I am trying to make a connection with the EthernetClient, but it always fails with code 0. I decided to write a small code just to test if the client ever becomes ready, and as I suspected it does not.

#include <SPI.h>
#include <Ethernet.h>

byte mac[] = {0x2A, 0x27, 0x55, 0x08, 0x1E, 0x6A};
IPAddress ip(172, 16, 48, 133);
EthernetClient client;

void setup() {
  Ethernet.begin(mac, ip);
  while (!Serial) {;}

  for (auto link = Ethernet.linkStatus(); link != LinkON; link = Ethernet.linkStatus()) {;}
  Serial.println("Link ON");

    Serial.println("Client not ready");

  Serial.println("Client ready!");

void loop() {}

If I try to run the code above, it gets stuck at the "Client not ready" loop, and I can't figure it out what I'm missing for it to work.

Obs.: I have already configured a web server with success with my shield, so I'm guessing the problem is not with it.

  • I'm not sure why you're evaluating !client. This maps to the boolean operator which tells you if it has a connected socket (github.com/arduino-libraries/Ethernet/blob/…). Since you have not called connect() on the client, this will always return false. What actual error do you have in your full sketch? – Maximilian Gerhardt Jan 10 at 20:22
  • Connect returns 0. I've looked the source and found multiple points where it might return 0, so it's hard to diagnose. – Lucas Noetzold Jan 10 at 20:25
  • @Maximilian the reference says only that it "Indicates if the specified Ethernet client is ready." – Lucas Noetzold Jan 10 at 20:29

An EthernetClient expects to connect to a remote socket. At no point do you instruct it to connect to a socket, so it never succeeds.

At some point before checking for a connection you need to initiate that connection:

// arduino.cc is
IPAddress server(100, 24, 172, 113);
client.connect(server, 80);

The if (client) operation relies on sockindex having been assigned a valid value, and that is only ever done either by EthernetClient::connect() or by being explicitly set by EthernetServer::available().

  • in this example this is done to check if the client is available – Lucas Noetzold Jan 10 at 20:34
  • @LucasNoetzold That example makes no sense. Like many on the Arduino website it's complete nonsense. You should only check if (client) to confirm a connection has been established. Normally only when using it as an incoming connection to a server. – Majenko Jan 10 at 20:40
  • As it states in the source code: // the next function allows us to use the client returned by // EthernetServer::available() as the condition in an if-statement. – Majenko Jan 10 at 20:42
  • I'll have to find why connect is returning 0 then. Thanks. – Lucas Noetzold Jan 10 at 20:54
  • 0 indicates DNS name resolve failure. github.com/arduino-libraries/Ethernet/blob/… @LucasNoetzold – Maximilian Gerhardt Jan 10 at 21:33

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