/*
 * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "DNSResponse.h"
#include "DNSPacket.h"
#include "DNSRequest.h"
#include <AK/IPv4Address.h>
#include <AK/StringBuilder.h>

static String parse_dns_name(const u8* data, size_t& offset, size_t max_offset, size_t recursion_level = 0);

class [[gnu::packed]] DNSRecordWithoutName {
public:
    DNSRecordWithoutName() { }

    u16 type() const { return m_type; }
    u16 record_class() const { return m_class; }
    u32 ttl() const { return m_ttl; }
    u16 data_length() const { return m_data_length; }

    void* data() { return this + 1; }
    const void* data() const { return this + 1; }

private:
    NetworkOrdered<u16> m_type;
    NetworkOrdered<u16> m_class;
    NetworkOrdered<u32> m_ttl;
    NetworkOrdered<u16> m_data_length;
};

static_assert(sizeof(DNSRecordWithoutName) == 10);

Optional<DNSResponse> DNSResponse::from_raw_response(const u8* raw_data, size_t raw_size)
{
    if (raw_size < sizeof(DNSPacket)) {
        dbgln("DNS response not large enough ({} out of {}) to be a DNS packet.", raw_size, sizeof(DNSPacket));
        return {};
    }

    auto& response_header = *(const DNSPacket*)(raw_data);
#ifdef LOOKUPSERVER_DEBUG
    dbgln("Got response (ID: {})", response_header.id());
    dbgln("  Question count: {}", response_header.question_count());
    dbgln("    Answer count: {}", response_header.answer_count());
    dbgln(" Authority count: {}", response_header.authority_count());
    dbgln("Additional count: {}", response_header.additional_count());
#endif

    DNSResponse response;
    response.m_id = response_header.id();
    response.m_code = response_header.response_code();

    if (response.code() != DNSResponse::Code::NOERROR)
        return response;

    size_t offset = sizeof(DNSPacket);

    for (u16 i = 0; i < response_header.question_count(); ++i) {
        auto name = parse_dns_name(raw_data, offset, raw_size);
        struct RawDNSAnswerQuestion {
            NetworkOrdered<u16> record_type;
            NetworkOrdered<u16> class_code;
        };
        auto& record_and_class = *(const RawDNSAnswerQuestion*)&raw_data[offset];
        response.m_questions.empend(name, record_and_class.record_type, record_and_class.class_code);
        offset += 4;
#ifdef LOOKUPSERVER_DEBUG
        auto& question = response.m_questions.last();
        dbgln("Question #{}: name=_{}_, type={}, class={}", i, question.name(), question.record_type(), question.class_code());
#endif
    }

    for (u16 i = 0; i < response_header.answer_count(); ++i) {
        auto name = parse_dns_name(raw_data, offset, raw_size);

        auto& record = *(const DNSRecordWithoutName*)(&raw_data[offset]);

        String data;

        offset += sizeof(DNSRecordWithoutName);
        if (record.type() == T_PTR) {
            size_t dummy_offset = offset;
            data = parse_dns_name(raw_data, dummy_offset, raw_size);
        } else if (record.type() == T_A) {
            auto ipv4_address = IPv4Address((const u8*)record.data());
            data = ipv4_address.to_string();
        } else {
            // FIXME: Parse some other record types perhaps?
            dbgln("data=(unimplemented record type {})", record.type());
        }
#ifdef LOOKUPSERVER_DEBUG
        dbgln("Answer   #{}: name=_{}_, type={}, ttl={}, length={}, data=_{}_", i, name, record.type(), record.ttl(), record.data_length(), data);
#endif
        response.m_answers.empend(name, record.type(), record.record_class(), record.ttl(), data);
        offset += record.data_length();
    }

    return response;
}

String parse_dns_name(const u8* data, size_t& offset, size_t max_offset, size_t recursion_level)
{
    if (recursion_level > 4)
        return {};
    Vector<char, 128> buf;
    while (offset < max_offset) {
        u8 ch = data[offset];
        if (ch == '\0') {
            ++offset;
            break;
        }
        if ((ch & 0xc0) == 0xc0) {
            if ((offset + 1) >= max_offset)
                return {};
            size_t dummy = (ch & 0x3f) << 8 | data[offset + 1];
            offset += 2;
            StringBuilder builder;
            builder.append(buf.data(), buf.size());
            auto okay = parse_dns_name(data, dummy, max_offset, recursion_level + 1);
            builder.append(okay);
            return builder.to_string();
        }
        for (size_t i = 0; i < ch; ++i)
            buf.append(data[offset + i + 1]);
        buf.append('.');
        offset += ch + 1;
    }
    return String::copy(buf);
}