Network interface selection for outgoing UDP multicast datagrams

Default  selection  is  the  loop-back interface.  Users  who  require
interoperation  between  WSJT-X   instances  cooperating  applications
running on different hosts should  select a suitable network interface
and  carefully choose  a multicast  group address,  and TTL,  that has
minimal scope covering the necessary  network(s). Using 224.0.0.1 is a
reasonable    strategy    if   all    hosts    are    on   the    same
subnet. Administratively  scoped multicast group addresses  like those
within 239.255.0.0/16  can cover larger  boundaries, but care  must be
taken if the local subnet has access to a multicast enabled router.

The  IPv4  broadcast  address  (255.255.255.255) may  be  used  as  an
alternative  to multicast  UDP, but  note that  WSJT-X will  only send
broadcast UDP datagrams  on the loop-back interface,  so all recipient
applications must be running on the same host system.

The reference UDP Message protocol  applications are being extended to
be configurable  with a list of  interfaces to join a  multicast group
address on. By default they will only join on the loop-back interface,
which is also  recommended for any applications designed  to take part
in the WSJT-X  UDP Message Protocol. This allows full  user control of
the  scope of  multicast  group membership  with  a very  conservative
default  mode  that will  work  with  all interoperating  applications
running on the same host system.

UDPExamples/MessageServer.hpp

@@ -2,6 +2,8 @@
 #define MESSAGE_SERVER_HPP__
 
 #include <QObject>
+#include <QString>
+#include <QStringList>
 #include <QTime>
 #include <QDateTime>
 #include <QHostAddress>
@@ -38,8 +40,9 @@ public:
   // start or restart the server, if the multicast_group_address
   // argument is given it is assumed to be a multicast group address
   // which the server will join
-  Q_SLOT void start (port_type port,
-                     QHostAddress const& multicast_group_address = QHostAddress {});
+  Q_SLOT void start (port_type port
+                     , QHostAddress const& multicast_group_address = QHostAddress {}
+                     , QStringList const& network_interface_names = QStringList {});
 
   // ask the client to clear one or both of the decode windows
   Q_SLOT void clear_decodes (QString const& id, quint8 window = 0);