Getting `[E][WiFiGeneric.cpp:658] hostByName(): DNS Failed` randomly

[GeorgeFlorian]

Hello !

I am using a HTTP Client inside loop to check for the body of a URL.
It worked fine until it didn't.
Today the ESP crashed and restarted. After the restart it connected to the WiFi using SSID and Password, so using DHCP. But it behaved like it did not connect.

• The web-pages that I have on the ESPAsyncWebServer didn't work.
• And I repeatedly got:

[E][WiFiGeneric.cpp:658] hostByName(): DNS Failed
[HTTP] GET... failed, error: connection refused

This is a part of the code that runs in void loop():

if (WiFi.status() == WL_CONNECTED && SPIFFS.exists("/configFile.txt")) {  //Check the current connection status 
      if(URL.length() > 15) {
        HTTPClient http;
        http.begin(URL); //Specify the URL
        int httpCode = http.GET(); //Make the request  
        if (httpCode > 0) { //Check for the returning code
          logOutput((String)"[HTTP] GET... code: " + httpCode);
          if(httpCode == HTTP_CODE_OK) {
              String payload = http.getString();
              logOutput(payload);
              signPlace = getPlaces(payload);
              oldSignPlace = signPlace;
            }
        } else {
            logOutput((String)"[HTTP] GET... failed, error: " + http.errorToString(httpCode).c_str());
          }  
        http.end(); //Free the resources
        } else {
            logOutput("URL Link is invalid ! Please enter another URL");
        }
    } // if (WiFi.status() == WL_CONNECTED)

Manually restarting the ESP, by pressing on the EN button, did fix the issue, however I still get this randomly. The only fix is to restart again !

[atanisoft]

can you confirm the value of URL is valid at all times? It may make sense to add a log output to print the URL being attempted.

[GeorgeFlorian]

can you confirm the value of URL is valid at all times? It may make sense to add a log output to print the URL being attempted.

URL starts as follows:
String URL = "INVALID_URL"; It's length is 11.

First condition comes from HTML:
<input id="text-input" type="url" inputmode="url" placeholder="Enter URL" name="getURL" value="" title = "URL">
I won't be able to input anything else aside a URL.

Second condition comes from the web-server:
if(URL != "INVALID_URL" && URL != NULL && URL.length() > 15)

Third condition is the one in void loop() :
if(URL.length() > 15) {}

URL = "INVALID_URL" won't be able to arrive at http.begin(URL);
The HTTP Client won't even be initialized until I have a valid URL.

I have tried with the same URL over and over again. I sometimes get the error and sometimes it works perfectly.

It may make sense to add a log output to print the URL being attempted.

I don't follow.

My logOutput is a method I made to push Strings into a buffer so that I can print over a %Placeholder% that's inside a HTML page.

void logOutput(String string1) {
	circle.push(string1);	
	Serial.println(string1);	
}

[atanisoft]

What I meant was can you add a printf (or Serial.println in your case likely) to dump the URL value just before you call http.begin(URL); I suspect that you are ending up with a partial value in the URL string that is causing the DNS failure.

[GeorgeFlorian]

What I meant was can you add a printf (or Serial.println in your case likely) to dump the URL value just before you call http.begin(URL); I suspect that you are ending up with a partial value in the URL string that is causing the DNS failure.

I've added:

        Serial.println((String)"DEBUG: URL = " + URL);
        http.begin(URL); //Specify the URL

For now it seems that it works.

[GeorgeFlorian]

@atanisoft
So it happened again.
Now I can confirm that the URL that reaches http.begin(URL); is correct and that I still I get the error after an ESP Crash.

[E][WiFiGeneric.cpp:658] hostByName(): DNS Failed
[HTTP] GET... failed, error: connection refused

My workaround is to have a counter each time if (httpCode > 0) is not true. When the counter hits a certain value I perform a restart:

void loop() {
if (httpCode > 0) {
\*
*\
} else {
            logOutput((String)"[HTTP] GET... failed, error: " + http.errorToString(httpCode).c_str());
            httpFailCounter++;
          }
\*
*\
  if(httpFailCounter > 5) {
    delay(100);
    ESP.restart();
  }
}

What do you think causes [E][WiFiGeneric.cpp:658] hostByName(): DNS Failed ?

[atanisoft]

That means it failed to resolve the hostname. There are a number of reasons for this and usually a retry should fix it.

[atanisoft]

You should also check the return value from http.begin() as it should return false when dns fails.

[GeorgeFlorian]

You should also check the return value from http.begin() as it should return false when dns fails.

