How to point to specific DNS with custom port in Java

Specifying a custom port to connect to a specific DNS server in Java is an advanced operation that typically involves network programming. To perform this operation in Java, we can use classes from the java.net package, such as InetSocketAddress and Socket.

Step 1: Create an InetSocketAddress instance

This class provides a combination of an IP address and port number for socket binding or connection. You can create this object using a domain name and port number.

Step 2: Use the Socket class to establish a connection

The Socket class creates a client socket that can connect to a specified IP address and port number via an InetSocketAddress instance.

Example Code

The following is a simple Java program demonstrating how to connect to a specific domain name and port number:

java
import java.net.InetSocketAddress;
import java.net.Socket;

public class DNSWithCustomPort {
    public static void main(String[] args) {
        String host = "example.com"; // Domain name to connect to
        int port = 12345; // Custom port number

        // Create a socket address object without immediate DNS resolution
        InetSocketAddress socketAddress = new InetSocketAddress(host, port);

        try (Socket socket = new Socket()) {
            // Connect to the remote address
            socket.connect(socketAddress);
            System.out.println("Successfully connected to " + host + " on port " + port);
        } catch (Exception e) {
            System.err.println("Failed to connect to the specified server: " + e.getMessage());
        }
    }
}

Notes

1. Error Handling: In network programming, properly handling network errors is essential. For instance, in the provided code, we utilize the try-with-resources statement to automatically close the socket and handle exceptions.

2. Network Permissions: When using privileged ports (typically below port 1024), administrator permissions may be necessary.

3. DNS Resolution: The InetSocketAddress class allows you to specify whether DNS resolution occurs immediately upon creation. For deferred resolution (e.g., resolving at connection time), you can use InetSocketAddress.createUnresolved(host, port).

By using this approach, you can establish network connections to specific domain names and ports in Java, which is highly useful for developing network applications or client-server models.