using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Linq;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
using System.Threading.Tasks;
using Jellyfin.Networking.Configuration;
using MediaBrowser.Common.Configuration;
using MediaBrowser.Common.Net;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Networking.Manager
{
    /// <summary>
    /// Class to take care of network interface management.
    /// Note: The normal collection methods and properties will not work with Collection{IPObject}. <see cref="MediaBrowser.Common.Net.NetworkExtensions"/>.
    /// </summary>
    public class NetworkManager : INetworkManager, IDisposable
    {
        /// <summary>
        /// Contains the description of the interface along with its index.
        /// </summary>
        private readonly Dictionary<string, int> _interfaceNames;

        /// <summary>
        /// Threading lock for network properties.
        /// </summary>
        private readonly object _intLock = new object();

        /// <summary>
        /// List of all interface addresses and masks.
        /// </summary>
        private readonly Collection<IPObject> _interfaceAddresses;

        /// <summary>
        /// List of all interface MAC addresses.
        /// </summary>
        private readonly List<PhysicalAddress> _macAddresses;

        private readonly ILogger<NetworkManager> _logger;

        private readonly IConfigurationManager _configurationManager;

        private readonly object _eventFireLock;

        /// <summary>
        /// Holds the bind address overrides.
        /// </summary>
        private readonly Dictionary<IPNetAddress, string> _publishedServerUrls;

        /// <summary>
        /// Used to stop "event-racing conditions".
        /// </summary>
        private bool _eventfire;

        /// <summary>
        /// Unfiltered user defined LAN subnets. (<see cref="NetworkConfiguration.LocalNetworkSubnets"/>)
        /// or internal interface network subnets if undefined by user.
        /// </summary>
        private Collection<IPObject> _lanSubnets;

        /// <summary>
        /// User defined list of subnets to excluded from the LAN.
        /// </summary>
        private Collection<IPObject> _excludedSubnets;

        /// <summary>
        /// List of interface addresses to bind the WS.
        /// </summary>
        private Collection<IPObject> _bindAddresses;

        /// <summary>
        /// List of interface addresses to exclude from bind.
        /// </summary>
        private Collection<IPObject> _bindExclusions;

        /// <summary>
        /// Caches list of all internal filtered interface addresses and masks.
        /// </summary>
        private Collection<IPObject> _internalInterfaces;

        /// <summary>
        /// Flag set when no custom LAN has been defined in the configuration.
        /// </summary>
        private bool _usingPrivateAddresses;

        /// <summary>
        /// True if this object is disposed.
        /// </summary>
        private bool _disposed;

        /// <summary>
        /// Initializes a new instance of the <see cref="NetworkManager"/> class.
        /// </summary>
        /// <param name="configurationManager">IServerConfigurationManager instance.</param>
        /// <param name="logger">Logger to use for messages.</param>
#pragma warning disable CS8618 // Non-nullable field is uninitialized. : Values are set in UpdateSettings function. Compiler doesn't yet recognise this.
        public NetworkManager(IConfigurationManager configurationManager, ILogger<NetworkManager> logger)
        {
            _logger = logger ?? throw new ArgumentNullException(nameof(logger));
            _configurationManager = configurationManager ?? throw new ArgumentNullException(nameof(configurationManager));

            _interfaceAddresses = new Collection<IPObject>();
            _macAddresses = new List<PhysicalAddress>();
            _interfaceNames = new Dictionary<string, int>();
            _publishedServerUrls = new Dictionary<IPNetAddress, string>();
            _eventFireLock = new object();

            UpdateSettings(_configurationManager.GetNetworkConfiguration());

            NetworkChange.NetworkAddressChanged += OnNetworkAddressChanged;
            NetworkChange.NetworkAvailabilityChanged += OnNetworkAvailabilityChanged;

            _configurationManager.NamedConfigurationUpdated += ConfigurationUpdated;
        }
#pragma warning restore CS8618 // Non-nullable field is uninitialized.

        /// <summary>
        /// Event triggered on network changes.
        /// </summary>
        public event EventHandler? NetworkChanged;

        /// <summary>
        /// Gets or sets a value indicating whether testing is taking place.
        /// </summary>
        public static string MockNetworkSettings { get; set; } = string.Empty;

        /// <summary>
        /// Gets or sets a value indicating whether IP6 is enabled.
        /// </summary>
        public bool IsIP6Enabled { get; set; }

        /// <summary>
        /// Gets or sets a value indicating whether IP4 is enabled.
        /// </summary>
        public bool IsIP4Enabled { get; set; }

        /// <inheritdoc/>
        public Collection<IPObject> RemoteAddressFilter { get; private set; }

        /// <summary>
        /// Gets a value indicating whether is all IPv6 interfaces are trusted as internal.
        /// </summary>
        public bool TrustAllIP6Interfaces { get; internal set; }

        /// <summary>
        /// Gets the Published server override list.
        /// </summary>
        public Dictionary<IPNetAddress, string> PublishedServerUrls => _publishedServerUrls;

        /// <summary>
        /// Creates a new network collection.
        /// </summary>
        /// <param name="source">Items to assign the collection, or null.</param>
        /// <returns>The collection created.</returns>
        public static Collection<IPObject> CreateCollection(IEnumerable<IPObject>? source = null)
        {
            var result = new Collection<IPObject>();
            if (source is not null)
            {
                foreach (var item in source)
                {
                    result.AddItem(item, false);
                }
            }

            return result;
        }

        /// <inheritdoc/>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <inheritdoc/>
        public IReadOnlyCollection<PhysicalAddress> GetMacAddresses()
        {
            // Populated in construction - so always has values.
            return _macAddresses;
        }

