Connecting to the daytime port (13)

The daytime port 13 will return a datagram containing the server time.
On many systems, this port is being disabled, fearing hackers may use it to perform a denial-of-service attack of some sort. Based on the date format that the server returns, the hacker may also find out about the operating system.

Just open a socket and read a line. It will contain the server time if the port is enabled.
DaytimeClient.java:

import java.util.*;
import java.net.*;
import java.io.*;
 
public class DaytimeClient {
   public static void main(String args[]) {
      if (args.length != 1) {
         System.out.println("java DaytimeClient <server>");
         System.exit(1);
      }
 
      try {
         Socket socket = new Socket(args[0], 13);    // 7 is echo port
         BufferedReader br = new BufferedReader(
                                new InputStreamReader(socket.getInputStream()));
  
         String ret = br.readLine();
         System.out.println(ret);
 
         socket.close();       
      }
      catch(Exception e) {
         e.printStackTrace();
      }
   }
}