Do DNS resolution before connection attempt

projects/RabbitMQ.Client/client/api/ITcpClient.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Net;
 using System.Net.Sockets;
 using System.Threading.Tasks;
 
@@ -17,6 +18,7 @@ public interface ITcpClient : IDisposable
         Socket Client { get; }
 
         Task ConnectAsync(string host, int port);
+        Task ConnectAsync(IPAddress host, int port);
 
         NetworkStream GetStream();
 

projects/RabbitMQ.Client/client/impl/SocketFrameHandler.cs

@@ -101,21 +101,38 @@ public SocketFrameHandler(AmqpTcpEndpoint endpoint,
             _channelReader = channel.Reader;
             _channelWriter = channel.Writer;
 
-            if (ShouldTryIPv6(endpoint))
+            // Resolve the hostname to know if it's even possible to even try IPv6
+            IPAddress[] adds = Dns.GetHostAddresses(endpoint.HostName);
+            IPAddress ipv6 = TcpClientAdapterHelper.GetMatchingHost(adds, AddressFamily.InterNetworkV6);
+
+            if (ipv6 == default(IPAddress))
+            {
+                if (endpoint.AddressFamily == AddressFamily.InterNetworkV6)
+                {
+                    throw new ConnectFailureException("Connection failed", new ArgumentException($"No IPv6 address could be resolved for {endpoint.HostName}"));
+                }
+            }
+            else if (ShouldTryIPv6(endpoint))
             {
                 try
                 {
-                    _socket = ConnectUsingIPv6(endpoint, socketFactory, connectionTimeout);
+                    _socket = ConnectUsingIPv6(new IPEndPoint(ipv6, endpoint.Port), socketFactory, connectionTimeout);
                 }
                 catch (ConnectFailureException)
                 {
+                    // We resolved to a ipv6 address and tried it but it still didn't connect, try IPv4
                     _socket = null;
                 }
             }
 
-            if (_socket == null && endpoint.AddressFamily != AddressFamily.InterNetworkV6)
+            if (_socket == null)
             {
-                _socket = ConnectUsingIPv4(endpoint, socketFactory, connectionTimeout);
+                IPAddress ipv4 = TcpClientAdapterHelper.GetMatchingHost(adds, AddressFamily.InterNetwork);
+                if (ipv4 == default(IPAddress))
+                {
+                    throw new ConnectFailureException("Connection failed", new ArgumentException($"No ip address could be resolved for {endpoint.HostName}"));
+                }
+                _socket = ConnectUsingIPv4(new IPEndPoint(ipv4, endpoint.Port), socketFactory, connectionTimeout);
             }
 
             Stream netstream = _socket.GetStream();
@@ -276,21 +293,21 @@ private static bool ShouldTryIPv6(AmqpTcpEndpoint endpoint)
             return Socket.OSSupportsIPv6 && endpoint.AddressFamily != AddressFamily.InterNetwork;
         }
 
-        private ITcpClient ConnectUsingIPv6(AmqpTcpEndpoint endpoint,
+        private ITcpClient ConnectUsingIPv6(IPEndPoint endpoint,
                                             Func<AddressFamily, ITcpClient> socketFactory,
                                             TimeSpan timeout)
         {
             return ConnectUsingAddressFamily(endpoint, socketFactory, timeout, AddressFamily.InterNetworkV6);
         }
 
-        private ITcpClient ConnectUsingIPv4(AmqpTcpEndpoint endpoint,
+        private ITcpClient ConnectUsingIPv4(IPEndPoint endpoint,
                                             Func<AddressFamily, ITcpClient> socketFactory,
                                             TimeSpan timeout)
         {
             return ConnectUsingAddressFamily(endpoint, socketFactory, timeout, AddressFamily.InterNetwork);
         }
 
-        private ITcpClient ConnectUsingAddressFamily(AmqpTcpEndpoint endpoint,
+        private ITcpClient ConnectUsingAddressFamily(IPEndPoint endpoint,
                                                     Func<AddressFamily, ITcpClient> socketFactory,
                                                     TimeSpan timeout, AddressFamily family)
         {
@@ -307,11 +324,11 @@ private ITcpClient ConnectUsingAddressFamily(AmqpTcpEndpoint endpoint,
             }
         }
 
-        private void ConnectOrFail(ITcpClient socket, AmqpTcpEndpoint endpoint, TimeSpan timeout)
+        private void ConnectOrFail(ITcpClient socket, IPEndPoint endpoint, TimeSpan timeout)
         {
             try
             {
-                socket.ConnectAsync(endpoint.HostName, endpoint.Port)
+                socket.ConnectAsync(endpoint.Address, endpoint.Port)
                      .TimeoutAfter(timeout)
                      .ConfigureAwait(false)
                      // this ensures exceptions aren't wrapped in an AggregateException

projects/RabbitMQ.Client/client/impl/TcpClientAdapter.cs

@@ -27,6 +27,12 @@ public virtual async Task ConnectAsync(string host, int port)
                 throw new ArgumentException($"No ip address could be resolved for {host}");
             }
 
+            await ConnectAsync(ep, port);
+        }
+
+        public virtual async Task ConnectAsync(IPAddress ep, int port)
+        {
+            AssertSocket();
 #if NET461
             await Task.Run(() => _sock.Connect(ep, port)).ConfigureAwait(false);
 #else

projects/Unit/APIApproval.Approve.verified.txt

@@ -493,6 +493,7 @@ namespace RabbitMQ.Client
         bool Connected { get; }
         System.TimeSpan ReceiveTimeout { get; set; }
         void Close();
+        System.Threading.Tasks.Task ConnectAsync(System.Net.IPAddress host, int port);
         System.Threading.Tasks.Task ConnectAsync(string host, int port);
         System.Net.Sockets.NetworkStream GetStream();
     }