Arduino HTTPClient Failure

I don’t know if anyone else has ever tried this, but I just spent the better part of a week trying to debug this little problem, so I thought maybe I’d save someone some time.

I was trying to use WiFiClientSecure along with HTTPClient to POST to an authentication server that is fronted by cloudflare. No matter what I did, I got error messages. I was sending a JSON payload, and it kept telling me that the parameters weren’t in the JSON. So I wrote my own mini server (using Golang, of course) to test against, and it was getting all the data correctly. Hmm…

The SREs in charge of the service were, oddly, not seeing any attempts by my device to connect to the service. Hmmmm … So my Arduino was telling me it was connecting, but the data I was sending was wrong. My Go server was telling me the data was sent correctly, and the owners of the service were telling me I wasn’t even connecting!

Let’s try OpenSSL … Let’s try anything!

So I wrote a straight-C OpenSSL client and it was able to connect to my server just fine, of course. But it was also able to connect, and authenticate, to the authentication server. Even more curious.

In a final fit of desperation, I dropped the HTTPClient part and just used the WiFiClientSecure to do writes, and lo and behold, Success!!

A solution at last

So a couple of things to note if you want to try this:

When you run openssl s_client -showcerts -connect HOST:PORT to a cloudflare service, you will get back 2 certs. The first cert is cloudflare’s cert. DO NOT USE THIS ONE. You want the second cert.

    const char C8SSLCA[] PROGMEM =  R"EOF(
    ------BEGIN CERTIFICATE-----
    -----END CERTIFICATE-----


    WiFiClientSecure *client = new WiFiClientSecure;
    if(client) {

to set the cert in the WiFiClientSecure

All the examples will have you then get a HTTPClient to do the work, but don’t fall for it! The HTTPClient will somehow fail you.

Do this instead:

    Serial.print("[HTTP] POST...\n");
    client->connect("", PORT);
    int tries = 0;
    while (!client->connected()) {
        Serial.printf("*** Can't connect. ***\n-------\n");
        client->connect("", PORT);
        if(tries > 10){
    client->print("POST /oauth/token HTTP/1.0\r\n"); // or whatever your path is
    client->print("Host:\r\n"); // this must match the host you used in connect()
    client->print("User-Agent: ESP8266\r\n"); // or make up your own
    client->print("Content-Length: ");
    client->print("Content-Type: application/json\r\n");
    client->print("Accept-encoding: *\r\n");
    client->print(pBuff); // this is a String
    uint32_t to = millis() + 10000;
    if (client->connected()) {
        Serial.println("Reading response ...");
        do { // wait until there is data
            int avail = client->available();
            if(avail > 0){
         } while (millis() < to); // but not forever
        to = millis() + 5000;
        do {
            char tmp[512]; // you might need more
            memset(tmp, 0, 512);
            int rlen = client->read((uint8_t*)tmp, sizeof(tmp) - 1);
            if (rlen < 0) {
                break; // we've reached the end
        } while (millis() < to);
        Serial.println("Finished reading");

Basically, all you’re doing here is doing the work of the HTTPClient since, at least as far as I can tell, it fails to do it’s job in this instance.

I hope this saves someone else 4 days of frustration because there’s 4 days of my life in it already.