        /// <inheritdoc/>
        public bool IsGatewayInterface(IPObject? addressObj)
        {
            var address = addressObj?.Address ?? IPAddress.None;
            return _internalInterfaces.Any(i => i.Address.Equals(address) && i.Tag < 0);
        }

        /// <inheritdoc/>
        public bool IsGatewayInterface(IPAddress? addressObj)
        {
            return _internalInterfaces.Any(i => i.Address.Equals(addressObj ?? IPAddress.None) && i.Tag < 0);
        }

        /// <inheritdoc/>
        public Collection<IPObject> GetLoopbacks()
        {
            Collection<IPObject> nc = new Collection<IPObject>();
            if (IsIP4Enabled)
            {
                nc.AddItem(IPAddress.Loopback);
            }

            if (IsIP6Enabled)
            {
                nc.AddItem(IPAddress.IPv6Loopback);
            }

            return nc;
        }

        /// <inheritdoc/>
        public bool IsExcluded(IPAddress ip)
        {
            return _excludedSubnets.ContainsAddress(ip);
        }

        /// <inheritdoc/>
        public bool IsExcluded(EndPoint ip)
        {
            return ip is not null && IsExcluded(((IPEndPoint)ip).Address);
        }

        /// <inheritdoc/>
        public Collection<IPObject> CreateIPCollection(string[] values, bool negated = false)
        {
            Collection<IPObject> col = new Collection<IPObject>();
            if (values is null)
            {
                return col;
            }

            for (int a = 0; a < values.Length; a++)
            {
                string v = values[a].Trim();

                try
                {
                    if (v.StartsWith('!'))
                    {
                        if (negated)
                        {
                            AddToCollection(col, v[1..]);
                        }
                    }
                    else if (!negated)
                    {
                        AddToCollection(col, v);
                    }
                }
                catch (ArgumentException e)
                {
                    _logger.LogWarning(e, "Ignoring LAN value {Value}.", v);
                }
            }

            return col;
        }

        /// <inheritdoc/>
        public Collection<IPObject> GetAllBindInterfaces(bool individualInterfaces = false)
        {
            int count = _bindAddresses.Count;

            if (count == 0)
            {
                if (_bindExclusions.Count > 0)
                {
                    // Return all the interfaces except the ones specifically excluded.
                    return _interfaceAddresses.Exclude(_bindExclusions, false);
                }

                if (individualInterfaces)
                {
                    return new Collection<IPObject>(_interfaceAddresses);
                }

                // No bind address and no exclusions, so listen on all interfaces.
                Collection<IPObject> result = new Collection<IPObject>();

                if (IsIP6Enabled && IsIP4Enabled)
                {
                    // Kestrel source code shows it uses Sockets.DualMode - so this also covers IPAddress.Any
                    result.AddItem(IPAddress.IPv6Any);
                }
                else if (IsIP4Enabled)
                {
                    result.AddItem(IPAddress.Any);
                }
                else if (IsIP6Enabled)
                {
                    // Cannot use IPv6Any as Kestrel will bind to IPv4 addresses.
                    foreach (var iface in _interfaceAddresses)
                    {
                        if (iface.AddressFamily == AddressFamily.InterNetworkV6)
                        {
                            result.AddItem(iface.Address);
                        }
                    }
                }

                return result;
            }

            // Remove any excluded bind interfaces.
            return _bindAddresses.Exclude(_bindExclusions, false);
        }

        /// <inheritdoc/>
        public string GetBindInterface(string source, out int? port)
        {
            if (IPHost.TryParse(source, out IPHost host))
            {
                return GetBindInterface(host, out port);
            }

            return GetBindInterface(IPHost.None, out port);
        }

        /// <inheritdoc/>
        public string GetBindInterface(IPAddress source, out int? port)
        {
            return GetBindInterface(new IPNetAddress(source), out port);
        }

        /// <inheritdoc/>
        public string GetBindInterface(HttpRequest source, out int? port)
        {
            string result;

            if (source is not null && IPHost.TryParse(source.Host.Host, out IPHost host))
            {
                result = GetBindInterface(host, out port);
                port ??= source.Host.Port;
            }
            else
            {
                result = GetBindInterface(IPNetAddress.None, out port);
                port ??= source?.Host.Port;
            }

            return result;
        }

        /// <inheritdoc/>
        public string GetBindInterface(IPObject source, out int? port)
        {
            port = null;
            ArgumentNullException.ThrowIfNull(source);

            // Do we have a source?
            bool haveSource = !source.Address.Equals(IPAddress.None);
            bool isExternal = false;

            if (haveSource)
            {
                if (!IsIP6Enabled && source.AddressFamily == AddressFamily.InterNetworkV6)
                {
                    _logger.LogWarning("IPv6 is disabled in Jellyfin, but enabled in the OS. This may affect how the interface is selected.");
                }

                if (!IsIP4Enabled && source.AddressFamily == AddressFamily.InterNetwork)
                {
                    _logger.LogWarning("IPv4 is disabled in Jellyfin, but enabled in the OS. This may affect how the interface is selected.");
                }

                isExternal = !IsInLocalNetwork(source);

                if (MatchesPublishedServerUrl(source, isExternal, out string res, out port))
                {
                    _logger.LogDebug("{Source}: Using BindAddress {Address}:{Port}", source, res, port);
                    return res;
                }
            }

