https://www.youtube.com/watch?v=gU5Vp0zCzak

mqtt-test.ino

#include <ESP8266WiFi.h>
#include <ESP8266mDNS.h>
#include <PubSubClient.h>

const char* cfg_wifi_ssid = "freifunk-myk.de";
const char* cfg_wifi_pwd = "";

const char* mqtt_server = "deinserver.local";
const unsigned int mqtt_port = 1883;
const char* mqtt_user =   "admin";
const char* mqtt_pass =   "test123";

//echo | openssl s_client -connect localhost:1883 | openssl x509 -fingerprint -noout
const char* mqtt_fprint = "D9:B2:A5:BB:E1:E5:4D:50:23:E1:84:24:29:44:BA:7C:54:01:BB:08";

WiFiClientSecure espClient;
PubSubClient client(espClient);

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("TestMQTT");

  WiFi.mode(WIFI_STA);
  WiFi.begin(cfg_wifi_ssid, cfg_wifi_pwd);

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

  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());

  espClient.setFingerprint(mqtt_fprint);
  client.setServer(mqtt_server, mqtt_port);
  client.setCallback(callback);

  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    verifyFingerprint();
    if (client.connect(WiFi.macAddress().c_str(), mqtt_user, mqtt_pass)) {
      Serial.println("connected");
      client.subscribe("/foo");
    }else{
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");

  char message[length + 1];
  for (int i = 0; i < length; i++) {
    message[i] = (char)payload[i];
  }
  message[length] = '\0';
  Serial.println(message);
}

void verifyFingerprint() {
  if(client.connected() || espClient.connected()) return; //Already connected
  
  Serial.print("Checking TLS @ ");
  Serial.print(mqtt_server);
  Serial.print("...");
  
  if (!espClient.connect(mqtt_server, mqtt_port)) {
    Serial.println("Connection failed. Rebooting.");
    Serial.flush();
    ESP.restart();
  }
  if (espClient.verify(mqtt_fprint, mqtt_server)) {
    Serial.print("Connection secure -> .");
  } else {
    Serial.println("Connection insecure! Rebooting.");
    Serial.flush();
    ESP.restart();
  }

  espClient.stop();

  delay(100);
}

unsigned int counter = 0;

void loop() {
  // put your main code here, to run repeatedly:

  client.loop();

  char buf[12];
  itoa(counter, buf, 11);
  
  client.publish("/test", buf);
  counter++;

  delay(1000);
}