user67819: I want to use hiveMQ public broker in my project that's the same broker as what available in PubSubClient esp8266 examples but when I change WiFi SSID and password in example and upload it I got this output:
Connecting to <SSID NAME>
........
WiFi connected
IP address: 
192.168.1.14
Attempting MQTT connection...failed, rc=-2 try again in 5 seconds
Attempting MQTT connection...failed, rc=-2 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...

I tried this with pubsub client v2.8/v2.6/v2.4 and esp32 latest version and esp8266 latest version and 2.3
in response to the comment as I said my code is esp8266_basic example in the PubSub client examples and here is my code:
#include <ESP8266WiFi.h>
#include <PubSubClient.h>

// Update these with values suitable for your network.

const char* ssid = "<SSID>";
const char* password = "<PASS>";
const char* mqtt_server = "broker.mqtt-dashboard.com"; //tested with broker.mqttdashboard.com

WiFiClient espClient;
PubSubClient client(espClient);
long lastMsg = 0;
char msg[50];
int value = 0;

void setup() {
  pinMode(BUILTIN_LED, OUTPUT);     // Initialize the BUILTIN_LED pin as an output
  Serial.begin(115200);
  setup_wifi();
  client.setServer(mqtt_server, 1883); //8000 and 1883 tested
  client.setCallback(callback);
}

void setup_wifi() {

  delay(10);
  // We start by connecting to a WiFi network
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);

  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());
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();

  // Switch on the LED if an 1 was received as first character
  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW);   // Turn the LED on (Note that LOW is the voltage level
    // but actually the LED is on; this is because
    // it is acive low on the ESP-01)
  } else {
    digitalWrite(BUILTIN_LED, HIGH);  // Turn the LED off by making the voltage HIGH
  }

}

void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Attempt to connect
    if (client.connect("ESP8266Client")) {
      Serial.println("connected");
      // Once connected, publish an announcement...
      client.publish("outTopic", "hello world");
      // ... and resubscribe
      client.subscribe("inTopic");
    } 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();
  }
  client.loop();

  long now = millis();
  if (now - lastMsg > 2000) {
    lastMsg = now;
    ++value;
    snprintf (msg, 75, "hello world #%ld", value);
    Serial.print("Publish message: ");
    Serial.println(msg);
    client.publish("outTopic", msg);
  }
}

on esp32 I remove pinMode(BUILTIN_LED, OUTPUT);     // Initialize the BUILTIN_LED pin as an output and
  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW);   // Turn the LED on (Note that LOW is the voltage level
    // but actually the LED is on; this is because
    // it is acive low on the ESP-01)
  } else {
    digitalWrite(BUILTIN_LED, HIGH);  // Turn the LED off by making the voltage HIGH
  }

sections and includes
and sometimes output is like this:
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected


user67819: all things are OK and the problem is about my network I think there blocked MQTT (cause I couldn't connect any broker and not just hiveMQ) or other related things cause also my notebook doesn't connect I changed my network provider and the problem fixed
if you ran into the same issue, check other stuff and finally check with a computer in your network (don't use things like http://www.hivemq.com/demos/websocket-client/)

user73520: I found this video as a problem solver, This may help you.
All MQTT Errors explained very well and Suggester Different troubleshooting techniques
https://youtu.be/CbodTTk-D18

67819: <p>I want to use hiveMQ public broker in my project that's the same broker as what available in PubSubClient esp8266 examples but when I change WiFi SSID and password in example and upload it I got this output:</p>
<pre><code>Connecting to &lt;SSID NAME&gt;
........
WiFi connected
IP address: 
192.168.1.14
Attempting MQTT connection...failed, rc=-2 try again in 5 seconds
Attempting MQTT connection...failed, rc=-2 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...
</code></pre>
<p><strong>I tried this with pubsub client v2.8/v2.6/v2.4 and esp32 latest version and esp8266 latest version and 2.3</strong></p>
<p>in response to the comment as I said my code is esp8266_basic example in the PubSub client examples and here is my code:</p>
<pre><code>#include &lt;ESP8266WiFi.h&gt;
#include &lt;PubSubClient.h&gt;

