title: How to receive UDP Multicast in VxWorks 5.5
body: <p>I have been unable to receive UDP multicast under VxWorks 5.5. I've joined the multicast group:</p>
  
  <pre><code>setsockopt(soc, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *) &amp;ipMreq, sizeof (ipMreq));
  </code></pre>
  
  <p>Similar code on an adjacent Windows machine does receive multicast.
  I am able to send multicast from VxWorks; ifShow() indicates the interface is multicast capable; MCAST_ROUTING is enabled in the kernel config, but still unable to receive multicast.</p>
  
  <p><strong>Edit</strong>: I needed to set a bit in the RealTek Ethernet drive RX configuration register to enable multicast to be passed on to the application layer.</p>
  
  <pre><code>#define RTL_RXCG_AM           0x04      /* Accept Multicast */
  </code></pre>