Andrei Andrei - 1 month ago 24
Python Question

MQTT Client not receiving messages

I'm using the following code from MQTT Paho project to subscribe to messages from my mqtt broker. I tested the connection using

mosquitto_sub
and I receive the messages there. However, when I run the following code it doesn't receive any messages and no output is printed. I checked the topic and host.

import paho.mqtt.client as mqtt

# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, rc):
print("Connected with result code "+str(rc))
# Subscribing in on_connect() means that if we lose the connection and
# reconnect then subscriptions will be renewed.
client.subscribe("test")

# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload))

client = mqtt.Client()

client.on_connect = on_connect
client.on_message = on_message

client.connect("localhost", 1883, 60)
client.loop_forever()


The following error is logged by the broker:

Invalid protocol "MQTT" in CONNECT from ::1.
Socket read error on client (null), disconnecting.


EDIT Thanks to @hardillb for pointing out the outdated MQTT version.

Everything worked after I did the following:


  1. sudo apt-get purge mosquitto

  2. sudo apt-add-repository ppa:mosquitto-dev/mosquitto-ppa

  3. sudo apt-get update

  4. sudo apt-get install mosquitto


Answer

This is most likely because you are using an old version of mosquitto and the python is expecting a newer build that supports MQTT 3.1.1

Try changing the code as shown:

import paho.mqtt.client as mqtt

# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, rc):
    print("Connected with result code "+str(rc))
    # Subscribing in on_connect() means that if we lose the connection and
    # reconnect then subscriptions will be renewed.
    client.subscribe("test")

# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))

# Change made HERE 
client = mqtt.Client(protocol=MQTTv31)

client.on_connect = on_connect
client.on_message = on_message

client.connect("localhost", 1883, 60)
client.loop_forever()

You should also upgrade your broker as soon as possible, that version is incredibly out of date and has a number of known issues, the current version is 1.4.10

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