How is the ESP32 (DOIT DevKit) finding another host in the same LAN via mDNS?

I have a Raspberry Pi connected to my Wifi LAN that responds to mDNS as mqtt-broker.local.

I can find it on my laptop with this command:

$ avahi-resolve-host-name -4 mqtt-broker.local
mqtt-broker.local   192.168.XXX.YYY

I have an ESP32 DOIT DevKit device that can send messages to the Raspberry Pi via Wifi if I use the IP address 192.168.XXX.YYY, however I would like my ESP32 to resolve the host using mDNS.

I am not able to get the mDNS working, the code at the bottom prints:

Finding the mDNS details...
No services found...
Done finding the mDNS details...

What's wrong with this code?
What should I put as service in MDNS.queryService("mqtt-broker", "tcp")? I have tried even with service mqtt with no luck, however this shouldn't matter, the mDNS stuff should work regardless what's exposed from the Raspberry Pi (HTTP server, MQTT, FTP whatever...)
Checking here https://github.com/espressif/arduino-esp32/blob/master/libraries/ESPmDNS/src/ESPmDNS.h#L98 there is not that much information about this "service" and "proto", and I am not that much familiar with low-level C/C++, what are these things?

This is the code I am using:

// import the headers
#include <ESPmDNS.h>

void findMyPi() {
  Serial.println("Finding the mDNS details...");

  // make sure we are connected to the Wifi
  while (WiFi.status() != WL_CONNECTED) {
    delay(250);
    Serial.println("Not yet connected to Wifi...");
  }

  if (!MDNS.begin("whatever_this_could_be_anything")) {
    Serial.println("Error setting up MDNS responder!");
  }

  // what should I put in here as "service"?
  int n = MDNS.queryService("mqtt-broker", "tcp");
  if (n == 0) {
    Serial.println("No services found...");
  }
  else {
    for (int i = 0; i < n; ++i) {
      // Print details for each service found
      Serial.print("  ");
      Serial.print(i + 1);
      Serial.print(": ");
      Serial.print(MDNS.hostname(i)); // "mqtt-broker" ??? How can I find it???
      Serial.print(" (");
      Serial.print(MDNS.IP(i));
      Serial.print(":");
      Serial.print(MDNS.port(i));
      Serial.println(")");
    }
  }
  Serial.println("Done finding the mDNS details...");
}

This function has been inspired by this example:

https://github.com/espressif/arduino-esp32/blob/master/libraries/ESPmDNS/examples/mDNS-SD_Extended/mDNS-SD_Extended.ino

Solution

Ended up using a different method from the class on that mDNS library provided by Espressif (ESPmDNS.h), a combination of:

IPAddress serverIp = MDNS.queryHost(mDnsHost);
while loop on this check serverIp.toString() == "0.0.0.0"

This is the code that glues up all together:

// on my laptop (Ubuntu) the equivalent command is: `avahi-resolve-host-name -4 mqtt-broker.local`
String findMDNS(String mDnsHost) { 
  // the input mDnsHost is e.g. "mqtt-broker" from "mqtt-broker.local"
  Serial.println("Finding the mDNS details...");
  // Need to make sure that we're connected to the wifi first
  while (WiFi.status() != WL_CONNECTED) {
    delay(250);
    Serial.print(".");
  }
  if (!MDNS.begin("esp32whatever")) {
    Serial.println("Error setting up MDNS responder!");
  } else {
    Serial.println("Finished intitializing the MDNS client...");
  }

  Serial.println("mDNS responder started");
  IPAddress serverIp = MDNS.queryHost(mDnsHost);
  while (serverIp.toString() == "0.0.0.0") {
    Serial.println("Trying again to resolve mDNS");
    delay(250);
    serverIp = MDNS.queryHost(mDnsHost);
  }
  Serial.print("IP address of server: ");
  Serial.println(serverIp.toString());
  Serial.println("Done finding the mDNS details...");
  return serverIp.toString();
}