Ethernet Client and Server

Hi!
I have an Arduino Uno with an Ethernet Shield and now it's running an program that every half hour does an "client.print" to an php page in my web site with the sensors values.
But now, I want to be able to at any time send a command to the Arduino via Ethernet form him to send the current sensors values at that time.

I know i can put the arduino running as a webserver and throw it's IP address I can send the command, but is it able to act as a client and a server at the same time?

thanks!

but is it able to act as a client and a server at the same time?

Of course not. Nothing can.

But it can be a server most of the time, accepting and acting on client requests, while also acting as a client as required.

Combined client/server test code.

//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 pins high/low
//use the ' in html instead of " to prevent having to escape the "
//address will look like http://192.168.1.102:84 when submited
//for use with W5100 based ethernet shields

#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[] = "web.comporium.net"; // (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
  Ethernet.begin(mac,ip,gateway,gateway,subnet); 
  server.begin();
  Serial.begin(9600); 
  Serial.println(F("server/client 1.0 test 7/03/12")); // keep track of what is loaded
  Serial.println(F("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 = Serial.read();
    if(inChar == 'g')
    {
      sendGET(); // call client sendGET function
    }
  }  

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

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

          //store characters to string 
          readString += c; 
          //Serial.print(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"));
            client.println();
            client.println();  
          }
          else {   
            client.println(F("HTTP/1.1 200 OK")); //send new page on browser request
            client.println(F("Content-Type: text/html"));
            client.println();

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

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

            // DIY buttons
            client.println(F("Pin5"));
            client.println(F("<a href=/?on2 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off3 target=inlineframe>OFF</a>

")); 

            client.println(F("Pin6"));
            client.println(F("<a href=/?on4 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off5 target=inlineframe>OFF</a>

")); 

            client.println(F("Pin7"));
            client.println(F("<a href=/?on6 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off7 target=inlineframe>OFF</a>

")); 

            client.println(F("Pin8"));
            client.println(F("<a href=/?on8 target=inlineframe>ON</a>")); 
            client.println(F("<a href=/?off9 target=inlineframe>OFF</a>

")); 

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

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

            client.println(F("</BODY>"));
            client.println(F("</HTML>"));
          }

          delay(1);
          //stopping client
          client.stop();

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

          //clearing string for next read
          readString="";

        }
      }
    }
  }
} 

//////////////////////////
void sendGET() //client function to send and receive GET data from external server.
{
  if (client.connect(serverName, 80)) {
    Serial.println(F("connected"));
    client.println(F("GET /~shb/arduino.txt HTTP/1.1"));
    client.println(F("Host: web.comporium.net"));
    client.println(F("Connection: close"));
    client.println();
  } 
  else {
    Serial.println(F("connection failed"));
    Serial.println();
  }

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

  Serial.println();
  Serial.println(F("disconnecting."));
  Serial.println(F("=================="));
  Serial.println();
  client.stop();

}

Thanks! it does seems to work. but only one at a time.
I found the problem and, if i have:

Ethernet.begin(mac,ip,gateway,gateway,subnet); 
server.begin();

i can open the arduino web site with the ip but can't connect to the char serverName[] = "web.comporium.net";
.

and if i put

if (Ethernet.begin(mac) == 0) {
Ethernet.begin(mac,ip,gateway,gateway,subnet); 
server.begin();

}

i can connect to the serverName but cant acess the arduino web site....

can you help me? thanks!

I retried the code I posted and it works as I expect it to. Not really sure what your expectations are.