Hi Sara
I am having difficulty connecting my esp32 to raspi mosquitto server.  This is my first time using Raspi but I have manged to get Mosquitto MQTT running on it using PuTTy and Mosquitto seems to be running ok.  I am using PlatformIO.  The ESP32 is connecting ok to the wifi but I get this error when attempting the Mqtt connection:-
Attempting MQTT connection…[E][WiFiClient.cpp:258] connect(): socket error on fd 56, errno: 104, “Connection reset by peer”
Here is my code:-

//System to monitor temperature humidity and moisture
//Send results to NodeRed dashboard via MQTT

#include <Arduino.h>
#include <DHT.h>
#include <WiFi.h>
#include <OneWire.h>
#include <Wire.h>
#include <DallasTemperature.h>

const char* ssid     = “ljhglhlhlhb”; // edited to hide SSID
const char* password = “lhjbljhvbljhvljh”;  //edited to hide password

//MQTT Setup Start
#include <PubSubClient.h>
#define mqtt_server “192.168.1.33”  // ip address of Pi
WiFiClient espClient;
PubSubClient client(espClient);
#define mqttTemp1 “Kiln/tempAmbient”
#define mqttHum1 “Kiln/HumidityIn”
#define mqttTemp2 “Kiln/tempunderkiln”
#define mqttHum2 “Kiln/HuminidtyExhaust”
#define mqttTemp3 “Kiln/tempGrain”
#define mqttTempOff “Kiln/AirOffTemp”
#define mqttGrainMCTop “Kiln/grainMCTop”
#define mqttGrainMCBtm “Kiln/grainMCBtm”
#define MQTT_SOCKET_TIMEOUT 1
//MQTT Setup End

const int oneWireBus = 18;

// Setup a oneWire instance to communicate with any OneWire devices
OneWire oneWire(oneWireBus);

// Pass our oneWire reference to Dallas Temperature sensor
DallasTemperature sensors(&oneWire);

uint8_t sensor1[8] = {0x28, 0x56, 0xD6, 0x95, 0xF0, 0x01, 0x3C, 0x74};//dallas DS temp sensor
uint8_t sensor2[8] = {0x28, 0x40, 0xC2, 0x95, 0xF0, 0x01, 0x3C, 0xFE};//dallas DS temp sensor
uint8_t sensor3[8] = {0x28, 0x69, 0x1D, 0x95, 0xF0, 0x01, 0x3C, 0xEF};//dallas DS temp sensor
#define MoistureProbeBtm 35 //GPIO 35
#define MoistureProbeTop 36 //GP1O 36

float AmbTemp1, Intakehum1, underKilntemp2, exhaustHum2, grainTemp, grainMCBtm, grainMCtop;

void setup() {
  Serial.begin(115200);
  Serial.println();
 
  // begin Wifi connect
  Serial.print(“Connecting to “);
  Serial.println(ssid);
  WiFi.mode(WIFI_STA);
  WiFi.disconnect();
  delay(2000);
  WiFi.begin(ssid, password);
 
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(“.”);
  }

  Serial.println(“”);
  Serial.println(“WiFi connected”);  
  Serial.println(“IP address: “);
  Serial.println(WiFi.localIP());
  //end Wifi connect

  client.setServer(mqtt_server, 1883);
 
   //Start Temp Sensors DS18B20
  sensors.begin();
  if(sensors.isParasitePowerMode()) Serial.println(“DS18B20 in Parasitic power mode  – ON”);
  else Serial.println(“DS18B20 in Parasitic power mode  – OFF”);
}

void getValues()
  {
  AmbTemp1 = sensors.getTempC(sensor1); // Gets the values of the temperature
  underKilntemp2 = sensors.getTempC(sensor2);
  grainTemp = sensors.getTempC(sensor3);
  Intakehum1 = 56;//set fixed test value until we humidity sensor
  exhaustHum2 = 87; // set fixed test value until we humidity sensor
  grainMCtop = analogRead(MoistureProbeTop);
  grainMCBtm = analogRead(MoistureProbeBtm);
   
  Serial.print(“AmbientKilnTemp = “);
  Serial.print(AmbTemp1);
  Serial.println(” *C”);
  Serial.print(“underKilntemp2 = “);
  Serial.print(underKilntemp2);
  Serial.println(” *C”);
  Serial.print(“Grain Temp = “);
  Serial.print(grainTemp);
  Serial.println(” *C”);
  Serial.print(“Grain Moisture Content = “);
  Serial.print(grainMCtop);
  Serial.println(” %”);
  Serial.print(grainMCBtm);
  Serial.println(” %”);

  /*Serial.print(“BME 2 Pressure = “);
  Serial.print(bme2.readPressure() / 100.0F);
  Serial.println(” hPa”);

  Serial.print(“BME 2 Humidity = “);
  Serial.print(hum2);
  Serial.println(” %”);
  */
  }

void reconnect() {
  // Loop until we’re reconnected
    String clientId = “ESPClient-“;
    clientId += String(random(0xffff), HEX);
  int counter = 0;
  while (!client.connected()) {
    if (counter==5){
      ESP.restart();
    }
    counter+=1;
    Serial.print(“Attempting MQTT connection…”);
    // Attempt to connect
   
    if (client.connect(“KilnEspController”)) {
      Serial.println(“connected”);
    } else {
      Serial.print(“failed, rc=”);
      Serial.print(client.state());
      Serial.println(” try again in 5 seconds”);
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void loop() {
  if (!client.connected()){
    reconnect();
  }
  getValues();

  client.publish(mqttTemp1, String(AmbTemp1).c_str(),true);
  client.publish(mqttHum1, String(Intakehum1).c_str(),true);
  client.publish(mqttTemp2, String(underKilntemp2).c_str(),true);
  client.publish(mqttHum2, String(exhaustHum2).c_str(),true);
  client.publish(mqttTemp3, String(grainTemp).c_str(),true);
  client.publish(mqttGrainMCTop, String(grainMCtop).c_str(),true);
  client.publish(mqttGrainMCBtm, String(grainMCBtm).c_str(),true);

  delay(5000);
}

The platformio.ini file is as follows:-
 

board = esp32dev
framework = arduino
monitor_speed = 115200
lib_deps =
    arduino-libraries/NTPClient@^3.1.0
    Adafruit Unified Sensor@^1.1.4
    bblanchon/ArduinoJson@^6.17.3
    https://github.com/milesburton/Arduino-Temperature-Control-Library.git
    https://github.com/adafruit/DHT-sensor-library.git
    knolleary/PubSubClient@^2.8

 
have you any suggestions of where I am going wrrong? 
Many thanks
 
Paul