done

Netch/3rd/DnsClient.NET/IpAddressExtensions.cs

@@ -0,0 +1,46 @@
+using System.Linq;
+using System.Net.Sockets;
+using System.Text;
+
+namespace System.Net
+{
+    /// <summary>
+    /// Extension methods for <see cref="IPAddress"/>.
+    /// </summary>
+    public static class IpAddressExtensions
+    {
+        /// <summary>
+        /// Translates a IPv4 or IPv6 <see cref="IPAddress"/> into an <see href="https://en.wikipedia.org/wiki/.arpa">arpa address</see>.
+        /// Used for reverse DNS lookup to get the domain name of the given <see cref="IPAddress"/>.
+        /// </summary>
+        /// <param name="ip">The address to translate.</param>
+        /// <returns>The arpa representation of the address.</returns>
+        /// <seealso cref="DnsClient.IDnsQuery.QueryReverse(IPAddress)"/>
+        /// <seealso cref="DnsClient.IDnsQuery.QueryReverseAsync(IPAddress, Threading.CancellationToken)"/>
+        /// <seealso href="https://en.wikipedia.org/wiki/.arpa"/>
+        public static string GetArpaName(this IPAddress ip)
+        {
+            var bytes = ip.GetAddressBytes();
+            Array.Reverse(bytes);
+
+            // check IP6
+            if (ip.AddressFamily == AddressFamily.InterNetworkV6)
+            {
+                // reveresed bytes need to be split into 4 bit parts and separated by '.'
+                var newBytes = bytes
+                    .SelectMany(b => new[] { (b >> 0) & 0xf, (b >> 4) & 0xf })
+                    .Aggregate(new StringBuilder(), (s, b) => s.Append(b.ToString("x")).Append(".")) + "ip6.arpa.";
+
+                return newBytes;
+            }
+            else if (ip.AddressFamily == AddressFamily.InterNetwork)
+            {
+                // else IP4
+                return string.Join(".", bytes) + ".in-addr.arpa.";
+            }
+
+            // never happens anyways!?
+            throw new ArgumentException($"Unsupported address family '{ip.AddressFamily}'.", nameof(ip));
+        }
+    }
+}