From linux I can send a socket message using netcat

echo "Hello" | nc 100

But I don't know how to setup the socket server for listening incoming messages using the ethernet shield.

What I have so far is one from a lot of examples I tried but still not showing anything

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

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

EthernetServer server(1000);

void setup() {
// Check for Ethernet hardware present
  if (Ethernet.hardwareStatus() == EthernetNoHardware) {
    Serial.println("Ethernet shield was not found.  Sorry, can't run without hardware. :(");
    while (true) {
      delay(1); // do nothing, no point running without Ethernet hardware
  if (Ethernet.linkStatus() == LinkOFF) {
    Serial.println("Ethernet cable is not connected.");

  // start the server
  Serial.print("server is at ");

void loop() {
  // listen for incoming clients
 EthernetClient client = server.available();
 if (client) {
   // an http request ends with a blank line
   boolean currentLineIsBlank = true;
   while (client.connected()) {
     while(client.available()) {
       char c = client.read();
       // if you've gotten to the end of the line (received a newline
       // character) and the line is blank, the http request has ended,
       // so you can send a reply
       if (c == '\n' && currentLineIsBlank) {

// Here is where the POST data is.  

         Serial.println("Sending response");
         // send a standard http response header
         //client.println("HTTP/1.0 200 OK");
         //client.println("Content-Type: text/html");
       else if (c == '\n') {
         // you're starting a new line
         currentLineIsBlank = true;
       else if (c != '\r') {
         // you've gotten a character on the current line
         currentLineIsBlank = false;
  • have you confirmed that your ethernet shield is functional?
    – jsotola
    Commented Jun 4, 2019 at 2:11
  • see the ChatServer example
    – Juraj
    Commented Jun 4, 2019 at 5:23
  • Where is your Ethernet.begin? Surely you still need one of those. have a look at the setup() in the docs here: EthernetServer Example
    – GMc
    Commented Jun 4, 2019 at 10:14
  • the shield is fully functional, I'm able to receive http post requests, that's how I figure it out, but I still needing the socket server, just to receive raw messaages Commented Jun 4, 2019 at 12:23
  • I updated the code, that's how actually receives http requests Commented Jun 4, 2019 at 12:33

1 Answer 1


I am learning to use the arduino library about sockets and I spent an hour detecting why your example was not working for me, I ended up noticing that you have one line wrong

if (c = '\n' && currentLineIsBlank) {

should be:

{ if (c != '\n' && !currentLineIsBlank) {

And you still have an error you are missing a return character because you are not printing it in the second loop.

Anyway thanks for your contributions.

  • there is == and that is right
    – Juraj
    Commented Feb 2, 2023 at 13:11

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