Bind error -8 on server restart if accessed during shutdown

[heluoly]

Platform

ESP32

IDE / Tooling

Arduino (IDE/CLI)

What happened?

When running the example program EndBegin.ino, I connected to the ESP32's WiFi and accessed http://192.168.4.1/ using a browser. While continuously refreshing the webpage, the program encountered the following error when executing server.begin() for the second time:
[20615][E][AsyncTCP.cpp:1521] begin(): bind error: -8.
At this point, the webpage returned net::ERR_CONNECTION_REFUSED.
I suspect this is caused by a port conflict. How can I resolve this issue?

Stack Trace

Software Info:

Compile Date/Time : Nov 21 2025 20:19:26
Compile Host OS : windows
ESP-IDF Version : v5.5.1-710-g8410210c9a
Arduino Version : 3.3.4

Board Info:

Arduino Board : ESP32S3_DEV
Arduino Variant : esp32s3
Arduino FQBN : esp32:esp32:esp32s3:UploadSpeed=921600,USBMode=hwcdc,CDCOnBoot=default,MSCOnBoot=default,DFUOnBoot=default,UploadMode=default,CPUFreq=240,FlashMode=qio,FlashSize=4M,PartitionScheme=default,DebugLevel=verbose,PSRAM=disabled,LoopCore=1,EventsCore=1,EraseFlash=none,JTAGAdapter=default,ZigbeeMode=default

[ 361][V][esp32-hal-periman.c:160] perimanSetPinBus(): Pin 44 successfully set to type UART_RX (2) with bus 0x3fc95e10
[ 372][V][esp32-hal-periman.c:235] perimanSetBusDeinit(): Deinit function for type UART_RX (2) successfully set to 0x42010d9c
[ 383][V][esp32-hal-periman.c:160] perimanSetPinBus(): Pin 43 successfully set to type UART_TX (3) with bus 0x3fc95e10
[ 394][V][esp32-hal-periman.c:235] perimanSetBusDeinit(): Deinit function for type UART_TX (3) successfully set to 0x42010d68
[ 405][V][esp32-hal-uart.c:716] uartBegin(): UART0 baud(115200) Mode(800001c) rxPin(44) txPin(43)
[ 414][V][esp32-hal-uart.c:814] uartBegin(): UART0 not installed. Starting installation
[ 422][V][esp32-hal-uart.c:824] uartBegin(): UART0 RX FIFO full threshold set to 120 (value requested: 120 || FIFO Max = 128)
[ 433][V][esp32-hal-uart.c:850] uartBegin(): Setting UART0 to use XTAL clock
[ 440][V][esp32-hal-uart.c:889] uartBegin(): UART0: RX and TX signals are set not inverted.
[ 449][V][esp32-hal-uart.c:917] uartBegin(): UART0 initialization done.
[ 455][V][esp32-hal-uart.c:968] uartSetRxFIFOFull(): UART0 RX FIFO Full value set to 120 from a requested value of 120
[ 480][V][NetworkEvents.cpp:120] _checkForEvent(): Network Event: 101 - WIFI_READY
[ 596][V][AP.cpp:108] _onApEvent(): AP Started
[ 600][V][NetworkEvents.cpp:120] _checkForEvent(): Network Event: 130 - AP_START
[ 608][V][AP.cpp:88] _onApArduinoEvent(): Arduino AP Event: 130 - AP_START
begin() - run: curl -v http://192.168.4.1/ => should succeed
[ 7806][V][AP.cpp:125] _onApEvent(): AP Station Connected: MAC: fa:20:6a:98:97:22, AID: 1
[ 7814][V][NetworkEvents.cpp:120] _checkForEvent(): Network Event: 132 - AP_STACONNECTED
[ 7822][V][AP.cpp:88] _onApArduinoEvent(): Arduino AP Event: 132 - AP_STACONNECTED
[ 8198][V][NetworkInterface.cpp:146] _onIpEvent(): ap Assigned IP: 192.168.4.2 to MAC: FA:20:6A:98:97:22
[ 8207][V][NetworkEvents.cpp:120] _checkForEvent(): Network Event: 134 - AP_STAIPASSIGNED
[ 8215][V][AP.cpp:88] _onApArduinoEvent(): Arduino AP Event: 134 - AP_STAIPASSIGNED
end()
[ 20615][E][AsyncTCP.cpp:1521] begin(): bind error: -8
begin() - run: curl -v http://192.168.4.1/ => should succeed

Minimal Reproductible Example (MRE)