            _logger.LogDebug("GetBindInterface: Source: {HaveSource}, External: {IsExternal}:", haveSource, isExternal);

            // No preference given, so move on to bind addresses.
            if (MatchesBindInterface(source, isExternal, out string result))
            {
                return result;
            }

            if (isExternal && MatchesExternalInterface(source, out result))
            {
                return result;
            }

            // Get the first LAN interface address that isn't a loopback.
            var interfaces = CreateCollection(
                _interfaceAddresses
                    .Exclude(_bindExclusions, false)
                    .Where(IsInLocalNetwork)
                    .OrderBy(p => p.Tag));

            if (interfaces.Count > 0)
            {
                if (haveSource)
                {
                    foreach (var intf in interfaces)
                    {
                        if (intf.Address.Equals(source.Address))
                        {
                            result = FormatIP6String(intf.Address);
                            _logger.LogDebug("{Source}: GetBindInterface: Has found matching interface. {Result}", source, result);
                            return result;
                        }
                    }

                    // Does the request originate in one of the interface subnets?
                    // (For systems with multiple internal network cards, and multiple subnets)
                    foreach (var intf in interfaces)
                    {
                        if (intf.Contains(source))
                        {
                            result = FormatIP6String(intf.Address);
                            _logger.LogDebug("{Source}: GetBindInterface: Has source, matched best internal interface on range. {Result}", source, result);
                            return result;
                        }
                    }
                }

                result = FormatIP6String(interfaces.First().Address);
                _logger.LogDebug("{Source}: GetBindInterface: Matched first internal interface. {Result}", source, result);
                return result;
            }

            // There isn't any others, so we'll use the loopback.
            result = IsIP6Enabled ? "::1" : "127.0.0.1";
            _logger.LogWarning("{Source}: GetBindInterface: Loopback {Result} returned.", source, result);
            return result;
        }

        /// <inheritdoc/>
        public Collection<IPObject> GetInternalBindAddresses()
        {
            int count = _bindAddresses.Count;

            if (count == 0)
            {
                if (_bindExclusions.Count > 0)
                {
                    // Return all the internal interfaces except the ones excluded.
                    return CreateCollection(_internalInterfaces.Where(p => !_bindExclusions.ContainsAddress(p)));
                }

                // No bind address, so return all internal interfaces.
                return CreateCollection(_internalInterfaces);
            }

            return new Collection<IPObject>(_bindAddresses.Where(a => IsInLocalNetwork(a)).ToArray());
        }

        /// <inheritdoc/>
        public bool IsInLocalNetwork(IPObject address)
        {
            return IsInLocalNetwork(address.Address);
        }

        /// <inheritdoc/>
        public bool IsInLocalNetwork(string address)
        {
            return IPHost.TryParse(address, out IPHost ipHost) && IsInLocalNetwork(ipHost);
        }

        /// <inheritdoc/>
        public bool IsInLocalNetwork(IPAddress address)
        {
            ArgumentNullException.ThrowIfNull(address);

            if (address.Equals(IPAddress.None))
            {
                return false;
            }

            // See conversation at https://github.com/jellyfin/jellyfin/pull/3515.
            if (TrustAllIP6Interfaces && address.AddressFamily == AddressFamily.InterNetworkV6)
            {
                return true;
            }

            // As private addresses can be redefined by Configuration.LocalNetworkAddresses
            return IPAddress.IsLoopback(address) || (_lanSubnets.ContainsAddress(address) && !_excludedSubnets.ContainsAddress(address));
        }

        /// <inheritdoc/>
        public bool IsPrivateAddressRange(IPObject address)
        {
            ArgumentNullException.ThrowIfNull(address);

            // See conversation at https://github.com/jellyfin/jellyfin/pull/3515.
            if (TrustAllIP6Interfaces && address.AddressFamily == AddressFamily.InterNetworkV6)
            {
                return true;
            }
            else
            {
                return address.IsPrivateAddressRange();
            }
        }

        /// <inheritdoc/>
        public bool IsExcludedInterface(IPAddress address)
        {
            return _bindExclusions.ContainsAddress(address);
        }

        /// <inheritdoc/>
        public Collection<IPObject> GetFilteredLANSubnets(Collection<IPObject>? filter = null)
        {
            if (filter is null)
            {
                return _lanSubnets.Exclude(_excludedSubnets, true).AsNetworks();
            }

            return _lanSubnets.Exclude(filter, true);
        }

        /// <inheritdoc/>
        public bool IsValidInterfaceAddress(IPAddress address)
        {
            return _interfaceAddresses.ContainsAddress(address);
        }

        /// <inheritdoc/>
        public bool TryParseInterface(string token, out Collection<IPObject>? result)
        {
            result = null;
            if (string.IsNullOrEmpty(token))
            {
                return false;
            }

            if (_interfaceNames is not null && _interfaceNames.TryGetValue(token.ToLower(CultureInfo.InvariantCulture), out int index))
            {
                result = new Collection<IPObject>();

                _logger.LogInformation("Interface {Token} used in settings. Using its interface addresses.", token);

