This code let’s me connect with a 2d camera.
I’m running Linux and I can read the messages from the TCP server on the camera.

But it never triggers :tcp_closed or :tcp_error which I’d expect to happen after unplugging the Ethernet cable (rj45).

When I do unplug the cable, the message stop coming in. After reconnecting the cable the messages come in again, without even mentioning the connection was temporarily severed.

defmodule TcpClient do
  use GenServer
  require Logger

  @con_opts [:binary, active: true, packet: :raw, send_timeout: 10000]
  @connect_timeout 2000
  @retry_interval 1000

  defmodule State do
    defstruct host: "localhost",
              port: 1234
  end

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts)
  end

  def init(opts) do
    state = opts_to_initial_state(opts)
    send(self(), :connect)
    {:ok, state}
  end

  def handle_info(:connect, state) do
    Logger.info("Connecting to #{state.host}:#{state.port}")

    case :gen_tcp.connect(state.host, state.port, @con_opts, @connect_timeout) do
      {:ok, _socket} ->
        Logger.info("Connected successfully to #{state.host}:#{state.port}")
        {:noreply, state}

      {:error, reason} ->
        Logger.warning(
          "Connection to #{state.host}:#{state.port} failed: #{inspect(reason)}. Will retry in #{@retry_interval}ms"
        )

        {:noreply, state, @retry_interval}
    end
  end

  def handle_info(:timeout, state) do
    send(self(), :connect)
    {:noreply, state}
  end

  def handle_info({:tcp_closed, _socket}, state) do
    Logger.info("Connection to #{state.host}:#{state.port} closed. Reconnecting...")
    send(self(), :connect)
    {:noreply, state}
  end

  def handle_info({:tcp_error, _socket, reason}, state) do
    Logger.info("Connection to #{state.host}:#{state.port} error: #{inspect(reason)}")
    send(self(), :connect)
    {:noreply, state}
  end

  def handle_info({:tcp, _socket, data}, state) do
    string =
      Regex.scan(~r/star;(\S+);stop/, data <> data, capture: :all_but_first)
      |> List.first()
      |> List.first()

    Logger.info("String: #{string}")
    {:noreply, state}
  end

  defp opts_to_initial_state(opts) do
    host = Keyword.get(opts, :host, "localhost") |> String.to_charlist()
    port = Keyword.fetch!(opts, :port)
    %State{host: host, port: port}
  end
end