// Update these with values suitable for your network.

const char* ssid = &quot;&lt;SSID&gt;&quot;;
const char* password = &quot;&lt;PASS&gt;&quot;;
const char* mqtt_server = &quot;broker.mqtt-dashboard.com&quot;; //tested with broker.mqttdashboard.com

WiFiClient espClient;
PubSubClient client(espClient);
long lastMsg = 0;
char msg[50];
int value = 0;

void setup() {
  pinMode(BUILTIN_LED, OUTPUT);     // Initialize the BUILTIN_LED pin as an output
  Serial.begin(115200);
  setup_wifi();
  client.setServer(mqtt_server, 1883); //8000 and 1883 tested
  client.setCallback(callback);
}

void setup_wifi() {

  delay(10);
  // We start by connecting to a WiFi network
  Serial.println();
  Serial.print(&quot;Connecting to &quot;);
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(&quot;.&quot;);
  }

  Serial.println(&quot;&quot;);
  Serial.println(&quot;WiFi connected&quot;);
  Serial.println(&quot;IP address: &quot;);
  Serial.println(WiFi.localIP());
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print(&quot;Message arrived [&quot;);
  Serial.print(topic);
  Serial.print(&quot;] &quot;);
  for (int i = 0; i &lt; length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();

  // Switch on the LED if an 1 was received as first character
  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW);   // Turn the LED on (Note that LOW is the voltage level
    // but actually the LED is on; this is because
    // it is acive low on the ESP-01)
  } else {
    digitalWrite(BUILTIN_LED, HIGH);  // Turn the LED off by making the voltage HIGH
  }

}

void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print(&quot;Attempting MQTT connection...&quot;);
    // Attempt to connect
    if (client.connect(&quot;ESP8266Client&quot;)) {
      Serial.println(&quot;connected&quot;);
      // Once connected, publish an announcement...
      client.publish(&quot;outTopic&quot;, &quot;hello world&quot;);
      // ... and resubscribe
      client.subscribe(&quot;inTopic&quot;);
    } else {
      Serial.print(&quot;failed, rc=&quot;);
      Serial.print(client.state());
      Serial.println(&quot; try again in 5 seconds&quot;);
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}
void loop() {

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

  long now = millis();
  if (now - lastMsg &gt; 2000) {
    lastMsg = now;
    ++value;
    snprintf (msg, 75, &quot;hello world #%ld&quot;, value);
    Serial.print(&quot;Publish message: &quot;);
    Serial.println(msg);
    client.publish(&quot;outTopic&quot;, msg);
  }
}
</code></pre>
<p>on esp32 I remove <code>pinMode(BUILTIN_LED, OUTPUT);     // Initialize the BUILTIN_LED pin as an output</code> and</p>
<pre><code>  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW);   // Turn the LED on (Note that LOW is the voltage level
    // but actually the LED is on; this is because
    // it is acive low on the ESP-01)
  } else {
    digitalWrite(BUILTIN_LED, HIGH);  // Turn the LED off by making the voltage HIGH
  }
</code></pre>
<p>sections and includes</p>
<p>and sometimes output is like this:</p>
<pre><code>Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...connected
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...failed, rc=-4 try again in 5 seconds
Attempting MQTT connection...connected
</code></pre>
 67819: <p>all things are OK and the problem is about my network I think there blocked MQTT (cause I couldn't connect any broker and not just hiveMQ) or other related things cause also my notebook doesn't connect I changed my network provider and the problem fixed</p>
<p>if you ran into the same issue, check other stuff and finally check with a computer in your network (don't use things like <a href="http://www.hivemq.com/demos/websocket-client/" rel="nofollow noreferrer">http://www.hivemq.com/demos/websocket-client/</a>)</p>
 73520: <p>I found this video as a problem solver, This may help you.
All MQTT Errors explained very well and Suggester Different troubleshooting techniques
<a href="https://youtu.be/CbodTTk-D18" rel="nofollow noreferrer">https://youtu.be/CbodTTk-D18</a></p>
