DNS SERVER:Phrase question section help

[swxsy]

I’m stuck on Stage #HD8.

Here are my logs:

And here’s a snippet of my code
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;

public class Main {
public static void main(String args) {
System.out.println(“Logs from your program will appear here!”);

    try (DatagramSocket serverSocket = new DatagramSocket(2053)) {
        while (true) {
            final byte[] buf = new byte[512];
            final DatagramPacket packet = new DatagramPacket(buf, buf.length);
            serverSocket.receive(packet);
            System.out.println("Received data");

            ByteBuffer requestBuffer = ByteBuffer.wrap(buf);
            requestBuffer.order(ByteOrder.BIG_ENDIAN);

            // Parse DNS header
            short transactionID = requestBuffer.getShort();
            short flags = requestBuffer.getShort();
            short qdCount = requestBuffer.getShort();
            short anCount = requestBuffer.getShort();
            short nsCount = requestBuffer.getShort();
            short arCount = requestBuffer.getShort();

            System.out.println("Transaction ID: " + transactionID);

            // Extract the RecursionDesired (RD) flag
            boolean recursionDesired = (flags & 0x0100) != 0;

            // Extract the Opcode from the flags (bits 1–4)
            int opcode = (flags >> 11) & 0xF;

            // Parse DNS question section
            String domainName = parseDomainName(requestBuffer);
            short qType = requestBuffer.getShort();
            short qClass = requestBuffer.getShort();

            System.out.println("Parsed domain name: " + domainName);
            System.out.println("Query Type: " + qType);
            System.out.println("Query Class: " + qClass);

            // Build the response
            ByteBuffer responseBuffer = ByteBuffer.allocate(512);
            responseBuffer.order(ByteOrder.BIG_ENDIAN);

            // Header section
            responseBuffer.putShort(transactionID); // Transaction ID

            // Set the Flags: QR=1 (response), Opcode=opcode, AA=0, TC=0, RD=recursionDesired, RA=1, Z=000, RCODE=0
            short responseFlags = (short) (0x8000 | (opcode << 11) | (recursionDesired ? 0x0100 : 0) | 0x0080); // QR=1, RD=recursionDesired, RA=1
            responseBuffer.putShort(responseFlags);
            
            responseBuffer.putShort(qdCount); // QDCOUNT
            responseBuffer.putShort((short) 1); // ANCOUNT (1 Answer Record)
            responseBuffer.putShort((short) 0); // NSCOUNT
            responseBuffer.putShort((short) 0); // ARCOUNT

            // Question section (echo the received question section)
            responseBuffer.put(domainNameToByteArray(domainName)); // Name
            responseBuffer.putShort(qType); // Type
            responseBuffer.putShort(qClass); // Class

            // Answer section
            responseBuffer.put(domainNameToByteArray(domainName)); // Name
            responseBuffer.putShort((short) 1); // Type (A record)
            responseBuffer.putShort((short) 1); // Class (IN)
            responseBuffer.putInt(60); // TTL (Time-To-Live)
            responseBuffer.putShort((short) 4); // Length of RDATA (4 bytes for IPv4 address)
            responseBuffer.put(new byte[]{(byte) 8, (byte) 8, (byte) 8, (byte) 8}); // RDATA (example IP address 8.8.8.8)

            final byte[] responseData = responseBuffer.array();
            DatagramPacket responsePacket = new DatagramPacket(responseData, responseData.length, packet.getSocketAddress());
            serverSocket.send(responsePacket);
            System.out.println("Response sent");
        }
    } catch (IOException e) {
        System.out.println("IOException: " + e.getMessage());
    }
}

private static String parseDomainName(ByteBuffer buffer) {
    StringBuilder domainName = new StringBuilder();
    int length;

    while ((length = buffer.get() & 0xFF) != 0) {
        if (domainName.length() > 0) {
            domainName.append('.');
        }
        byte[] label = new byte[length];
        buffer.get(label);
        domainName.append(new String(label));
    }

    return domainName.toString();
}

private static byte[] domainNameToByteArray(String domainName) {
    String[] labels = domainName.split("\\.");
    ByteBuffer buffer = ByteBuffer.allocate(domainName.length() + 2);
    for (String label : labels) {
        buffer.put((byte) label.length());
        buffer.put(label.getBytes());
    }
    buffer.put((byte) 0); // Null byte to terminate domain name
    return buffer.array();
}

}

[andy1li]

Looks like your code can pass the latest stage Parse question section #hd8, but cannot pass the previous stage Parse header section #uc8 anymore.

The problem is an incorrect Rcode:

Expected Rcode field to be 4 got 0

I suggest going back and reviewing your implementation of the header logic, particularly how you set the Rcode.

[andy1li]

Closing this thread due to inactivity. If you still need assistance, feel free to reopen or start a new discussion!