/*
Technitium DNS Server
Copyright (C) 2022  Shreyas Zare (shreyas@technitium.com)

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

*/

using DnsServerCore.ApplicationCommon;
using Newtonsoft.Json;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;
using TechnitiumLibrary;
using TechnitiumLibrary.Net;
using TechnitiumLibrary.Net.Dns;
using TechnitiumLibrary.Net.Dns.ResourceRecords;

namespace SplitHorizon
{
    public class SimpleAddress : IDnsApplication, IDnsAppRecordRequestHandler
    {
        #region IDisposable

        public void Dispose()
        {
            //do nothing
        }

        #endregion

        #region public

        public Task InitializeAsync(IDnsServer dnsServer, string config)
        {
            //no config needed
            return Task.CompletedTask;
        }

        public Task<DnsDatagram> ProcessRequestAsync(DnsDatagram request, IPEndPoint remoteEP, DnsTransportProtocol protocol, bool isRecursionAllowed, string zoneName, uint appRecordTtl, string appRecordData)
        {
            DnsQuestionRecord question = request.Question[0];
            switch (question.Type)
            {
                case DnsResourceRecordType.A:
                case DnsResourceRecordType.AAAA:
                    dynamic jsonAppRecordData = JsonConvert.DeserializeObject(appRecordData);
                    dynamic jsonAddresses = null;

                    foreach (dynamic jsonProperty in jsonAppRecordData)
                    {
                        string name = jsonProperty.Name;

                        if ((name == "public") || (name == "private"))
                            continue;

                        NetworkAddress networkAddress = NetworkAddress.Parse(name);
                        if (networkAddress.Contains(remoteEP.Address))
                        {
                            jsonAddresses = jsonProperty.Value;
                            break;
                        }
                    }

                    if (jsonAddresses is null)
                    {
                        if (NetUtilities.IsPrivateIP(remoteEP.Address))
                            jsonAddresses = jsonAppRecordData.@private;
                        else
                            jsonAddresses = jsonAppRecordData.@public;

                        if (jsonAddresses is null)
                            return Task.FromResult<DnsDatagram>(null);
                    }

                    List<DnsResourceRecord> answers = new List<DnsResourceRecord>();

                    switch (question.Type)
                    {
                        case DnsResourceRecordType.A:
                            foreach (dynamic jsonAddress in jsonAddresses)
                            {
                                IPAddress address = IPAddress.Parse(jsonAddress.Value);

                                if (address.AddressFamily == AddressFamily.InterNetwork)
                                    answers.Add(new DnsResourceRecord(question.Name, DnsResourceRecordType.A, DnsClass.IN, appRecordTtl, new DnsARecordData(address)));
                            }
                            break;

                        case DnsResourceRecordType.AAAA:
                            foreach (dynamic jsonAddress in jsonAddresses)
                            {
                                IPAddress address = IPAddress.Parse(jsonAddress.Value);

                                if (address.AddressFamily == AddressFamily.InterNetworkV6)
                                    answers.Add(new DnsResourceRecord(question.Name, DnsResourceRecordType.AAAA, DnsClass.IN, appRecordTtl, new DnsAAAARecordData(address)));
                            }
                            break;
                    }

                    if (answers.Count == 0)
                        return Task.FromResult<DnsDatagram>(null);

                    if (answers.Count > 1)
                        answers.Shuffle();

                    return Task.FromResult(new DnsDatagram(request.Identifier, true, request.OPCODE, true, false, request.RecursionDesired, isRecursionAllowed, false, false, DnsResponseCode.NoError, request.Question, answers));

                default:
                    return Task.FromResult<DnsDatagram>(null);
            }
        }

        #endregion

        #region properties

        public string Description
        { get { return "Returns A or AAAA records with different set of IP addresses for clients querying over public, private, or other specified networks."; } }

        public string ApplicationRecordDataTemplate
        {
            get
            {
                return @"{
  ""public"": [
    ""1.1.1.1"", 
    ""2.2.2.2""
  ],
  ""private"": [
    ""192.168.1.1"", 
    ""::1""
  ],
  ""10.0.0.0/8"": [
    ""10.1.1.1""
  ]
}";
            }
        }

        #endregion
    }
}