Java networking - how to permanently ban IP address?

See more:

Java

server

networking

I'm making a program where the user logs into the server with a username and password only the server knows. They have 4 tries to get the correct username and password. If they do not enter the correct login information in 4 tries, the server will close connection to the client.

The next part of the program which I need help with is permanently banning the user from connecting for further attempts. When the user is logging in for the first time and gets all 4 attempts wrong, their ip address is written to a file called "userIP.txt".

What I tried to do was read the file and if it matches the user's IP address, they will be banned from the program. It doesn't work - when they come back to the program it lets them log in again.
I know this may not be the best way to ban a user from the server, but my assignment requires me to ban their ip address.

Any ideas how I can fix this?

Here is part of the server code:

What I have tried:

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

class Server {
	public static void main(String args[]) throws FileNotFoundException {
		String welcome = "Welcome! The server is now connected.";
		String login = "Enter username and password: ";
		String message; 
		PrintWriter writer = new PrintWriter("userIP.txt");

		try {
			//Detecting the localhost's ip address
			InetAddress localaddr = InetAddress.getLocalHost();
			System.out.println("SERVER\n");
			System.out.println ("Local hostnameIP: " + localaddr );

			// Creating a server socket for connection
			ServerSocket srvr = new ServerSocket(1234);
			System.out.println("Waiting for connection on "+localaddr);
			// Accept incoming connection
			Socket skt = srvr.accept();
			System.out.print("Server has connected!\n");
			// get Input and Output streams
			PrintWriter out = new PrintWriter(skt.getOutputStream(), true);
			out.flush();
			BufferedReader in = new BufferedReader(new InputStreamReader(skt.getInputStream()));
			BufferedReader log = new BufferedReader(new InputStreamReader(skt.getInputStream())); //read input for login
			System.out.print("Sending string: '" + welcome + "'\n");
			out.println(welcome);
			InetAddress clientInetAddress = skt.getInetAddress();
			String ip = clientInetAddress.getHostAddress();
			
			//read file

			String checkIP = "userIP.txt";
			String line = null;
			try {
				FileReader readFile = new FileReader (checkIP);
				BufferedReader br = new BufferedReader (readFile);
				while ((line = br.readLine())!= null) {
					System.out.println("reading file: " + line);
					
					if (line.equals(ip)) {
						System.out.println("IP MATCHES");
						
						//closing server
//						out.println("You are banned. Server closing.");
//						out.close();
//						skt.close();
//						srvr.close();
					}
				}
				br.close();
				
				
			}
			catch (FileNotFoundException ex) {
				System.out.println("Unable to open file '" + checkIP + "'");
			}
			catch(IOException ex) {
				System.out.println("Error reading file '" + checkIP + "'");
			}
			
			//login attempts
			int tries = 4;
			while (tries>0) {
				out.println(login);
				
				//login
				String username = in.readLine();
				System.out.println("Client's username: " + username);

				String password = in.readLine();
				System.out.println("Client's password: " + password);

				if (username.equals("hello123") && password.equals("mypass")) {
					out.println("Correct login!");
					System.out.println ("Client's IP Address: " + ip);
					tries=-1;
				}
				
				else  { //if wrong login - give 3 more tries
					
					tries--;
					System.out.println("Number of tries left: " + tries);
					out.println("Try again. Login attempts left - " + tries);
					
				}
			}
			
			
				if (tries==0){
				out.println("Wrong login - server closing");
				out.close();
				skt.close();
				srvr.close();
				
				//ban ip address permanently 
				System.out.println("local: " + localaddr.getHostAddress()); 
				System.out.println("client: " + ip);
				 
				 //write ip address to file
				writer.println(ip);
				writer.close();
				
			}