Arduino does not connect to MQTT local broker

Question

I have an Arduino Mega with Ethernet connected to a router by cable. Also I have a PC connected to the router (by Wifi) where also the local MQTT broker is installed (mosquitto). Althought the Arduino Ethernet connection seems ok, the MQTT connection does not: I get the return code -2

This is the full code, there are several functions but the MQTT init is the one that fails:

#include <SPI.h>
#include <Ethernet.h>
#include <PubSubClient.h> // Librería para MQTT
byte mac[] = { 0xA8, 0x61, 0x0A, 0xAE, 0x3A, 0xC4 };
byte ip[] = { 192, 168, 1, 100 }; 
byte gateway[] = { 192, 168, 1, 2 }; 
byte subnet[] = { 255, 255, 255, 0 };
EthernetClient ethClient;
PubSubClient mqttClient(ethClient);
const char *MQTT_BROKER_ADRESS = "192.168.1.101";
const uint16_t MQTT_PORT = 1883;
const char *MQTT_CLIENT_NAME = "mqttx_7c0a0ed3";
void SuscribeMqtt()
{
 mqttClient.subscribe("hello/world");
}
void OnMqttReceived(char *topic, byte *payload, unsigned int length)
{
 Serial.print("Received on ");
 Serial.print(topic);
 Serial.print(": ");
 String content = "";
 for (size_t i = 0; i < length; i++)
 {
 content.concat((char)payload[i]);
 }
 Serial.print(content);
 Serial.println();
}
void InitMqtt()
{
 mqttClient.setServer(MQTT_BROKER_ADRESS, MQTT_PORT);
 mqttClient.setCallback(OnMqttReceived);
}
void ConnectMqtt()
{
 Serial.print("Starting MQTT connection...");
 if (mqttClient.connect(MQTT_CLIENT_NAME))
 {
 SuscribeMqtt();
 mqttClient.publish("connected","hello/world");
 }
 else
 {
 Serial.print("Failed MQTT connection, rc=");
 Serial.print(mqttClient.state());
 Serial.println(" try again in 5 seconds");
 delay(5000);
 }
}
void HandleMqtt()
{
 if (!mqttClient.connected())
 {
 ConnectMqtt();
 }
 mqttClient.loop();
}
void CheckLinkStatus() {
 
 auto link = Ethernet.linkStatus();
 
 Serial.print("Link status: ");
 
 switch (link) {
 case Unknown:
 Serial.println("Unknown");
 break;
 case LinkON:
 Serial.println("ON");
 break;
 case LinkOFF:
 Serial.println("OFF");
 break;
 }
 delay(1000);
}
void setup() {
 
 // You can use Ethernet.init(pin) to configure the CS pin
 //Ethernet.init(10); // Most Arduino shields
 Serial.begin(9600);
 Ethernet.begin(mac, ip, gateway, subnet);
 Serial.print("server is at ");
 Serial.println(Ethernet.localIP());
 delay(1500);
 
 InitMqtt();
 
}
// put your main code here, to run repeatedly:
void loop() {
 CheckLinkStatus();
 HandleMqtt();
 
}

I have done ping from the PC IP (192.168.1.101) to the Arduino IP (192.168.1.100) and it is ok. What could be the issue?.

• StarCat: What is in the logs of the MQTT broker?
• bardulia: @StarCat how do I check the log file? I am not able to find it.
• bardulia: @StarCat maybe I can't find it because I am executing mosquitto as a service? also I am not able to modify conf file
• bardulia: @timemage I have installed MQTT Explorer on the PC, it is very useful to check the traffic. Thanks!
• bardulia: @StarCat I was finally able to run mosquitto correctly, also I am writing now the log into a text file. In my answer I have put all the code. Thanks.

Answer 1

The issue was coming from mosquitto local broker, which was being blocked by windows 10 firewall. When I executed mosquitto with the correct permissions on its folder (read and write for "all users") and with correct mosquitto configuration file, the following window popped up and I clicked on to allow both private and public connections:

n

I also use the following batch file to load mosquitto and it works all the time:

cd "C:\Program Files\mosquitto"
del mosquitto.log
mosquitto -v -c mosquitto.conf

This is the content of my mosquitto configuration file:

listener 1883
allow_anonymous true
log_type error
log_type warning
log_type notice
log_type information
connection_messages true
log_timestamp true
log_dest file C:\Program Files\mosquitto\mosquitto.log

Now I am able to use MQTT from Arduino connected to a router.

Answer 2

With a bit of research you can see that return code two is:

2 Connection refused: client identifier not allowed by server.

My guess is that you forgot to add the login details in the code, or the code fails because the client ID is not a c-string. Try something like this (replace with appropriate values):

if (client.connect(clientId.c_str(), mqttUsername, mqttPassword)){

• bardulia: It was a firewall isse when executing mosquitto, but thanks anyway!