[DNS Server] stage 7 (#YC9) passed but the response is malformed

Platform Bug Reports
1

My code passed #YC9 even if the answer section doesn’t contain proper response

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"net"
)

/*
*	 0   1 2 3 4    5  6  7  8   9 10 11   12 13 14 15
*  |QR|   Opcode   |AA|TC|RD|RA|    Z    |    RCODE   |
*
*	QR      	Query/Response Indicator, 1 bit
* 	OPCODE  	Operation Code, 4 bit
* 	AA      	Authoritative Answer, 1 bit
* 	TC      	Truncation, 1 bit
* 	RD      	Recursion Desired, 1 bit
* 	RA      	Recursion Available, 1 bit
* 	Z       	Reserved, 3 bit
* 	RCODE   	Response Code, 4 bit
 */
type DNSHeader struct {
	ID      uint16 // Packet Identifier (bytes 0-1)
	FLAGS   uint16 // Bits in the middle packed into a 2 byte flag (bytes 2-3)
	QDCOUNT uint16 // Question Count (bytes 4-5)
	ANCOUNT uint16 // Answer Record Count (bytes 6-7)
	NSCOUNT uint16 // Authority Record Count (bytes 8-9)
	ARCOUNT uint16 // Additional Record Count (bytes 10-11)
}

func (h *DNSHeader) ToBytes() []byte {
	buf := new(bytes.Buffer)
	binary.Write(buf, binary.BigEndian, h)
	return buf.Bytes()
}

func main() {
	udpAddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:2053")
	if err != nil {
		fmt.Println("Failed to resolve UDP address:", err)
		return
	}

	udpConn, err := net.ListenUDP("udp", udpAddr)
	if err != nil {
		fmt.Println("Failed to bind to address:", err)
		return
	}
	defer udpConn.Close()

	buf := make([]byte, 512)

	for {
		_, source, err := udpConn.ReadFromUDP(buf)
		if err != nil {
			fmt.Println("Error receiving data:", err)
			break
		}

		header := buf[:12]
		// Set QR bit to make it a response
		header[2] |= 0x80 // 10000000 in binary

		flags := binary.BigEndian.Uint16(header[2:4])
		opcode := (flags >> 11) & 0x0F // 0x0F = 0b1111 in binary, can also (flags >> 11) & 15
		if opcode != 0 {
			// Clear the bottom 4 bits (RCODE) and set to 4
			flags = (flags & 0xFFF0) | 4                   // 0xFFF0 = 0b1111111111110000
			binary.BigEndian.PutUint16(header[2:4], flags) // Write the updated flags back to the header
		}

		qdCount := binary.BigEndian.Uint16(header[4:6])
		binary.BigEndian.PutUint16(header[6:8], qdCount) // Set ANCOUNT (bytes 6-7)

		names, questionEnd := getQuestionEnd(buf, qdCount)
		question := buf[12:questionEnd]

		answer := []byte{}
		for _, name := range names {
			answer = append(answer, name...)                   // Copy name first
			answer = binary.BigEndian.AppendUint16(answer, 1)  // Type: 1 (A record)
			answer = binary.BigEndian.AppendUint16(answer, 1)  // Class: 1 (IN)
			answer = binary.BigEndian.AppendUint32(answer, 60) // TTL: 60
			answer = binary.BigEndian.AppendUint16(answer, 4)  // RDLENGTH: 4 bytes
			answer = append(answer, 8, 8, 8, 8)                // RDATA: 8.8.8.8
		}

		response := bytes.Join([][]byte{header, question, answer}, nil)

		_, err = udpConn.WriteToUDP(response, source)
		if err != nil {
			fmt.Println("Failed to send response:", err)
		}
	}
}

func getQuestionEnd(buf []byte, qdCount uint16) ([][]byte, int) {
	pos := 12 // start after header

	currQd := 0
	names := [][]byte{}
	nameMap := make(map[int][]byte)

	// iterate through domain labels till we get a null terminator
	for currQd != int(qdCount) {
		// if 1st 2 bit of 1st byte is 11, then it is pointer, the next byte is label start pos
		// if 1st 2 bit of 1st byte is 00, then treat it as length of label
		start := pos
		firstByte := buf[pos]
		if firstByte == 0xc0 { // 0b1100_0000
			pos += 1
			names = append(names, nameMap[int(buf[pos])])
			pos += 1
		} else if firstByte != 0 {
			for buf[pos] != 0 {
				labelLen := int(buf[pos])
				pos += labelLen + 1
			}

			pos += 1                        // null terminator itself
			nameMap[start] = buf[start:pos] // domain name with
			fmt.Println("found name:", string(nameMap[start]))
			names = append(names, nameMap[start])
		}

		currQd++
		pos += 4 // QTYPE and QCLASS
	}

	return names, pos
}

[tester::#YC9] Querying the following in the same request (Messages with >> prefix are part of this log)
[tester::#YC9] >> ;abc.longassdomainname.com. IN A
[tester::#YC9] >> ;def.longassdomainname.com. IN A
[tester::#YC9] Sending Request: (Messages with >>> prefix are part of this log)
[tester::#YC9] >>> ;; opcode: QUERY, status: NOERROR, id: 28447
[tester::#YC9] >>> ;; flags: rd; QUERY: 2, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
[tester::#YC9] >>>
[tester::#YC9] >>> ;; QUESTION SECTION:
[tester::#YC9] >>> ;abc.longassdomainname.com. IN A
[tester::#YC9] >>> ;def.longassdomainname.com. IN A
[tester::#YC9] >>>
[your_program] found name: abclongassdomainnamecom
[your_program] found name: def�
[tester::#YC9] Received Response: (Messages with >>> prefix are part of this log)
[tester::#YC9] >>> ;; opcode: QUERY, status: NOERROR, id: 28447
[tester::#YC9] >>> ;; flags: qr rd; QUERY: 2, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0
[tester::#YC9] >>>
[tester::#YC9] >>> ;; QUESTION SECTION:
[tester::#YC9] >>> ;abc.longassdomainname.com. IN A
[tester::#YC9] >>> ;def.longassdomainname.com. IN A
[tester::#YC9] >>>
[tester::#YC9] >>> ;; ANSWER SECTION:
[tester::#YC9] >>> . 0 CLASS0 TYPE0 # 0
[tester::#YC9] >>> . 0 CLASS0 TYPE0 # 0
[tester::#YC9] >>>
[tester::#YC9] Test passed.
[tester::#YC9] Terminating program
[tester::#YC9] Shutting down DNS resolver server…
[tester::#YC9] Program terminated successfully