                // Replace interface tags with the interface IP's.
                foreach (IPNetAddress iface in _interfaceAddresses)
                {
                    if (Math.Abs(iface.Tag) == index
                        && ((IsIP4Enabled && iface.Address.AddressFamily == AddressFamily.InterNetwork)
                            || (IsIP6Enabled && iface.Address.AddressFamily == AddressFamily.InterNetworkV6)))
                    {
                        result.AddItem(iface, false);
                    }
                }

                return true;
            }

            return false;
        }

        /// <inheritdoc/>
        public bool HasRemoteAccess(IPAddress remoteIp)
        {
            var config = _configurationManager.GetNetworkConfiguration();
            if (config.EnableRemoteAccess)
            {
                // Comma separated list of IP addresses or IP/netmask entries for networks that will be allowed to connect remotely.
                // If left blank, all remote addresses will be allowed.
                if (RemoteAddressFilter.Count > 0 && !IsInLocalNetwork(remoteIp))
                {
                    // remoteAddressFilter is a whitelist or blacklist.
                    return RemoteAddressFilter.ContainsAddress(remoteIp) == !config.IsRemoteIPFilterBlacklist;
                }
            }
            else if (!IsInLocalNetwork(remoteIp))
            {
                // Remote not enabled. So everyone should be LAN.
                return false;
            }

            return true;
        }

        /// <summary>
        /// Reloads all settings and re-initialises the instance.
        /// </summary>
        /// <param name="configuration">The <see cref="NetworkConfiguration"/> to use.</param>
        public void UpdateSettings(object configuration)
        {
            NetworkConfiguration config = (NetworkConfiguration)configuration ?? throw new ArgumentNullException(nameof(configuration));

            IsIP4Enabled = Socket.OSSupportsIPv4 && config.EnableIPV4;
            IsIP6Enabled = Socket.OSSupportsIPv6 && config.EnableIPV6;

            if (!IsIP6Enabled && !IsIP4Enabled)
            {
                _logger.LogError("IPv4 and IPv6 cannot both be disabled.");
                IsIP4Enabled = true;
            }

            TrustAllIP6Interfaces = config.TrustAllIP6Interfaces;

            if (string.IsNullOrEmpty(MockNetworkSettings))
            {
                InitialiseInterfaces();
            }
            else // Used in testing only.
            {
                // Format is <IPAddress>,<Index>,<Name>: <next interface>. Set index to -ve to simulate a gateway.
                var interfaceList = MockNetworkSettings.Split('|');
                foreach (var details in interfaceList)
                {
                    var parts = details.Split(',');
                    var address = IPNetAddress.Parse(parts[0]);
                    var index = int.Parse(parts[1], CultureInfo.InvariantCulture);
                    address.Tag = index;
                    _interfaceAddresses.AddItem(address, false);
                    _interfaceNames[parts[2]] = Math.Abs(index);
                }
            }

            InitialiseLAN(config);
            InitialiseBind(config);
            InitialiseRemote(config);
            InitialiseOverrides(config);
        }

        /// <summary>
        /// Protected implementation of Dispose pattern.
        /// </summary>
        /// <param name="disposing"><c>True</c> to dispose the managed state.</param>
        protected virtual void Dispose(bool disposing)
        {
            if (!_disposed)
            {
                if (disposing)
                {
                    _configurationManager.NamedConfigurationUpdated -= ConfigurationUpdated;
                    NetworkChange.NetworkAddressChanged -= OnNetworkAddressChanged;
                    NetworkChange.NetworkAvailabilityChanged -= OnNetworkAvailabilityChanged;
                }

                _disposed = true;
            }
        }

        /// <summary>
        /// Tries to identify the string and return an object of that class.
        /// </summary>
        /// <param name="addr">String to parse.</param>
        /// <param name="result">IPObject to return.</param>
        /// <returns><c>true</c> if the value parsed successfully, <c>false</c> otherwise.</returns>
        private static bool TryParse(string addr, out IPObject result)
        {
            if (!string.IsNullOrEmpty(addr))
            {
                // Is it an IP address
                if (IPNetAddress.TryParse(addr, out IPNetAddress nw))
                {
                    result = nw;
                    return true;
                }

                if (IPHost.TryParse(addr, out IPHost h))
                {
                    result = h;
                    return true;
                }
            }

            result = IPNetAddress.None;
            return false;
        }

        /// <summary>
        /// Converts an IPAddress into a string.
        /// Ipv6 addresses are returned in [ ], with their scope removed.
        /// </summary>
        /// <param name="address">Address to convert.</param>
        /// <returns>URI safe conversion of the address.</returns>
        private static string FormatIP6String(IPAddress address)
        {
            var str = address.ToString();
            if (address.AddressFamily == AddressFamily.InterNetworkV6)
            {
                int i = str.IndexOf("%", StringComparison.OrdinalIgnoreCase);
                if (i != -1)
                {
                    str = str.Substring(0, i);
                }

                return $"[{str}]";
            }

            return str;
        }

        private void ConfigurationUpdated(object? sender, ConfigurationUpdateEventArgs evt)
        {
            if (evt.Key.Equals(NetworkConfigurationStore.StoreKey, StringComparison.Ordinal))
            {
                UpdateSettings((NetworkConfiguration)evt.NewConfiguration);
            }
        }