Ok. I will further investigate using this:
logOutput(http.begin(URL) ? "DNS Resolved" : "DNS Fail");

My concern is that the DNS fails only after a crash.
I get a [HTTP] GET... failed, error: connection refused from time to time, without [E][WiFiGeneric.cpp:658] hostByName(): DNS Failed, but this seems normal and it works fine next loop.

[GeorgeFlorian]

@atanisoft
Sorry to bother you, but I get a strange behavior when disconnecting the ESP and then plugging it back.
Using this code:

    if (WiFi.status() == WL_CONNECTED && SPIFFS.exists("/configFile.txt")) {  //Check the current connection status
      logOutput((String)"IP ADDRESS: " + IP_copy.toString());
      if(URL.length() > 15) {
        HTTPClient http;
        Serial.println((String)"DEBUG: URL = " + URL);
        logOutput(http.begin(URL) ? "DNS Resolved" : "DNS Failed");
        int httpCode = http.GET(); //Make the request  
        // Serial.println("int httpCode = http.GET();");
        if (httpCode > 0) { //Check for the returning code
          // Serial.println("if (httpCode > 0)");
          logOutput((String)"[HTTP] GET... code: " + httpCode);
          if(httpCode == HTTP_CODE_OK) {
            // Serial.println("if(httpCode == HTTP_CODE_OK)");
            httpFailCounter = 0;
            String payload = http.getString();
            logOutput(payload);
            signPlace = getPlaces(payload);
            oldSignPlace = signPlace;
          }
        } else {
            logOutput((String)"[HTTP] GET... failed, error: " + http.errorToString(httpCode).c_str());
            httpFailCounter++;
          }  
        http.end(); //Free the resources
        } else {
            logOutput("URL Link is invalid ! Please enter another URL");
        }
    } // if (WiFi.status() == WL_CONNECTED)

Serial.println((String)"DEBUG: URL = " + URL); returns the correct URL.
logOutput(http.begin(URL) ? "DNS Resolved" : "DNS Failed"); returns DNS Resolved

And then comes [HTTP] GET... failed, error: connection refused.
The only thing that fixes this is pressing the EN button to reset the ESP or , what I am currently doing, using a counter to count the number of errors and on 4 straight errors the ESP restarts using ESP.restart().

Do you have any idea why is this happening ?

[atanisoft]

Unfortunately I'm not sure why it is doing that

[maxpromer]

I have same this bug. and now i use counter to check dns fail if over 6 times i use ESP.restart() to restart ESP. after restart ESP can connect to server without this bug.

i think fix this bug without restart ESP is better.

[justoke]

I can confirm this issue, as I have been investigating the same problem for a couple of weeks now. These are my observations:

Devices and Configuration

ESP32 Dev Module
PubSubClient - MQTT library
MQTT Broker - cloudmqtt.com
Arduino platform on Platform IO

Error Logged

[E][WiFiGeneric.cpp:652] hostByName(): DNS Failed for m20.cloudmqtt.com

Behaviour

Once the ESP32 has connected to WiFi and MQTT client is connected, the code runs tasks that read sensors and other typical microcontroller tasks. I usually have the MQTT web interface open so I can see messages going in and out of the ESP32. Now the dns failure can happen randomly. And sometimes hours or even just a few minutes after the device has been powered up or reset.

WiFi Stack Game Over

However, once it has failed it will not recover. I had to implement a similar regime as @maxpromer and do an ESP.restart() after a set number of failures. I tried reconnecting WiFi client but this issue seems to break the WiFi stack and only a reset will get it working.

Host is online, DNS resolution is fine

When it is in this DNS failed state, I can nslookup the domain from my PC and the MQTT web interface is working fine. This issue affects the fundamental operation of an IOT device - it must be able to maintain its connection to the WiFi AP so that it can access resources such as MQTT brokers, http APIs and so forth. I now make the assumption that the internet is down when I implement my code, as the WiFi stack is not reliable enough to maintain a connection or recover from outages and failed DNS resolution.

Proposed Change

It seems it would be safer to assume that once DNS has been resolved, that the resolved IP address could be cached. The IP address for mqtt.com, for example, has not changed in over a year. The DNS failure could raise a callback, that would pass the last successfully resolved IP address and the developer could then attempt to connection using the LastSuccessfulIPAddress, instead of the host name. If this fails to connect then, a few retries of the same patterns and we could assume with some certainty that the host may well be offline or unavailable.

ESP8266 By Comparison

I have a couple of ESP8266 running the same MQTT client and they been online for 18 months now and I have never had to reset them, not even once. They have survived internet outages, MQTT outages, simply I assume because the WiFi stack recovers from these issues and keeps a healthy connection.