title: Boost asio socket multicast to a specific ethernet interface
body: <p>I thought I had found the answer in the following example, but not quite.</p>
  
  <pre><code>boost::asio::ip::udp::socket socket(io_service); 
  ...
  boost::asio::ip::address_v4 local_interface =
      boost::asio::ip::address_v4::from_string("1.2.3.4");
  boost::asio::ip::multicast::outbound_interface option(local_interface);
  socket.set_option(option);
  </code></pre>
  
  <p>How do I map <code>eth0</code> to the appropriate outbound_interface option?</p>