#include <Arduino.h>
#if defined(ESP32) || defined(LIBRETINY)
#include <AsyncTCP.h>
#include <WiFi.h>
#elif defined(ESP8266)
#include <ESP8266WiFi.h>
#include <ESPAsyncTCP.h>
#elif defined(TARGET_RP2040) || defined(TARGET_RP2350) || defined(PICO_RP2040) || defined(PICO_RP2350)
#include <RPAsyncTCP.h>
#include <WiFi.h>
#endif

#include <ESPAsyncWebServer.h>

static AsyncWebServer server(80);

void setup() {
Serial.begin(115200);

#if SOC_WIFI_SUPPORTED || CONFIG_ESP_WIFI_REMOTE_ENABLED || LT_ARD_HAS_WIFI || CONFIG_ESP32_WIFI_ENABLED
WiFi.mode(WIFI_AP);
WiFi.softAP("esp-captive");
#endif

server.on("/", HTTP_GET, [](AsyncWebServerRequest *request) {
request->send(200, "text/plain", "Hello, world");
});

server.begin();
Serial.println("begin() - run: curl -v http://192.168.4.1/ => should succeed");
delay(20000);
// Connect to the WiFi and continuously access http://192.168.4.1/ during this period.
Serial.println("end()");
server.end();
server.begin();
// The error message [ 20615][E][AsyncTCP.cpp:1521] begin(): bind error: -8 appeared, and the web page could no longer be accessed.
Serial.println("begin() - run: curl -v http://192.168.4.1/ => should succeed");
}

// not needed
void loop() {
delay(100);
}

I confirm that:

• I have read the documentation.
• I have searched for similar discussions.
• I have searched for similar issues.
• I have looked at the examples.
• I have upgraded to the lasted version of ESPAsyncWebServer (and AsyncTCP for ESP32).

[mathieucarbou]

@heluoly : if the example is not working anymore, we probably have a big. I will check.

@WillMiles @vortigont FYI

[mathieucarbou]

I am able to reproduce. Indeed the server is not restarting.

[mathieucarbou]

I am able to reproduce. Indeed the server is not restarting.

@heluoly : and now I am not able to.. I suspect this is a timing issue in LWIP: LWIP has an async task so maybe the begin is happening too soon when lwip did not have to free the resources yet for the previous opened port.

Can you try add a delay and see if you can reproduce ? I've only been able to reproduce once and did try 6 times total with all arduino core versions from 3.3.1 to 3.3.4.

  Serial.println("end()");
  server.end();

  Serial.println("waiting before restarting server..."); // LINE ADDED
  delay(200); // LINE ADDED

  server.begin();
  Serial.println("begin() - run: curl -v http://192.168.4.1/ => should succeed");

[heluoly]

I added a 200ms delay between the service end and start, then conducted further testing. Using Arduino Core version 3.3.4, I tested ESP Async WebServer versions 3.8.1 and 3.9.0. With version 3.8.1, I tested by connecting my phone to the ESP32-S3's WiFi and encountered no issues. However, when using a laptop to connect to the WiFi and accessing it via the Edge browser, I was able to reproduce the issue almost every time. With version 3.9.0, regardless of whether I used a mobile browser or a laptop browser, the issue could be reproduced nearly every time. Since I only have this one ESP32-S3 development board on hand, I'm not sure whether this is an isolated case with my particular board or not.

[vortigont]

reusing socket on same port could be a bit tricky.
I'll take a closer look into this case, meanwhile cat you explain your use case why do you need to execute this sequence?

server.end();
server.begin();

[mathieucarbou]

@heluoly : the correct way to handle this situation is to have begin() called once and use .filter() or middlewares to deactivate handlers depending on WiFi mode.

[mathieucarbou]

@heluoly : the correct way to handle this situation is to have begin() called once and use .filter() or middlewares to deactivate handlers depending on WiFi mode.

update: or add a small delay (i.e. i remember a use case where a user had to do that because of a change in his interfaces).

[heluoly]

@heluoly : the correct way to handle this situation is to have begin() called once and use or middlewares to deactivate handlers depending on WiFi mode..filter()

I appreciate the correction. I will proceed to implement the suggested change.

[vortigont]

@heluoly : the correct way to handle this situation is to have begin() called once and use .filter() or middlewares to deactivate handlers depending on WiFi mode.

right!
Another option is to use reset() and then reassign needed on() handlers.
But anyway I'll keep this issue in mind. Thanks for reporting!

[heluoly]

@heluoly : the correct way to handle this situation is to have begin() called once and use .filter() or middlewares to deactivate handlers depending on WiFi mode.

right! Another option is to use reset() and then reassign needed on() handlers. But anyway I'll keep this issue in mind. Thanks for reporting!

That does seem like a simpler approach. Thanks for the tip!