feat: support reverse address mapping

README.md

@@ -20,6 +20,7 @@ service or service instance.
 - CI on Circle (Debian GNU/Linux), Travis (Ubuntu Xenial and OSX) and AppVeyor (Windows Server 2016)
 - Detects new and/or removed network interfaces
 - Supports multicasting on multiple network interfaces
+- Supports reverse address mapping
 - Handles legacy unicast queries, see #61
 
 ## Getting started

src/ServiceDiscovery.cs

@@ -192,6 +192,10 @@ public void QueryUnicastServiceInstances(string service)
         /// <remarks>
         ///   Any queries for the service or service instance will be answered with
         ///   information from the profile.
+        ///   <para>
+        ///   Besides adding the profile's resource records to the <see cref="Catalog"/> PTR records are
+        ///   created to support DNS-SD and reverse address mapping (DNS address lookup).
+        ///   </para>
         /// </remarks>
         public void Advertise(ServiceProfile service)
         {
@@ -209,6 +213,8 @@ public void Advertise(ServiceProfile service)
             {
                 catalog.Add(r, authoritative: true);
             }
+
+            catalog.IncludeReverseLookupRecords();
         }
 
         /// <summary>
@@ -255,7 +261,9 @@ void OnAnswer(object sender, MessageEventArgs e)
             }
 
             // Any DNS-SD answers?
-            var sd = msg.Answers.OfType<PTRRecord>();
+            var sd = msg.Answers
+                .OfType<PTRRecord>()
+                .Where(ptr => ptr.Name.EndsWith(".local"));
             foreach (var ptr in sd)
             {
                 if (ptr.Name == ServiceName)

test/ServiceDiscoveryTest.cs

@@ -321,5 +321,49 @@ public void Unadvertise()
                 mdns.Stop();
             }
         }
+        [TestMethod]
+        public void ReverseAddressMapping()
+        {
+            var service = new ServiceProfile("x9", "_sdtest-1._udp", 1024, new[] { IPAddress.Loopback, IPAddress.IPv6Loopback });
+            var arpaAddress = IPAddress.Loopback.GetArpaName();
+            var done = new ManualResetEvent(false);
+
+            var mdns = new MulticastService();
+            Message response = null;
+            mdns.NetworkInterfaceDiscovered += (s, e) =>
+                mdns.SendQuery(arpaAddress, DnsClass.IN, DnsType.PTR);
+            mdns.AnswerReceived += (s, e) =>
+            {
+                var msg = e.Message;
+                if (msg.Answers.OfType<PTRRecord>().Any(p => p.Name == arpaAddress))
+                {
+                    response = msg;
+                    done.Set();
+                }
+            };
+            try
+            {
+                using (var sd = new ServiceDiscovery(mdns))
+                {
+                    sd.Advertise(service);
+                    mdns.Start();
+                    Assert.IsTrue(done.WaitOne(TimeSpan.FromSeconds(1)), "query timeout");
+                    var answers = response.Answers
+                        .OfType<PTRRecord>()
+                        .Where(ptr => service.HostName == ptr.DomainName);
+                    foreach (var answer in answers)
+                    {
+                        Assert.AreEqual(arpaAddress, answer.Name);
+                        Assert.IsTrue(answer.TTL > TimeSpan.Zero);
+                        Assert.AreEqual(DnsClass.IN, answer.Class);
+                    }
+                }
+            }
+            finally
+            {
+                mdns.Stop();
+            }
+        }
+
     }
 }