        /// <summary>
        /// Checks the string to see if it matches any interface names.
        /// </summary>
        /// <param name="token">String to check.</param>
        /// <param name="index">Interface index numbers that match.</param>
        /// <returns><c>true</c> if an interface name matches the token, <c>False</c> otherwise.</returns>
        private bool TryGetInterfaces(string token, [NotNullWhen(true)] out List<int>? index)
        {
            index = null;

            // Is it the name of an interface (windows) eg, Wireless LAN adapter Wireless Network Connection 1.
            // Null check required here for automated testing.
            if (_interfaceNames is not null && token.Length > 1)
            {
                bool partial = token[^1] == '*';
                if (partial)
                {
                    token = token[..^1];
                }

                foreach ((string interfc, int interfcIndex) in _interfaceNames)
                {
                    if ((!partial && string.Equals(interfc, token, StringComparison.OrdinalIgnoreCase))
                        || (partial && interfc.StartsWith(token, true, CultureInfo.InvariantCulture)))
                    {
                        index ??= new List<int>();
                        index.Add(interfcIndex);
                    }
                }
            }

            return index is not null;
        }

        /// <summary>
        /// Parses a string and adds it into the collection, replacing any interface references.
        /// </summary>
        /// <param name="col"><see cref="Collection{IPObject}"/>Collection.</param>
        /// <param name="token">String value to parse.</param>
        private void AddToCollection(Collection<IPObject> col, string token)
        {
            // Is it the name of an interface (windows) eg, Wireless LAN adapter Wireless Network Connection 1.
            // Null check required here for automated testing.
            if (TryGetInterfaces(token, out var indices))
            {
                _logger.LogInformation("Interface {Token} used in settings. Using its interface addresses.", token);

                // Replace all the interface tags with the interface IP's.
                foreach (IPNetAddress iface in _interfaceAddresses)
                {
                    if (indices.Contains(Math.Abs(iface.Tag))
                        && ((IsIP4Enabled && iface.Address.AddressFamily == AddressFamily.InterNetwork)
                            || (IsIP6Enabled && iface.Address.AddressFamily == AddressFamily.InterNetworkV6)))
                    {
                        col.AddItem(iface);
                    }
                }
            }
            else if (TryParse(token, out IPObject obj))
            {
                // Expand if the ip address is "any".
                if ((obj.Address.Equals(IPAddress.Any) && IsIP4Enabled)
                    || (obj.Address.Equals(IPAddress.IPv6Any) && IsIP6Enabled))
                {
                    foreach (IPNetAddress iface in _interfaceAddresses)
                    {
                        if (obj.AddressFamily == iface.AddressFamily)
                        {
                            col.AddItem(iface);
                        }
                    }
                }
                else if (!IsIP6Enabled)
                {
                    // Remove IP6 addresses from multi-homed IPHosts.
                    obj.Remove(AddressFamily.InterNetworkV6);
                    if (!obj.IsIP6())
                    {
                        col.AddItem(obj);
                    }
                }
                else if (!IsIP4Enabled)
                {
                    // Remove IP4 addresses from multi-homed IPHosts.
                    obj.Remove(AddressFamily.InterNetwork);
                    if (obj.IsIP6())
                    {
                        col.AddItem(obj);
                    }
                }
                else
                {
                    col.AddItem(obj);
                }
            }
            else
            {
                _logger.LogDebug("Invalid or unknown object {Token}.", token);
            }
        }

        /// <summary>
        /// Handler for network change events.
        /// </summary>
        /// <param name="sender">Sender.</param>
        /// <param name="e">A <see cref="NetworkAvailabilityEventArgs"/> containing network availability information.</param>
        private void OnNetworkAvailabilityChanged(object? sender, NetworkAvailabilityEventArgs e)
        {
            _logger.LogDebug("Network availability changed.");
            OnNetworkChanged();
        }

        /// <summary>
        /// Handler for network change events.
        /// </summary>
        /// <param name="sender">Sender.</param>
        /// <param name="e">An <see cref="EventArgs"/>.</param>
        private void OnNetworkAddressChanged(object? sender, EventArgs e)
        {
            _logger.LogDebug("Network address change detected.");
            OnNetworkChanged();
        }

        /// <summary>
        /// Async task that waits for 2 seconds before re-initialising the settings, as typically these events fire multiple times in succession.
        /// </summary>
        /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
        private async Task OnNetworkChangeAsync()
        {
            try
            {
                await Task.Delay(2000).ConfigureAwait(false);
                InitialiseInterfaces();
                // Recalculate LAN caches.
                InitialiseLAN(_configurationManager.GetNetworkConfiguration());

                NetworkChanged?.Invoke(this, EventArgs.Empty);
            }
            finally
            {
                _eventfire = false;
            }
        }

        /// <summary>
        /// Triggers our event, and re-loads interface information.
        /// </summary>
        private void OnNetworkChanged()
        {
            lock (_eventFireLock)
            {
                if (!_eventfire)
                {
                    _logger.LogDebug("Network Address Change Event.");
                    // As network events tend to fire one after the other only fire once every second.
                    _eventfire = true;
                    OnNetworkChangeAsync().GetAwaiter().GetResult();
                }
            }
        }

