ESP32 Switch Between Promiscuous and STA Mode

Question

I'm working on a project based on the ESP32 platform. The aim is to count the number of MAC addresses in the area, and transmit this information over WiFi (using an http POST request).

The first task is achieved by sniffing WIFI packets and collecting the contained addresses, following this example: https://blog.podkalicki.com/esp32-wifi-sniffer/ I believe that the code which "sniffs" the packets sets the ESP to run in promiscuous mode, and therefore I cannot connect to any AP anymore.

I've tried several solutions, first starting with timer interrupts. However this approach always led to a Core Panic and reset of the chip. I also learnt I could use RTOS to run different tasks in parallel on the two cores of the CPU, but that didn't help to solve the problem.

void wifi_sniffer_packet_handler(void* buff, wifi_promiscuous_pkt_type_t type)
{
  if (type != WIFI_PKT_MGMT)//aggiungere filtro su RSSI a questa altezza.
    return;

  const wifi_promiscuous_pkt_t *ppkt = (wifi_promiscuous_pkt_t *)buff;
  const wifi_ieee80211_packet_t *ipkt = (wifi_ieee80211_packet_t *)ppkt->payload;
  const wifi_ieee80211_mac_hdr_t *hdr = &ipkt->hdr;

//some analysis and then print the MAC address  

}

void setup() {
  Serial.begin(115200);
  timer = timerBegin(0, 80, true);
  timerAttachInterrupt(timer, &chUpdate, true);
  timerAlarmWrite(timer, 1000000, true);//timer, arr_val, reload=true
  delay(4000);
  wifi_sniffer_init();
  timerAlarmEnable(timer);
}

// the loop function runs over and over again forever
void loop() {
  //Serial.print("inside loop");
  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Establishing connection to WiFi..");
  }
  Serial.println("Connected to network");

}

I also noticed that the code in the loop gets stuck into the while, and is restarted every time the packet handler is run (I nevere get to see "Connected to network", but i see "Establishing connection to WiFi.." several times.

Anyone can explain me what's going on? Is there a different approach to achieve this result? Thank you.

Answer 1

You may have two tasks and two cores, but the ESP32 still has only one wifi chip. The way your code is written (at least, the code you shared), you'll be trying to connect to a wifi network at the same time as you're trying to run promiscuous mode. You can do only one of those things at a time.

You'll need to stop promiscuous mode before you attempt to connect to an access point. Right now your code constantly attempt to connect to a wifi access point. Use a volatile variable to store the current mode - promiscuous or connected. Change it when you need to change states. Only attempt to connect to wifi when the variable says you want to be in connected mode.

There may be some code you need to run to turn off promiscuous mode when you change states, before you connect to a wifi access point.

If you're using wifi_sniffer_init() from the example you linked to, that code isn't meant to be run in an Arduino Core application. It does some network initialization that the Arduino Core will also do. It may not be safe to do that twice (it might work, it might not... but it's definitely not intended to be done that way).

You're setting an interrupt handle chUpdate() which you didn't share. I'd bet that's the cause of your Core Panics. You can do very little in an interrupt handler. You definitely can't call most Arduino Core functions or most ESP-IDF functions. Most code isn't protected against interrupts, so the timer interrupt can occur while data structures are in an inconsistent state. Re-entering code can corrupt the data structures and cause the kind of crash you described. You're best off setting a volatile variable and waking up a task that will do the work you need done while not in the interrupt handler.

Finally, you should call WiFi.mode(WIFI_STA); before you call WiFi.begin().

For anyone confused about why you can't connect to a wifi network while in promiscuous mode - what ESP8266 and ESP32 call "promiscuous mode" is really "wifi monitor mode", which lets you monitor a wifi radio channel and see all wifi frames sent on it. They use the term "promiscuous mode" differently from the rest of the industry. Usually "promiscuous mode" means seeing all the packets being sent on the network (wifi or hardwired) that you're connected to.

Answer 2

I know that it's been just over a year but THANKS to the info provided by romkey I think I solved this problem within my app by calling this routine before connecting to WiFi to upload MAC data.

void end_Scan_WiFi() {
  esp_wifi_set_promiscuous(false);
  esp_wifi_stop();
}

Followed by this ...

WiFi.mode(WIFI_STA);
WiFi.begin(ssid,pass);