I was trying to write a Telnet server that would be able to accept commands of sorts using the Arduino Ethernet library, and in order to access what the client was saying (to compare it to strings to check what command the client entered), I made a buffer string that would read until \n, which is part of the standard Telnet library. However, the string would never get printed to the serial connection (just for debugging purposes). Here's my code:

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

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xFE, 0xED };  // this client's MAC address

EthernetServer server(23);  // telnet is 23
boolean alreadyConnected = false;

void setup() {


  server.begin();  // open the socket

  while (!Serial) {
    ; // wait for serial to start, Leonardo only

  Serial.print("Connect to ");


void loop() {

  EthernetClient c = server.available();

  if (c) {
    String messageBuffer = "";

    if (!alreadyConnected) {
      Serial.println("New client");
      c.println("test post, please ignore");
      alreadyConnected = true;
    if (c.available() > 0) {

      char thisChar = c.read();


      //buffer until the client sends a new line
      if (thisChar == '\n') {
        messageBuffer = "";
      } else {
        messageBuffer += thisChar;

Most of this code was copied from the Telnet chat server example, but I changed the Serial.write(thisChar) line, because I wanted to put it into a string first. When this is run, and a user telnets to the IP, the Arduino prints New client, but when the user sends data, it doesn't print anything.

Any suggestions for what I'm doing wrong?

  • Can you send the complete code? – user6647 Jan 10 '15 at 19:26
  • That is the complete code. As I've said, I fixed the problem. – Piccolo Jan 13 '15 at 4:29

Well, I asked on the #arduino IRC channel, and realized my stupid mistake. I was setting the buffer to "" inside the loop.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.