Ethical Hacking Institute Course in Pune-India
Extreme Hacking | Sadik Shaikh | Cyber Suraksha Abhiyan

This article describes a very basic one-way Client and Server setup where a Client connects, sends messages to server and the server shows them using socket connection. There’s a lot of low-level stuff that needs to happen for these things to work but the Java API networking package (java.net) takes care of all of that, making network programming very easy for programmers.

Client Side Programming

Establish a Socket Connection

To connect to other machine we need a socket connection. A socket connection means the two machines have information about each other’s network location (IP Address) and TCP port.The java.net.Socket class represents a Socket. To open a socket:

Socket socket = new Socket(“127.0.0.1”, 5000)
  • First argument – IP address of Server. ( 127.0.0.1  is the IP address of localhost, where code will run on single stand-alone machine).
  • Second argument – TCP Port. (Just a number representing which application to run on a server. For example, HTTP runs on port 80. Port number can be from 0 to 65535)

Communication

To communicate over a socket connection, streams are used to both input and output the data.

Closing the connection

The socket connection is closed explicitly once the message to server is sent.

In the program, Client keeps reading input from user and sends to the server until “Over” is typed.

Java Implementation

// A Java program for a Client
import java.net.*;
import java.io.*;
publicclassClient
{
    // initialize socket and input output streams
    privateSocket socket            = null;
    privateDataInputStream  input   = null;
    privateDataOutputStream out     = null;
    // constructor to put ip address and port
    publicClient(String address, intport)
    {
        // establish a connection
        try
        {
            socket = newSocket(address, port);
            System.out.println("Connected");
            // takes input from terminal
            input  = newDataInputStream(System.in);
            // sends output to the socket
            out    = newDataOutputStream(socket.getOutputStream());
        }
        catch(UnknownHostException u)
        {
            System.out.println(u);
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
        // string to read message from input
        String line = "";
        // keep reading until "Over" is input
        while(!line.equals("Over"))
        {
            try
            {
                line = input.readLine();
                out.writeUTF(line);
            }
            catch(IOException i)
            {
                System.out.println(i);
            }
        }
        // close the connection
        try
        {
            input.close();
            out.close();
            socket.close();
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
    }
    publicstaticvoidmain(String args[])
    {
        Client client = newClient("127.0.0.1", 5000);
    }
}

Establish a Socket Connection

To write a server application two sockets are needed.

  • A ServerSocket which waits for the client requests (when a client makes a new Socket())
  • A plain old Socket socket to use for communication with the client.

Communication

getOutputStream() method is used to send the output through the socket.

Close the Connection

After finishing,  it is important to close the connection by closing the socket as well as input/output streams.

// A Java program for a Server
importjava.net.*;
importjava.io.*;
publicclassServer
{
    //initialize socket and input stream
    privateSocket          socket   = null;
    privateServerSocket    server   = null;
    privateDataInputStream in       =  null;
    // constructor with port
    publicServer(intport)
    {
        // starts server and waits for a connection
        try
        {
            server = newServerSocket(port);
            System.out.println("Server started");
            System.out.println("Waiting for a client ...");
            socket = server.accept();
            System.out.println("Client accepted");
            // takes input from the client socket
            in = newDataInputStream(
                newBufferedInputStream(socket.getInputStream()));
            String line = "";
            // reads message from client until "Over" is sent
            while(!line.equals("Over"))
            {
                try
                {
                    line = in.readUTF();
                    System.out.println(line);
                }
                catch(IOException i)
                {
                    System.out.println(i);
                }
            }
            System.out.println("Closing connection");
            // close connection
            socket.close();
            in.close();
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
    }
    publicstaticvoidmain(String args[])
    {
        Server server = newServer(5000);
    }
}

 

Important Points

  • Server application makes a ServerSocket on a specific port which is 5000. This starts our Server listening for client requests coming in for port 5000.
  • Then Server makes a new Socket to communicate with the client.
socket = server.accept()
  • The accept() method blocks(just sits there) until a client connects to the server.
  • Then we take input from the socket using getInputStream() method. Our Server keeps receiving messages until the Client sends “Over”.
  • After we’re done we close the connection by closing the socket and the input stream.
  • To run the Client and Server application on your machine, compile both of them. Then first run the server application and then run the Client application.

To run on Terminal or Command Prompt

Open two windows one for Server and another for Client

1. First run the Server application as ,

$ java Server

Server started
Waiting for a client …

2. Then run the Client application on another terminal as,

$ java Client

It will show – Connected and the server accepts the client and shows,

Client accepted

3. Then you can start typing messages in the Client window. Here is a sample input to the Client

Hello
I made my first socket connection
Over

Which the Server simultaneously receives and shows,

Hello
I made my first socket connection
Over
Closing connection

Notice that sending “Over” closes the connection between the Client and the Server just like said before.

If you’re using Eclipse or likes of such-

  1.  Compile both of them on two different terminals or tabs
  2.  Run the Server program first
  3.  Then run the Client program
  4.  Type messages in the Client Window which will be received and showed by the Server Window simultaneously.
  5.  Type Over to end.

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