# Strange test #YC9, C++

**URL:** <https://forum.codecrafters.io/t/strange-test-yc9-c/4408>\
**Category:** Bug Reports\
**Tags:** challenge:dns-server\
**Created:** [January 23, 2025, 2:27am UTC](https://forum.codecrafters.io/t/strange-test-yc9-c/4408 "2025-01-23T02:27:00Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Germenzi](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/germenzi/32/4485_2.png) [@Germenzi](https://forum.codecrafters.io/u/Germenzi)\
**Post date:** [January 23, 2025, 2:27am UTC](https://forum.codecrafters.io/t/strange-test-yc9-c/4408/1 "2025-01-23T02:27:00Z")

</div>

I’m stuck(???) on Stage #YC9.

There is a very strange thing I found for me. Some days ago, I successfully passed test #YC9 about parsing compressed packets in DNS server challenge. Today, I started doing next stage, #GT1, about forwarding messages and noticed, that my code passes #GT1 but doesn’t pass #YC9. After some hours of debugging, I found that problem is in parsing compressed question. Also, I found that my program breaks at pointer octet, and I logged it. It looks like this: `11000000`, so the pointer is 0 and programs goes to the begging of the buffer, thinking that it is a label sequence.

I rolled back my code to the last commit that passes previous stage (#YC9), and the programs had broken again! In my logs below you can see that octet at 47 index is `11000000`, and if you calculate octets, you will see that exactly this octet must be a pointer (12 to header + 1 + 3 + 1 + 17 + 1 + 3 + 1 + 2 + 2 +1 + 3 = 47, so 46th octet is the `f`, last octet of the `def` label). It goes after label, it is not ASCII symbol (so there is no doubts about this octet is not a part of label) and it begins with two 1.

In newer code (not as below) I found myself believing that I don’t modify the buffer I parsing by marking every buffer pointer by `const`, so the buffer is exactly how it was read.

So my question is: other people (and me, lol) passed this test. Can someone check him test? Am I doing something wrong?

Here are my logs (previous tests have been passed):

```auto
[33m[tester::#YC9] [94mRunning tests for Stage #YC9 (Parse compressed packet)
[33m[tester::#YC9] [94mStarting DNS server on 127.0.0.1:2053
[33m[tester::#YC9] [94mConnecting to 127.0.0.1:2053 using UDP
[33m[your_program] Logs from your program will appear here!
[33m[your_program] 
[33m[your_program] Request question labels (1):
[33m[your_program] codecrafters.io.
[33m[your_program] Written 64 bytes
[33m[tester::#YC9] [94mQuerying the following in the same request (Messages with >> prefix are part of this log)
[33m[tester::#YC9] [94m>> ;abc.longassdomainname.com.	IN A
[33m[tester::#YC9] [94m>> ;def.longassdomainname.com.	IN A
[33m[tester::#YC9] [91mDNS query failed: dns: bad rdata.
[33m[tester::#YC9] [91mIf you are seeing this after a while then it is likely that your server is not responding with appropriate id
[33m[tester::#YC9] [91mTest failed (try setting 'debug: true' in your codecrafters.yml to see more details)
[33m[your_program] 
[33m[your_program] BYTE: 47
[33m[your_program] BITSET: 11000000
[33m[your_program] PTR: 0
[33m[your_program] Request question labels (2):
[33m[your_program] abc.longassdomainname.com.
[33m[your_program] def.�

```

BYTE and so on was written when my code faced pointer byte. Below, you can see my parsing code:

```c++

inline bool byte_get_bit(std::byte byte, char bit) {
    std::byte mask = std::byte{1} << bit;
    return (byte & mask) != std::byte{0};
}

DNS::dns_message DNS::dns_message::parse_from_buffer(void *buffer) {
    dns_message result {};

    std::byte *byte_buf = reinterpret_cast<std::byte*>(buffer);

    result.packet_id = ntohs(reinterpret_cast<std::uint16_t*>(byte_buf)[0]);
    result.flag_qr = byte_get_bit(byte_buf[2], 7);
    result.opcode = std::to_integer<std::uint8_t>((byte_buf[2] & std::byte{0b0111'1000}) >> 3);
    result.flag_aa = byte_get_bit(byte_buf[2], 2);
    result.flag_tc = byte_get_bit(byte_buf[2], 1);
    result.flag_rd = byte_get_bit(byte_buf[2], 0);
    result.flag_ra = byte_get_bit(byte_buf[3], 7);
    result.reserved = std::to_integer<std::uint8_t>((byte_buf[3] & std::byte{0b0111'0000}) >> 4);
    result.rcode = std::to_integer<std::uint8_t>(byte_buf[3] & std::byte{0b0000'1111});

    std::uint16_t qdcount = ntohs(reinterpret_cast<std::uint16_t*>(byte_buf+4)[0]);
    std::uint16_t ancount = ntohs(reinterpret_cast<std::uint16_t*>(byte_buf+6)[0]); // to parse answers
    result.nscount_ = ntohs(reinterpret_cast<std::uint16_t*>(byte_buf+8)[0]);
    result.arcount_ = ntohs(reinterpret_cast<std::uint16_t*>(byte_buf+10)[0]);

    std::byte *read_byte = byte_buf + 12;
    for (int i = 0; i < qdcount; ++i) {
        std::byte *old_read_byte_compressed;
        bool read_compressed = false;
        dns_question q;

        bool first_write = true;
        while (*read_byte != std::byte{0}) { // WARNING: memory issue if invalid packet format
            if ((*read_byte & std::byte{0xC0}) == std::byte{0xC0}) {
                std::uint8_t ptr = std::to_integer<std::uint8_t>(*read_byte & std::byte{0x3F});

                if (first_write) {
                    std::cout << "BYTE: " << read_byte - byte_buf << std::endl;
                    std::cout << "BITSET: " << std::bitset<8>((std::uint8_t)*read_byte) << std::endl;
                    std::cout << "PTR: " << (int)ptr << std::endl;
                    first_write = false;
                }

                old_read_byte_compressed = read_byte;
                read_byte = byte_buf + ptr;
                read_compressed = true;
            } 

            int label_size = std::to_integer<int>(*read_byte++);
            std::string label(label_size, '\0');
            for (char &ch : label) ch = std::to_integer<int>(*read_byte++);
            q.labels.push_back(label);
        }

        if (read_compressed) read_byte = old_read_byte_compressed;

        ++read_byte; // skip zero octet or pointer octet

        q.qtype = ntohs(reinterpret_cast<std::uint16_t*>(read_byte)[0]);
        read_byte += 2;

        q.qclass = ntohs(reinterpret_cast<std::uint16_t*>(read_byte)[0]);
        read_byte += 2;

        result.questions.push_back(q);
    }

    return result;
}

```

It is used simply like this:

```c++
        // (In while)
        // Receive data
        bytesRead = recvfrom(udpSocket, buffer, sizeof(buffer), 0, reinterpret_cast<struct sockaddr*>(&clientAddress), &clientAddrLen);
        if (bytesRead == -1) {
            perror("Error receiving data");
            break;
        }

        DNS::dns_message request = DNS::dns_message::parse_from_buffer(buffer);

```

I feel that there must be something simple, but… I can’t figure it out.

---

<div class="post-metadata">

**Author:** ![andy1li](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/andy1li/32/10_2.png) [@andy1li](https://forum.codecrafters.io/u/andy1li)\
**Post date:** [January 23, 2025, 1:22pm UTC](https://forum.codecrafters.io/t/strange-test-yc9-c/4408/2 "2025-01-23T13:22:02Z")

</div>

Hi @Germenzi, thanks for highlighting the issue! We’ll investigate it and get back to you as soon as possible.

---

<div class="post-metadata">

**Author:** ![andy1li](https://yyz1.discourse-cdn.com/flex003/user_avatar/forum.codecrafters.io/andy1li/32/10_2.png) [@andy1li](https://forum.codecrafters.io/u/andy1li)\
**Post date:** [December 4, 2025, 1:09pm UTC](https://forum.codecrafters.io/t/strange-test-yc9-c/4408/5 "2025-12-04T13:09:14Z")

</div>

We’ve noted this issue and will circle back once we’re working on the DNS challenge. Closing this for now.

---

<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex003/uploads/codecrafters/original/3X/7/0/700657133935c15703e22c7c8870394f6e6dc27d.svg) [@system](https://forum.codecrafters.io/u/system)\
**Post date:** [December 11, 2025, 1:28pm UTC](https://forum.codecrafters.io/t/strange-test-yc9-c/4408/7 "2025-12-11T13:28:03Z")

</div>

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