OK, so we do need a default client, but make sure we only create it w…

…hen we actually need it.

src/Enclave.ARSoft.Tools.Net/Dns/DnsClient.cs

@@ -36,10 +36,17 @@ namespace ARSoft.Tools.Net.Dns
 	/// </summary>
 	public class DnsClient : DnsClientBase
 	{
+        private DnsClient _defaultClient;
+
 		/// <summary>
-		///   Gets or sets a value indicationg whether queries can be sent using UDP.
+		/// Provides a default client.
 		/// </summary>
-		public new bool IsUdpEnabled
+        public DnsClient Default => _defaultClient ??= new DnsClient(GetLocalConfiguredDnsServers(), 10000) { IsResponseValidationEnabled = true };
+
+        /// <summary>
+        ///   Gets or sets a value indicationg whether queries can be sent using UDP.
+        /// </summary>
+        public new bool IsUdpEnabled
 		{
 			get { return base.IsUdpEnabled; }
 			set { base.IsUdpEnabled = value; }
@@ -60,7 +67,7 @@ public class DnsClient : DnsClientBase
 		/// <param name="dnsServer"> The IPAddress of the dns server to use </param>
 		/// <param name="queryTimeout"> Query timeout in milliseconds </param>
 		public DnsClient(IPAddress dnsServer, int queryTimeout)
-			: this(new List<IPAddress> { dnsServer }, queryTimeout) { }
+			: this(new List<IPAddress> { dnsServer }, queryTimeout) {}
 
 		/// <summary>
 		///   Provides a new instance with custom dns servers and query timeout
@@ -202,5 +209,89 @@ public DnsUpdateMessage SendUpdate(DnsUpdateMessage message)
 
 			return SendMessageAsync(message, token);
 		}
+
+		/// <summary>
+		///   Returns a list of the local configured DNS servers.
+		/// </summary>
+		/// <returns></returns>
+		public static List<IPAddress> GetLocalConfiguredDnsServers()
+		{
+			List<IPAddress> res = new List<IPAddress>();
+
+			try
+			{
+				foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
+				{
+					if ((nic.OperationalStatus == OperationalStatus.Up) && (nic.NetworkInterfaceType != NetworkInterfaceType.Loopback))
+					{
+						foreach (IPAddress dns in nic.GetIPProperties().DnsAddresses)
+						{
+							// only use servers defined in draft-ietf-ipngwg-dns-discovery if they are in the same subnet
+							// fec0::/10 is marked deprecated in RFC 3879, so nobody should use these addresses
+							if (dns.AddressFamily == AddressFamily.InterNetworkV6)
+							{
+								IPAddress unscoped = new IPAddress(dns.GetAddressBytes());
+								if (unscoped.Equals(IPAddress.Parse("fec0:0:0:ffff::1"))
+								    || unscoped.Equals(IPAddress.Parse("fec0:0:0:ffff::2"))
+								    || unscoped.Equals(IPAddress.Parse("fec0:0:0:ffff::3")))
+								{
+									if (!nic.GetIPProperties().UnicastAddresses.Any(x => x.Address.GetNetworkAddress(10).Equals(IPAddress.Parse("fec0::"))))
+										continue;
+								}
+							}
+
+							if (!res.Contains(dns))
+								res.Add(dns);
+						}
+					}
+				}
+			}
+			catch (Exception e)
+			{
+				Trace.TraceError("Configured nameserver couldn't be determined: " + e);
+			}
+
+			// try parsing resolv.conf since getting data by NetworkInterface is not supported on non-windows mono
+			if ((res.Count == 0) && ((Environment.OSVersion.Platform == PlatformID.Unix) || (Environment.OSVersion.Platform == PlatformID.MacOSX)))
+			{
+				try
+				{
+					using (StreamReader reader = File.OpenText("/etc/resolv.conf"))
+					{
+						string line;
+						while ((line = reader.ReadLine()) != null)
+						{
+							int commentStart = line.IndexOf('#');
+							if (commentStart != -1)
+							{
+								line = line.Substring(0, commentStart);
+							}
+
+							string[] lineData = line.Split(new[] { ' ', '\t' }, StringSplitOptions.RemoveEmptyEntries);
+							IPAddress dns;
+							if ((lineData.Length == 2) && (lineData[0] == "nameserver") && (IPAddress.TryParse(lineData[1], out dns)))
+							{
+								res.Add(dns);
+							}
+						}
+					}
+				}
+				catch (Exception e)
+				{
+					Trace.TraceError("/etc/resolv.conf could not be parsed: " + e);
+				}
+			}
+
+			if (res.Count == 0)
+			{
+				// fallback: use the public dns-resolvers of google
+				res.Add(IPAddress.Parse("2001:4860:4860::8844"));
+				res.Add(IPAddress.Parse("2001:4860:4860::8888"));
+				res.Add(IPAddress.Parse("8.8.4.4"));
+				res.Add(IPAddress.Parse("8.8.8.8"));
+			}
+
+			return res.OrderBy(x => x.AddressFamily == AddressFamily.InterNetworkV6 ? 1 : 0).ToList();
+		}
 	}
 }