        /// <summary>
        /// Parses the user defined overrides into the dictionary object.
        /// Overrides are the equivalent of localised publishedServerUrl, enabling
        /// different addresses to be advertised over different subnets.
        /// format is subnet=ipaddress|host|uri
        /// when subnet = 0.0.0.0, any external address matches.
        /// </summary>
        private void InitialiseOverrides(NetworkConfiguration config)
        {
            lock (_intLock)
            {
                _publishedServerUrls.Clear();
                string[] overrides = config.PublishedServerUriBySubnet;
                if (overrides is null)
                {
                    return;
                }

                foreach (var entry in overrides)
                {
                    var parts = entry.Split('=');
                    if (parts.Length != 2)
                    {
                        _logger.LogError("Unable to parse bind override: {Entry}", entry);
                    }
                    else
                    {
                        var replacement = parts[1].Trim();
                        if (string.Equals(parts[0], "all", StringComparison.OrdinalIgnoreCase))
                        {
                            _publishedServerUrls[new IPNetAddress(IPAddress.Broadcast)] = replacement;
                        }
                        else if (string.Equals(parts[0], "external", StringComparison.OrdinalIgnoreCase))
                        {
                            _publishedServerUrls[new IPNetAddress(IPAddress.Any)] = replacement;
                        }
                        else if (TryParseInterface(parts[0], out Collection<IPObject>? addresses) && addresses is not null)
                        {
                            foreach (IPNetAddress na in addresses)
                            {
                                _publishedServerUrls[na] = replacement;
                            }
                        }
                        else if (IPNetAddress.TryParse(parts[0], out IPNetAddress result))
                        {
                            _publishedServerUrls[result] = replacement;
                        }
                        else
                        {
                            _logger.LogError("Unable to parse bind ip address. {Parts}", parts[1]);
                        }
                    }
                }
            }
        }

        /// <summary>
        /// Initialises the network bind addresses.
        /// </summary>
        private void InitialiseBind(NetworkConfiguration config)
        {
            lock (_intLock)
            {
                string[] lanAddresses = config.LocalNetworkAddresses;

                // Add virtual machine interface names to the list of bind exclusions, so that they are auto-excluded.
                if (config.IgnoreVirtualInterfaces)
                {
                    // each virtual interface name must be prepended with the exclusion symbol !
                    var virtualInterfaceNames = config.VirtualInterfaceNames.Split(',').Select(p => "!" + p).ToArray();
                    if (lanAddresses.Length > 0)
                    {
                        var newList = new string[lanAddresses.Length + virtualInterfaceNames.Length];
                        Array.Copy(lanAddresses, newList, lanAddresses.Length);
                        Array.Copy(virtualInterfaceNames, 0, newList, lanAddresses.Length, virtualInterfaceNames.Length);
                        lanAddresses = newList;
                    }
                    else
                    {
                        lanAddresses = virtualInterfaceNames;
                    }
                }

                // Read and parse bind addresses and exclusions, removing ones that don't exist.
                _bindAddresses = CreateIPCollection(lanAddresses).ThatAreContainedInNetworks(_interfaceAddresses);
                _bindExclusions = CreateIPCollection(lanAddresses, true).ThatAreContainedInNetworks(_interfaceAddresses);
                _logger.LogInformation("Using bind addresses: {0}", _bindAddresses.AsString());
                _logger.LogInformation("Using bind exclusions: {0}", _bindExclusions.AsString());
            }
        }

        /// <summary>
        /// Initialises the remote address values.
        /// </summary>
        private void InitialiseRemote(NetworkConfiguration config)
        {
            lock (_intLock)
            {
                RemoteAddressFilter = CreateIPCollection(config.RemoteIPFilter);
            }
        }

        /// <summary>
        /// Initialises internal LAN cache settings.
        /// </summary>
        private void InitialiseLAN(NetworkConfiguration config)
        {
            lock (_intLock)
            {
                _logger.LogDebug("Refreshing LAN information.");

                // Get configuration options.
                string[] subnets = config.LocalNetworkSubnets;

                // Create lists from user settings.

                _lanSubnets = CreateIPCollection(subnets);
                _excludedSubnets = CreateIPCollection(subnets, true).AsNetworks();

                // If no LAN addresses are specified - all private subnets are deemed to be the LAN
                _usingPrivateAddresses = _lanSubnets.Count == 0;

                // NOTE: The order of the commands generating the collection in this statement matters.
                // Altering the order will cause the collections to be created incorrectly.
                if (_usingPrivateAddresses)
                {
                    _logger.LogDebug("Using LAN interface addresses as user provided no LAN details.");
                    // Internal interfaces must be private and not excluded.
                    _internalInterfaces = CreateCollection(_interfaceAddresses.Where(i => IsPrivateAddressRange(i) && !_excludedSubnets.ContainsAddress(i)));

                    // Subnets are the same as the calculated internal interface.
                    _lanSubnets = new Collection<IPObject>();

                    if (IsIP6Enabled)
                    {
                        _lanSubnets.AddItem(IPNetAddress.Parse("fc00::/7")); // ULA
                        _lanSubnets.AddItem(IPNetAddress.Parse("fe80::/10")); // Site local
                    }

                    if (IsIP4Enabled)
                    {
                        _lanSubnets.AddItem(IPNetAddress.Parse("10.0.0.0/8"));
                        _lanSubnets.AddItem(IPNetAddress.Parse("172.16.0.0/12"));
                        _lanSubnets.AddItem(IPNetAddress.Parse("192.168.0.0/16"));
                    }
                }
                else
                {
                    // Internal interfaces must be private, not excluded and part of the LocalNetworkSubnet.
                    _internalInterfaces = CreateCollection(_interfaceAddresses.Where(IsInLocalNetwork));
                }

