fixed multicast updated tests

Source/Components/Interfaces/UDPStream/UDPSClient.h

@@ -111,6 +111,7 @@ public:
      *  - ServerAddr      (char*)  Server IPv4 address. Required.
      *  - Port            (uint16) Server UDP port (unicast) or TCP listen port (multicast). Required.
      *  - MulticastGroup  (char*)  IPv4 multicast address; presence enables multicast mode.
+     *  - Interface       (char*)  Network interface for multicast join (e.g. "lo"). Required when MulticastGroup is set.
      *  - DataPort        (uint16) UDP multicast data port (defaults to Port+1).
      *  - SilenceTimeout  (uint32) Seconds of no data before reconnect. Default 5.
      *  - ReconnectDelay  (uint32) Seconds to wait between reconnect attempts. Default 2.

Source/Components/Interfaces/UDPStream/UDPSServer.cpp

@@ -11,8 +11,10 @@
 #include "MemoryOperationsHelper.h"
 #include "StreamString.h"
 
+#include <arpa/inet.h>
 #include <poll.h>
 #include <sys/select.h>
+#include <sys/socket.h>
 
 namespace MARTe {
 
@@ -125,9 +127,17 @@ bool UDPSServer::Start() {
     if (ok) {
       tcpListener.SetBlocking(false);
     }
-    // UDP data socket connected to multicast group
+    // UDP data socket connected to multicast group.
+    // Set IP_MULTICAST_IF so outgoing datagrams leave on the specified
+    // interface rather than whichever the kernel routing table picks.
     ok &= dataSocket.Open();
-    ok &= dataSocket.Join(multicastGroup.Buffer(), interface.Buffer());
+    if (ok) {
+      struct in_addr localIf;
+      localIf.s_addr = inet_addr(interface.Buffer());
+      int fd = static_cast<int>(dataSocket.GetWriteHandle());
+      ok = (setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF,
+                       &localIf, static_cast<socklen_t>(sizeof(localIf))) == 0);
+    }
     ok &= dataSocket.Connect(multicastGroup.Buffer(), dataPort);
     if (!ok) {
       REPORT_ERROR_STATIC(