eolas/zk/mosquitto_mqtt_client.md

---
tags:
  - Linux
  - Debian
  - mqtt
---

## Config

At `/etc/mosquitto/mosquitto.conf`.

Most important lines:

```
listener 1883
allow_anonymous true
```

This means `mosquitto` will run on port:1883 and that I am not using
authentication with requests to the broker.

### Websockets

In order to enable [websockets](./Web_sockets.md), must create a config file at
`/etc/mosquitto/mosquitto.conf/websockets.conf`:

```
# /etc/mosquitto/mosquitto.conf/websockets.conf`
listener 8083
protocol websockets
```

This is only relevant if you require access from outside of the local network.

## Basic usage

## Create a topic

```sh
mosquitto_sub -d -t test_topic
```

This then enters listening mode and takes control of terminal:

```
$ mosquitto_sub -d -t test_topic
Client (null) sending CONNECT
Client (null) received CONNACK (0)
Client (null) sending SUBSCRIBE (Mid: 1, Topic: test_topic, QoS: 0, Options: 0x00)
Client (null) received SUBACK
Subscribed (mid: 1): 0
```

## Publish to a topic

From my other machine I install `mosquitto` and:

```sh
mosquitto_pub --host 192.168.68.53 --port 1883 --topic test_topic --message 'hello world'
```

This registers on the broker device:

```
Client (null) received PUBLISH (d0, q0, r0, m0, 'test_topic', ... (11 bytes))
hello world
```

## Topics are inherently ephemeral

Once you exit from `mosquitto_sub` the topic ceases to exist. You can't then
relisten to the topic later as it hasn't persisted.

Topics are ephemeral routing labels, not persistent database entities. Think of
them more like radio frequencies. They only exist whilst they are transmitting.