                _logger.LogInformation("Defined LAN addresses: {0}", _lanSubnets.AsString());
                _logger.LogInformation("Defined LAN exclusions: {0}", _excludedSubnets.AsString());
                _logger.LogInformation("Using LAN addresses: {0}", _lanSubnets.Exclude(_excludedSubnets, true).AsNetworks().AsString());
            }
        }

        /// <summary>
        /// Generate a list of all the interface ip addresses and submasks where that are in the active/unknown state.
        /// Generate a list of all active mac addresses that aren't loopback addresses.
        /// </summary>
        private void InitialiseInterfaces()
        {
            lock (_intLock)
            {
                _logger.LogDebug("Refreshing interfaces.");

                _interfaceNames.Clear();
                _interfaceAddresses.Clear();
                _macAddresses.Clear();

                try
                {
                    IEnumerable<NetworkInterface> nics = NetworkInterface.GetAllNetworkInterfaces()
                        .Where(i => i.SupportsMulticast && i.OperationalStatus == OperationalStatus.Up);

                    foreach (NetworkInterface adapter in nics)
                    {
                        try
                        {
                            IPInterfaceProperties ipProperties = adapter.GetIPProperties();
                            PhysicalAddress mac = adapter.GetPhysicalAddress();

                            // populate mac list
                            if (adapter.NetworkInterfaceType != NetworkInterfaceType.Loopback && mac is not null && mac != PhysicalAddress.None)
                            {
                                _macAddresses.Add(mac);
                            }

                            // populate interface address list
                            foreach (UnicastIPAddressInformation info in ipProperties.UnicastAddresses)
                            {
                                if (IsIP4Enabled && info.Address.AddressFamily == AddressFamily.InterNetwork)
                                {
                                    IPNetAddress nw = new IPNetAddress(info.Address, IPObject.MaskToCidr(info.IPv4Mask))
                                    {
                                        // Keep the number of gateways on this interface, along with its index.
                                        Tag = ipProperties.GetIPv4Properties().Index
                                    };

                                    int tag = nw.Tag;
                                    if (ipProperties.GatewayAddresses.Count > 0 && !nw.IsLoopback())
                                    {
                                        // -ve Tags signify the interface has a gateway.
                                        nw.Tag *= -1;
                                    }

                                    _interfaceAddresses.AddItem(nw, false);

                                    // Store interface name so we can use the name in Collections.
                                    _interfaceNames[adapter.Description.ToLower(CultureInfo.InvariantCulture)] = tag;
                                    _interfaceNames["eth" + tag.ToString(CultureInfo.InvariantCulture)] = tag;
                                }
                                else if (IsIP6Enabled && info.Address.AddressFamily == AddressFamily.InterNetworkV6)
                                {
                                    IPNetAddress nw = new IPNetAddress(info.Address, (byte)info.PrefixLength)
                                    {
                                        // Keep the number of gateways on this interface, along with its index.
                                        Tag = ipProperties.GetIPv6Properties().Index
                                    };

                                    int tag = nw.Tag;
                                    if (ipProperties.GatewayAddresses.Count > 0 && !nw.IsLoopback())
                                    {
                                        // -ve Tags signify the interface has a gateway.
                                        nw.Tag *= -1;
                                    }

                                    _interfaceAddresses.AddItem(nw, false);

                                    // Store interface name so we can use the name in Collections.
                                    _interfaceNames[adapter.Description.ToLower(CultureInfo.InvariantCulture)] = tag;
                                    _interfaceNames["eth" + tag.ToString(CultureInfo.InvariantCulture)] = tag;
                                }
                            }
                        }
#pragma warning disable CA1031 // Do not catch general exception types
                        catch (Exception ex)
                        {
                            // Ignore error, and attempt to continue.
                            _logger.LogError(ex, "Error encountered parsing interfaces.");
                        }
#pragma warning restore CA1031 // Do not catch general exception types
                    }
                }
                catch (Exception ex)
                {
                    _logger.LogError(ex, "Error in InitialiseInterfaces.");
                }

                // If for some reason we don't have an interface info, resolve our DNS name.
                if (_interfaceAddresses.Count == 0)
                {
                    _logger.LogError("No interfaces information available. Resolving DNS name.");
                    IPHost host = new IPHost(Dns.GetHostName());
                    foreach (var a in host.GetAddresses())
                    {
                        _interfaceAddresses.AddItem(a);
                    }

                    if (_interfaceAddresses.Count == 0)
                    {
                        _logger.LogWarning("No interfaces information available. Using loopback.");
                    }
                }

                if (IsIP4Enabled)
                {
                    _interfaceAddresses.AddItem(IPNetAddress.IP4Loopback);
                }

                if (IsIP6Enabled)
                {
                    _interfaceAddresses.AddItem(IPNetAddress.IP6Loopback);
                }

                _logger.LogDebug("Discovered {0} interfaces.", _interfaceAddresses.Count);
                _logger.LogDebug("Interfaces addresses: {0}", _interfaceAddresses.AsString());
            }
        }

        /// <summary>
        /// Attempts to match the source against a user defined bind interface.
        /// </summary>
        /// <param name="source">IP source address to use.</param>
        /// <param name="isInExternalSubnet">True if the source is in the external subnet.</param>
        /// <param name="bindPreference">The published server url that matches the source address.</param>
        /// <param name="port">The resultant port, if one exists.</param>
        /// <returns><c>true</c> if a match is found, <c>false</c> otherwise.</returns>
        private bool MatchesPublishedServerUrl(IPObject source, bool isInExternalSubnet, out string bindPreference, out int? port)
        {
            bindPreference = string.Empty;
            port = null;

