Client and Server runing at the same time


I was wondering is it possible to run a server and a client at the same time with the Ethernet shield



I was wondering is it possible to run a server and a client at the same time with the Ethernet shield

No, but it is possible for a server to make GET requests of other servers, while it is not in the middle of servicing a client request.

Two things can't happen AT THE SAME TIME on an Arduino.

So, you are saying that i can use a Server to serve the requests that arrive and i need to make some GET to a specific URL i can still use that same server? Assuming that the GET do not collide with serving the requests...

This way i don´t have to instantiate a Client. Correct?

So, you are saying that i can use a Server to serve the requests that arrive and i need to make some GET to a specific URL i can still use that same server?

Yes. The Arduino-as-server's focus, when a client connects, needs to be to answer that client's request. When the server has no client requests to deal with, it can initiate GET requests of it's own.

It's a little unusual, and you need to be careful doing it, but it can be done.

What do you think i should be carefull with?

Only with the question of loosing requests? is that it? Or are you envisioning other problems?

Test code that has both client and server functions. The code runs in a loop alternately checking for input to the web server and input on the serial port. If a g is received via the serial port, a get request is made to another web server and result displayed in the serial monitor. If the arduino web server receives a get request from the contained web page , then the arduino pins are made high/low per the request.

//zoomkat 7-03-12, combined client and server
//simple button GET with iframe code
//for use with IDE 1.0
//open serial monitor and send an g to test client and
//see what the arduino client/server receives
//web page buttons make pin 5 high/low
//use the ' in html instead of " to prevent having to escape the "
//address will look like when submited
//for use with W5100 based ethernet shields
//note that the below bug fix may be required

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

byte mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED }; //assign arduino mac address
byte ip[] = {192, 168, 1, 102 }; // ip in lan assigned to arduino
byte gateway[] = {192, 168, 1, 1 }; // internet access via router
byte subnet[] = {255, 255, 255, 0 }; //subnet mask
EthernetServer server(84); //server port arduino server will use
EthernetClient client;
char serverName[] = ""; // (DNS) zoomkat's test web page server
//byte serverName[] = { 208, 104, 2, 86 }; // (IP) zoomkat web page server IP address

String readString; //used by server to capture GET request 


void setup(){

  pinMode(5, OUTPUT); //pin selected to control
  pinMode(6, OUTPUT); //pin selected to control
  pinMode(7, OUTPUT); //pin selected to control
  pinMode(8, OUTPUT); //pin selected to control

  //pinMode(5, OUTPUT); //pin 5 selected to control
  Serial.println("server/client 1.0 test 7/03/12"); // keep track of what is loaded
  Serial.println("Send an g in serial monitor to test client"); // what to do to test client

void loop(){
  // check for serial input
  if (Serial.available() > 0) 
    byte inChar;
    inChar =;
    if(inChar == 'g')
      sendGET(); // call client sendGET function

  EthernetClient client = server.available();
  if (client) {
    while (client.connected()) {
      if (client.available()) {
        char c =;

        //read char by char HTTP request
        if (readString.length() < 100) {

          //store characters to string 
          readString += c; 

        //if HTTP request has ended
        if (c == '\n') {

          Serial.print(readString); //print to serial monitor for debuging 

            //now output HTML data header
          if(readString.indexOf('?') >=0) { //don't send new page
            client.println(F("HTTP/1.1 204 Zoomkat"));
          else {   
            client.println(F("HTTP/1.1 200 OK")); //send new page on browser request
            client.println(F("Content-Type: text/html"));

            client.println(F("<TITLE>Arduino GET test page</TITLE>"));

            client.println(F("<H1>Zoomkat's simple Arduino 1.0 button</H1>"));

            // DIY buttons
            client.println(F("<a href=/?on2 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off3 target=inlineframe>OFF</a>


            client.println(F("<a href=/?on4 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off5 target=inlineframe>OFF</a>


            client.println(F("<a href=/?on6 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off7 target=inlineframe>OFF</a>


            client.println(F("<a href=/?on8 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off9 target=inlineframe>OFF</a>


            client.println(F("&nbsp;<a href=/?off2468 target=inlineframe>ALL ON</a>")); 
            client.println(F("&nbsp;<a href=/?off3579 target=inlineframe>ALL OFF</a>


                      // mousedown buttons
          client.println(F("<input type=button value=ON onmousedown=location.href='/?on4;' target=inlineframe>")); 
          client.println(F("<input type=button value=OFF onmousedown=location.href='/?off5;' target=inlineframe>"));        
          client.println(F("&nbsp;<input type=button value='ALL OFF' onmousedown=location.href='/?off3579;' target=inlineframe>

          // mousedown radio buttons
          client.println(F("<input type=radio onmousedown=location.href='/?on6;' target=inlineframe>ON</>")); 
          client.println(F("<input type=radio onmousedown=location.href='/?off7; target=inlineframe'>OFF</>")); 
          client.println(F("&nbsp;<input type=radio onmousedown=location.href='/?off3579;' target=inlineframe>ALL OFF</>

          // custom buttons
          client.print(F("<input type=submit value=ON target=inlineframe style=width:100px;height:45px onClick=location.href='/?on8;'>"));
          client.print(F("<input type=submit value=OFF target=inlineframe style=width:100px;height:45px onClick=location.href='/?off9;' target=inlineframe>"));
          client.print(F("&nbsp;<input type=submit value='ALL OFF' target=inlineframe style=width:100px;height:45px onClick=location.href='/?off3579;' target=inlineframe>"));

            client.println(F("<IFRAME name=inlineframe style='display:none'>"));          


          //stopping client

          ///////////////////// control arduino pin
          if(readString.indexOf('2') >0)//checks for 2
            digitalWrite(5, HIGH);    // set pin 5 high
            Serial.println("Led 5 On");
          if(readString.indexOf('3') >0)//checks for 3
            digitalWrite(5, LOW);    // set pin 5 low
            Serial.println("Led 5 Off");
          if(readString.indexOf('4') >0)//checks for 4
            digitalWrite(6, HIGH);    // set pin 6 high
            Serial.println("Led 6 On");
          if(readString.indexOf('5') >0)//checks for 5
            digitalWrite(6, LOW);    // set pin 6 low
            Serial.println("Led 6 Off");
          if(readString.indexOf('6') >0)//checks for 6
            digitalWrite(7, HIGH);    // set pin 7 high
            Serial.println("Led 7 On");
          if(readString.indexOf('7') >0)//checks for 7
            digitalWrite(7, LOW);    // set pin 7 low
            Serial.println("Led 7 Off");
          if(readString.indexOf('8') >0)//checks for 8
            digitalWrite(8, HIGH);    // set pin 8 high
            Serial.println("Led 8 On");
          if(readString.indexOf('9') >0)//checks for 9
            digitalWrite(8, LOW);    // set pin 8 low
            Serial.println("Led 8 Off");

          //clearing string for next read


void sendGET() //client function to send and receive GET data from external server.
  if (client.connect(serverName, 80)) {
    client.println("GET /~shb/arduino.txt HTTP/1.0");
  else {
    Serial.println("connection failed");

  while(client.connected() && !client.available()) delay(1); //waits for data
  while (client.connected() || client.available()) { //connected or data available
    char c =;