DnsAnswer.cs

namespace DnsMessageParser;

public record DnsAnswerItem(string DomainName, DnsType DnsType, DnsClass DnsClass, uint TimeToLive, string Data);

public class DnsAnswer(byte[] udpPacket)
{
    public readonly List<DnsAnswerItem> DnsAnswers = ParseDnsAnswers(udpPacket);

    public void AddDnsAnswer(DnsAnswerItem dnsAnswerItem)
    {
        DnsAnswers.Add(dnsAnswerItem);
    }

    private static List<DnsAnswerItem> ParseDnsAnswers(byte[] udpPacket)
    {
        var dnsAnswers = new List<DnsAnswerItem>();
    
        var position = 12;
        position = udpPacket.QuestionSectionSkippedPosition(position);
        
        var answerCount = (udpPacket[6] << 8) | udpPacket[7];
        for (var i = 0; i < answerCount; i++)
        {
            // Read domain name (can be compressed)
            var domainName = udpPacket.ReadDomainName(ref position);

            // Read DNS Type (2 bytes)
            position += 2;
            // Read DNS Class (2 bytes)
            position += 2;
            // Read TTL (4 bytes)
            var ttl = (uint)((udpPacket[position] << 24) | (udpPacket[position + 1] << 16) |
                             (udpPacket[position + 2] << 8) | udpPacket[position + 3]);
            position += 4;

            // Read data length (2 bytes)
            var dataLength = (ushort)((udpPacket[position] << 8) | udpPacket[position + 1]);
            position += 2;

            var ipAddress =
                $"{udpPacket[position]}." +
                $"{udpPacket[position + 1]}." +
                $"{udpPacket[position + 2]}." +
                $"{udpPacket[position + 3]}";

            dnsAnswers.Add(new DnsAnswerItem(domainName, DnsType.A, DnsClass.In, ttl, ipAddress));
            position += dataLength;
        }

        return dnsAnswers;
    }
}