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Ping a serverTag(s): Networking


It's not possible to really "ping" a machine to check if it's alive or not (it's a long story, but to keep it short I will just say that the Socket class is not low-level enough for that operation). But we can emulate a ping by talking the "echo port". On a server, the echo port is always port 7. We write a string to that port and the server will echo the string. 
import java.io.*;
import java.net.*;

public class PseudoPing {
  public static void main(String args[]) {
    try {
      Socket t = new Socket(args[0], 7);
      DataInputStream dis = new DataInputStream(t.getInputStream());
      PrintStream ps = new PrintStream(t.getOutputStream());
      ps.println("Hello");
      String str = dis.readLine();
      if (str.equals("Hello"))
        System.out.println("Alive!") ;
      else
        System.out.println("Dead or echo port not responding");              
      t.close();
    }
    catch (IOException e) {
      e.printStackTrace();}
    }
  }
}
NOTE: To make this a more "complete PING", you may want to check this How-to to display the response time.

Since JDK1.5, java.net.InetAddress.isReachable(int) can be used to check if a server is reachable or not. 

import java.io.*;
import java.net.*;

public class ReachableTest {
 public static void main(String args[]) {
     try {
       InetAddress address = InetAddress.getByName("web.mit.edu");
       System.out.println("Name: " + address.getHostName());
       System.out.println("Addr: " + address.getHostAddress());
       System.out.println("Reach: " + address.isReachable(3000));
     }
     catch (UnknownHostException e) {
       System.err.println("Unable to lookup web.mit.edu");
     }
     catch (IOException e) {
       System.err.println("Unable to reach web.mit.edu");
     }
   }
}
isReachable() will use ICMP ECHO REQUESTs if the privilege can be obtained, otherwise it will try to establish a TCP connection on port 7 (Echo) of the destination host. But most Internet sites have disabled the service or blocked the requests (except some university web sites like the example above) .