Failure to send POST requests over SSL with TinyGSM

[reusables-official]

Hello, I am observing repeated failures trying to send a post request via SSLClient using my SIMCOM7600G modem with TinyGSM.

The most common and consistent error message:

[318168][E][ssl__client.cpp:401] init_tcp_connection(): Connection to server failed, is the signal good, server available at this address and timeout sufficient? reusables.hyperether.com:443
[319175][E][ssl__client.cpp:46] _handle_error(): [start_ssl_client():371]: (-2) BIGNUM - An error occurred while reading from or writing to a file
[319177][E][SSLClient.cpp:242] connect(): start_ssl_client failed: 0

I have tried to set headers to adjust the time out like so:

client.connectionKeepAlive();
client.setHttpWaitForDataDelay(5000);
client.sendHeader("Keep-Alive", "timeout=1000, max=5000");
int err = client.post(API_ENDPOINTS.container, "application/json", body);

where body is a relatively small json payload:

{
  "mode": "return-containers",
  "data": {
    "containerIds": [
      "64367339246bf8633c782348"
    ],
    "locationId": "6621a0099cf68973c58f68bf"
  }
}

Here is my setup code:

#define TINY_GSM_MODEM_SIM7600
#define TINY_GSM_RX_BUFFER 2048

#include <TinyGsmClient.h>
#include <ThingerTinyGSM.h>

HardwareSerial gsmSerial(1);
TinyGsm modem(gsmSerial);
TinyGsmClient gsmClient(modem, 0);

SSLClient secure_layer_lte(&gsmClient);
HttpClient lte_ssl_client = HttpClient(secure_layer_lte, API_HOST, PORT);

void lteInit() {
  gsmSerial.begin(BAUD_RATE, SERIAL_8N1, LTE_RX, LTE_TX, false);
  secure_layer_lte.setCACert(api_root_ca);
  modem.init();
  modem.setNetworkMode(38);
  modem.sendAT("AT+CMNB=1");
  modem.gprsConnect(APN);
};

Then I use a RTOS task to monitor the connection and reconnect as needed:

void lteConnect(void * parameter) {
  lteInit();
  for (;;) {
    if (modem.isNetworkConnected()) {
      if (modem.isGprsConnected()) {
        vTaskDelay(20000 / portTICK_PERIOD_MS);
      } else {
        modem.gprsConnect(APN);
      };
    } else {
      bool networkConnected = modem.waitForNetwork();
    };
    vTaskDelay(5000 / portTICK_PERIOD_MS);
  };
  vTaskDelete(NULL);
};

What confuses me the most is that sometimes the request will go through and I'll get a 200 back as if nothing is wrong, but the vast majority of the time, I get the error pasted above.

I have run a test on signal quality to assess, and I will get a mix of 99 back as a value and anything from 28-31 at other points. Running on an interval of one second might produce output like: [99, 31, 31, 31, 99, 27, 99, 99, 99, 28] which also confuses me. My understanding is that a value of 99 for signal quality from TinyGSM means it could not retrieve the value, 31 is the best quality possible and that a value of 0 would mean there was no reception at all.

Any ideas what I'm doing wrong or inconsistently? I know the error is being thrown by this library, but very possible some other culprit is to blame here. I have the same functionality working perfectly over WiFi, where I provide the WiFi client to the "client.post" function I pasted above, also with SSL also via this library.

Very puzzled. Any thoughts greatly appreciated. I commented on a recent PR for this library that I thought might address the issue but it has not been resolved in my case, I am on version 1.2.0.

Platform.ini

[env:esp32-s3-devkitc-1]
platform = espressif32
board = esp32-s3-devkitc-1
framework = arduino
monitor_speed = 115200
monitor_filters = esp32_exception_decoder
lib_deps = 
	bblanchon/[email protected]
	vshymanskyy/[email protected]
	makuna/[email protected]
	arduino-libraries/[email protected]
	digitaldragon/[email protected]
	thinger-io/[email protected]
	bblanchon/[email protected]