      {:nerves_hub_link, "~> 2.2"},
      ...
    ] ++ system(target)
  end

Connecting your device to NervesHub

Shared secret device authentication

Important: Shared Secret authentication is a new feature under active development.

Shared Secrets use HMAC cryptography to generate an authentication token used during websocket connection.

This has been built with simple device registration in mind, an ideal fit for hobby projects or projects under early R&D.

You can generate a key and secret in your NervesHub Product settings which you then include in your NervesHubLink settings.

A full example config:

config :nerves_hub_link,
  host: "your.nerveshub.host",
  shared_secret: [
    product_key: "<product_key>",
    product_secret: "<product_secret>",
  ]

NervesKey (with cert based auth)

Important: This is recommended for production device fleets.

If your project is using NervesKey, you can tell NervesHubLink to read those certificates and key from the chip and assign the SSL options for you by adding it as a dependency:

def deps() do
  [
    {:nerves_key, "~> 1.2"}
  ]
end

This allows your config to be simplified to:

config :nerves_hub_link,
  host: "your.nerveshub.host"

NervesKey will default to using I2C bus 1 and the :primary certificate pair (:primary is one-time configurable and :aux may be updated). You can customize these options to use a different bus and certificate pair:

config :nerves_hub_link, :nerves_key,
  certificate_pair: :aux,
  i2c_bus: 0

Certificate device authentication

If you would like to use certificate device authentication, but you are not using NervesKey, you can tell NervesHubLink to read the certificate and key from the file system by using:

config :nerves_hub_link,
  host: "your.nerveshub.host",
  configurator: NervesHubLink.Configurator.LocalCertKey

By default the configurator will use a certificate found at /data/nerves_hub/cert.pem and a key found at /data/nerves_hub/key.pem. If these are stored somewhere differently then you can specify certfile and keyfile in the ssl config, e.g.:

config :nerves_hub_link,
  host: "your.nerveshub.host",
  configurator: NervesHubLink.Configurator.LocalCertKey,
  ssl: [
    certfile: "/path/to/certfile.pem",
    keyfile: "/path/to/keyfile.key"
  ]

For more information on how to generate device certificates, please read the "Initializing devices" section in the NervesHubCLI readme.

Additional notes

Any valid Erlang ssl socket option can go in the :ssl key. These options are passed to Mint by Slipstream, which NervesHubLink uses for websocket connections.

Runtime configuration

NervesHubLink also supports runtime configuration via the NervesHubLink.Configurator behavior. This is called during application startup to build the configuration that is to be used for the connection. When implementing the behavior, you'll receive the initial default config read in from the application environment and you can modify it however you need.

This is useful for cases like:

• selectively choosing which cert/key to use
• reading a certificate file stored on the device which isn't available during compilation

For example:

defmodule MyApp.Configurator do
  @behaviour NervesHubLink.Configurator

  @impl NervesHubLink.Configurator
  def build(config) do
    ssl = [certfile: "/root/ssl/cert.pem", keyfile: "/root/ssl/key.pem"]
    %{config | ssl: ssl}
  end
end

Then you specify which configurator NervesHubLink should use in config.exs:

config :nerves_hub_link, configurator: MyApp.Configurator

Advanced features

Conditionally applying updates

It's not always appropriate to apply a firmware update immediately. Custom logic can be added to the device by implementing the NervesHubLink.Client behaviour and telling the NervesHubLink OTP application about it.

Here's an example implementation:

defmodule MyApp.NervesHubLinkClient do
   @behaviour NervesHubLink.Client

   # May return:
   #  * `:apply` - apply the action immediately
   #  * `:ignore` - don't apply the action, don't ask again.
   #  * `{:reschedule, timeout_in_milliseconds}` - call this function again later.

