Send UDP multicast on the network interface the transport joins

docs/PubSub.md

@@ -108,12 +108,10 @@ a [Security Key Service](#security-key-service-sks). Two warnings are expected:
   with the metadata configured on its DataSetReader instead.
 
 The sample's default endpoint, `opc.udp://239.0.0.1:4840`, is a multicast
-address. On one computer, the subscriber can miss multicast messages: the UDP
-transport disables multicast loopback by default
-(`UdpTransportOptions.MulticastLoopback`), and the publisher sends through the
-operating system's default multicast interface, which can differ from the
-interface that the subscriber joins. The unicast loopback address avoids both
-limitations. See [UDP / UADP](#udp--uadp) for the multicast options.
+address. On one computer, the subscriber can miss multicast messages because
+the UDP transport disables multicast loopback by default
+(`UdpTransportOptions.MulticastLoopback`). The unicast loopback address avoids
+this limitation. See [UDP / UADP](#udp--uadp) for the multicast options.
 
 A subscriber decodes a message only when it matches the publisher's settings:
 
@@ -587,12 +585,15 @@ defaults favor containment over reach:
 | --- | --- | --- |
 | `Ttl` | `1` | Multicast and unicast time-to-live; keeps multicast on the local subnet. |
 | `MulticastLoopback` | `false` | Whether this host receives copies of its own multicast messages. |
-| `PreferredNetworkInterface` | `null` | NIC name or local IP address used to join multicast groups when the connection address does not name one; otherwise the first operational interface for the address family. |
-
-A receiving connection joins its multicast group on that interface. A
-sending connection uses the operating system's default multicast interface.
-For a publisher and subscriber on one computer, use a unicast loopback address,
-as in [Run a first publisher and subscriber](#run-a-first-publisher-and-subscriber).
+| `PreferredNetworkInterface` | `null` | NIC name or local IP address used for multicast when the connection address does not name one; otherwise the first operational interface for the address family, preferring a non-loopback interface. |
+
+A receiving connection joins its multicast group on that interface. A sending
+connection sends its multicast messages and discovery announcements through
+the same interface. On a computer with several network interfaces, name the
+interface that reaches the other PubSub applications, either in the
+connection address (`NetworkInterface`) or in `PreferredNetworkInterface`. For
+a publisher and subscriber on one computer, use a unicast loopback address, as
+in [Run a first publisher and subscriber](#run-a-first-publisher-and-subscriber).
 
 ### Ethernet / UADP (`opc.eth://`)
 

src/Opc.Ua.PubSub.Udp/UdpDatagramTransport.cs

@@ -237,6 +237,21 @@ internal IPEndPoint? RemoteEndpoint
             }
         }
 
+        /// <summary>
+        /// The socket of the open transport, or <see langword="null"/> while the
+        /// transport is closed. Exposed for tests that inspect the socket options.
+        /// </summary>
+        internal Socket? SocketForTest
+        {
+            get
+            {
+                lock (m_sync)
+                {
+                    return m_socket;
+                }
+            }
+        }
+
         internal void SetAuthenticatedRemoteEndpoint(IPEndPoint remoteEndpoint)
         {
             if (remoteEndpoint is null)
@@ -873,9 +888,49 @@ private void ConfigureSocket(Socket socket)
             {
                 m_logger.SettingMulticastLoopbackFailed(ex, m_connection.Name);
             }
+            SetMulticastSendInterface(socket);
             ApplyQosCategory(socket);
         }
 
+        /// <summary>
+        /// Sends multicast datagrams, including discovery announcements, on the network
+        /// interface that the transport joins multicast groups on. Without the option the
+        /// operating system picks its default multicast route, which can be another
+        /// interface than the one subscribers listen on (OPC 10000-14 7.3.2.2).
+        /// </summary>
+        private void SetMulticastSendInterface(Socket socket)
+        {
+            try
+            {
+                if (socket.AddressFamily == AddressFamily.InterNetwork)
+                {
+                    IPAddress? localAddress = SelectLocalIPv4(m_networkInterface);
+                    if (localAddress is not null)
+                    {
+                        socket.SetSocketOption(
+                            SocketOptionLevel.IP,
+                            SocketOptionName.MulticastInterface,
+                            localAddress.GetAddressBytes());
+                    }
+                }
+                else if (socket.AddressFamily == AddressFamily.InterNetworkV6)
+                {
+                    int interfaceIndex = SelectIPv6InterfaceIndex(m_networkInterface);
+                    if (interfaceIndex != 0)
+                    {
+                        socket.SetSocketOption(
+                            SocketOptionLevel.IPv6,
+                            SocketOptionName.MulticastInterface,
+                            interfaceIndex);
+                    }
+                }
+            }
+            catch (SocketException ex)
+            {
+                m_logger.SettingMulticastInterfaceFailed(ex, m_connection.Name);
+            }
+        }
+
         private void ApplyQosCategory(Socket socket)
         {
             if (string.IsNullOrEmpty(m_v2Settings.QosCategory))
@@ -1387,6 +1442,13 @@ public static partial void UdpUnicastAlreadyConnected(
             Exception exception,
             string? connection,
             string endpoint);
+
+        [LoggerMessage(EventId = PubSubUdpEventIds.UdpDatagramTransport + 19, Level = LogLevel.Debug,
+            Message = "Setting IP_MULTICAST_IF failed for connection '{Connection}'.")]
+        public static partial void SettingMulticastInterfaceFailed(
+            this ILogger logger,
+            Exception exception,
+            string? connection);
     }
 
 }

src/Opc.Ua.PubSub.Udp/UdpNetworkInterfaceResolver.cs

@@ -44,8 +44,9 @@ namespace Opc.Ua.PubSub.Udp
     /// <remarks>
     /// Implements the NIC-selection guidance in
     /// <see href="https://reference.opcfoundation.org/specs/OPC-10000-14/v1.05.06/7.3.2.2">
-    /// Part 14 §7.3.2.2 UDP multicast / broadcast</see> — multicast
-    /// joins must specify the interface to avoid the OS picking an
+    /// Part 14 §7.3.2.2 UDP multicast / broadcast</see>: the transport
+    /// joins multicast groups and sends multicast datagrams on the
+    /// resolved interface, so the operating system does not pick an
     /// unrelated route.
     /// </remarks>
     public static class UdpNetworkInterfaceResolver

src/Opc.Ua.PubSub.Udp/UdpTransportOptions.cs

@@ -108,7 +108,10 @@ public sealed class UdpTransportOptions
         /// <c>NetworkInterface.Description</c>) or a literal IP
         /// address bound to a local NIC. When <see langword="null"/>
         /// or empty the transport picks the first up-and-running
-        /// interface that supports the target address family.
+        /// interface that supports the target address family. The
+        /// transport joins multicast groups and sends multicast
+        /// datagrams, including discovery announcements, on this
+        /// interface.
         /// </summary>
         public string? PreferredNetworkInterface { get; set; }
     }