            // Check for user override.
            foreach (var addr in _publishedServerUrls)
            {
                // Remaining. Match anything.
                if (addr.Key.Address.Equals(IPAddress.Broadcast))
                {
                    bindPreference = addr.Value;
                    break;
                }
                else if ((addr.Key.Address.Equals(IPAddress.Any) || addr.Key.Address.Equals(IPAddress.IPv6Any)) && isInExternalSubnet)
                {
                    // External.
                    bindPreference = addr.Value;
                    break;
                }
                else if (addr.Key.Contains(source))
                {
                    // Match ip address.
                    bindPreference = addr.Value;
                    break;
                }
            }

            if (string.IsNullOrEmpty(bindPreference))
            {
                return false;
            }

            // Has it got a port defined?
            var parts = bindPreference.Split(':');
            if (parts.Length > 1)
            {
                if (int.TryParse(parts[1], out int p))
                {
                    bindPreference = parts[0];
                    port = p;
                }
            }

            return true;
        }

        /// <summary>
        /// Attempts to match the source against a user defined bind interface.
        /// </summary>
        /// <param name="source">IP source address to use.</param>
        /// <param name="isInExternalSubnet">True if the source is in the external subnet.</param>
        /// <param name="result">The result, if a match is found.</param>
        /// <returns><c>true</c> if a match is found, <c>false</c> otherwise.</returns>
        private bool MatchesBindInterface(IPObject source, bool isInExternalSubnet, out string result)
        {
            result = string.Empty;
            var addresses = _bindAddresses.Exclude(_bindExclusions, false);

            int count = addresses.Count;
            if (count == 1 && (_bindAddresses[0].Equals(IPAddress.Any) || _bindAddresses[0].Equals(IPAddress.IPv6Any)))
            {
                // Ignore IPAny addresses.
                count = 0;
            }

            if (count != 0)
            {
                // Check to see if any of the bind interfaces are in the same subnet.

                IPAddress? defaultGateway = null;
                IPAddress? bindAddress = null;

                if (isInExternalSubnet)
                {
                    // Find all external bind addresses. Store the default gateway, but check to see if there is a better match first.
                    foreach (var addr in addresses.OrderBy(p => p.Tag))
                    {
                        if (defaultGateway is null && !IsInLocalNetwork(addr))
                        {
                            defaultGateway = addr.Address;
                        }

                        if (bindAddress is null && addr.Contains(source))
                        {
                            bindAddress = addr.Address;
                        }

                        if (defaultGateway is not null && bindAddress is not null)
                        {
                            break;
                        }
                    }
                }
                else
                {
                    // Look for the best internal address.
                    bindAddress = addresses
                        .Where(p => IsInLocalNetwork(p) && (p.Contains(source) || p.Equals(IPAddress.None)))
                        .OrderBy(p => p.Tag)
                        .FirstOrDefault()?.Address;
                }

                if (bindAddress is not null)
                {
                    result = FormatIP6String(bindAddress);
                    _logger.LogDebug("{Source}: GetBindInterface: Has source, found a match bind interface subnets. {Result}", source, result);
                    return true;
                }

                if (isInExternalSubnet && defaultGateway is not null)
                {
                    result = FormatIP6String(defaultGateway);
                    _logger.LogDebug("{Source}: GetBindInterface: Using first user defined external interface. {Result}", source, result);
                    return true;
                }

                result = FormatIP6String(addresses[0].Address);
                _logger.LogDebug("{Source}: GetBindInterface: Selected first user defined interface. {Result}", source, result);

                if (isInExternalSubnet)
                {
                    _logger.LogWarning("{Source}: External request received, however, only an internal interface bind found.", source);
                }

                return true;
            }

            return false;
        }

        /// <summary>
        /// Attempts to match the source against an external interface.
        /// </summary>
        /// <param name="source">IP source address to use.</param>
        /// <param name="result">The result, if a match is found.</param>
        /// <returns><c>true</c> if a match is found, <c>false</c> otherwise.</returns>
        private bool MatchesExternalInterface(IPObject source, out string result)
        {
            result = string.Empty;
            // Get the first WAN interface address that isn't a loopback.
            var extResult = _interfaceAddresses
                .Exclude(_bindExclusions, false)
                .Where(p => !IsInLocalNetwork(p))
                .OrderBy(p => p.Tag)
                .ToList();

            if (extResult.Any())
            {
                // Does the request originate in one of the interface subnets?
                // (For systems with multiple internal network cards, and multiple subnets)
                foreach (var intf in extResult)
                {
                    if (!IsInLocalNetwork(intf) && intf.Contains(source))
                    {
                        result = FormatIP6String(intf.Address);
                        _logger.LogDebug("{Source}: GetBindInterface: Selected best external on interface on range. {Result}", source, result);
                        return true;
                    }
                }

                result = FormatIP6String(extResult.First().Address);
                _logger.LogDebug("{Source}: GetBindInterface: Selected first external interface. {Result}", source, result);
                return true;
            }

            _logger.LogDebug("{Source}: External request received, but no WAN interface found. Need to route through internal network.", source);
            return false;
        }
    }
}