   @impl NervesHubLink.Client
   def update_available(data) do
    if SomeInternalAPI.is_now_a_good_time_to_update?(data) do
      :apply
    else
      {:reschedule, 60_000}
    end
   end
end

To have NervesHubLink invoke it, update your config.exs as follows:

config :nerves_hub_link, client: MyApp.NervesHubLinkClient

Reporting update progress

See the previous section for implementing a client behaviour.

defmodule MyApp.NervesHubLinkClient do
  @behaviour NervesHubLink.Client
  #  argument can be:
  #   {:ok, non_neg_integer(), String.t()}
  #   {:warning, non_neg_integer(), String.t()}
  #   {:error, non_neg_integer(), String.t()}
  #   {:progress, 0..100}
  def handle_fwup_message({:ok, _, _}) do
    Logger.error("Firmware update complete")
    :ok
  end

  def handle_fwup_message({:warning, code, message}) do
    Logger.error("Warning while applying firmware update (#{code)}): #{message}")
    :ok
  end

  def handle_fwup_message({:error, _, message}) do
    Logger.error("Error while applying firmware update #(#{code}): {message}")
    :ok
  end

  def handle_fwup_message({:progress, progress}) when rem(progress, 10) do
    Logger.info("Update progress: #{progress}%")
    :ok
  end

  def handle_fwup_message({:progress, _}) do
    :ok
  end
end

Enabling remote IEx access

It's possible to remotely log into your device via the NervesHub web interface. This feature is disabled by default. To enable, add the following to your config.exs:

config :nerves_hub_link, remote_iex: true

The remote IEx process is started on the first data request from NervesHub and is terminated after 5 minutes of inactivity. You can adjust this by setting :remote_iex_timeout value in seconds in your config.exs:

config :nerves_hub_link, remote_iex_timeout: 900 # 15 minutes

You may also need additional permissions on NervesHub to see the device and to use the remote IEx feature.

Alarms

This application can set and clear the following alarms:

• NervesHubLink.Disconnected
  • set: An issue is preventing a connection to NervesHub or one just hasn't been made yet
  • clear: Currently connected to NervesHub
• NervesHubLink.UpdateInProgress
  • set: A new firmware update is being downloaded or applied
  • clear: No updates are happening

CA Certificates

The CA certificates installed on the device are used by default.

If you include the CAStore in your project, then that will be selected and used.

Otherwise you can configure nerves_hub_link to use custom CA certificates, which is useful if you are running your own NervesHub instance with self signed SSL certificates. Use the :ca_store option to specify a module with a ca_certs/0 function that returns a list of DER encoded certificates:

config :nerves_hub_link, ca_store: MyModule

Or if you have the certificates in DER format, you can also explicitly set them in the :ssl option:

my_der_list = [<<213, 34, 234, 53, 83, 8, 2, ...>>]
config :nerves_hub_link, ssl: [cacerts: my_der_list]

Verifying network availability

NervesHubLink will attempt to verify that the network is available before initiating the first connection attempt. This is done by checking if the NervesHub host address (config.host) can be resolved. If the network isn't available then the check will be run again in 2 seconds.

You can disable this behaviour with the following config:

config :nerves_hub_link, connect_wait_for_network: false

Disable NervesHubLink during testing

To disable NervesHubLink connecting to NervesHub when testing, you can add:

config :nerves_hub_link, connect: false

to your config/test.exs

Debugging errors

TLS client errors

If you see the following in your logs:

14:26:06.926 [info]  ['TLS', 32, 'client', 58, 32, 73, 110, 32, 115, 116, 97, 116, 101, 32, 'cipher', 32, 'received SERVER ALERT: Fatal - Unknown CA', 10]

This probably indicates that the signing certificate hasn't been uploaded to NervesHub so the device can't be authenticated. Double check that you ran:

mix nerves_hub.ca_certificate register my-signer.cert

Another possibility is that the device wasn't provisioned with the certificate that's on NervesHub.

See also NervesHubWeb: Potential SSL Issues

Internal API Versions

NervesHubLink.Configurator includes two internal versions for NervesHub to determine what features are available on the device socket.

device_api_version

• 1.0.0 - Updating firmware, status updates, reboot device
• 2.0.0 - Identify a device, archives
• 2.1.0 - Run scripts on a device separate from the console, sync firmware keys and archive keys

console_version

• 1.0.0 - Remote IEx console
• 2.0.0 - Send and receive files from a device